docs/services/cloudformation.md

@@ -160,7 +160,7 @@ cross-resource references.
 | Kinesis Data Firehose | `DeliveryStream` |
 | IoT Core | `DomainConfiguration` (`ServerCertificates` resolves to a JSON string), `Policy` (deleted after detaching it from its principals; on AWS the delete fails with `DeleteConflictException` while the policy is attached), `Thing`, `TopicRule` |
 | CloudFront | `CachePolicy`, `Distribution`, `OriginAccessControl`, `OriginRequestPolicy`, `ResponseHeadersPolicy` |
-| CloudWatch | `Alarm`, `Dashboard` |
+| CloudWatch | `Alarm`, `CompositeAlarm`, `Dashboard`. Composite alarms have backing definitions with name Ref and Arn GetAtt, rollback, and replacement cleanup. Composite actions are stored only; suppression properties are rejected. |
 | CloudWatch Logs | `LogGroup`, `LogStream`, `MetricFilter` |
 | WAFv2 | `WebACL` |
 | Config | `ConfigRule` |

docs/services/cloudwatch.md

@@ -311,6 +311,7 @@ aws logs put-retention-policy \
 | `GetMetricStatistics` | Get metric statistics (Average, Sum, etc.) |
 | `GetMetricData` | Query metrics with math expressions |
 | `PutMetricAlarm` | Create a metric alarm |
+| `PutCompositeAlarm` | Create or update a composite alarm rule |
 | `DescribeAlarms` | List alarms |
 | `DeleteAlarms` | Delete alarms |
 | `SetAlarmState` | Manually set alarm state |
@@ -331,6 +332,11 @@ aws logs put-retention-policy \
 Metric streams are stored as definitions with their `running` or `stopped` state. Floci never
 delivers metrics to the Firehose delivery stream a metric stream names.
 
+Composite alarms support boolean rules over alarm states. Request `AlarmTypes` containing
+`CompositeAlarm` when describing them; the default describes metric alarms. Composite
+action lists are stored but not executed. Action suppression is not implemented and is
+rejected with `ValidationError`.
+
 ### SNS alarm actions
 
 State transitions publish to same-account standard SNS topics in `AlarmActions`, `OKActions`, or

src/main/java/io/github/hectorvent/floci/services/cloudformation/provisioners/CloudWatchCompositeAlarmCfnProvisioner.java

@@ -0,0 +1,183 @@
+package io.github.hectorvent.floci.services.cloudformation.provisioners;
+
+import com.fasterxml.jackson.core.JsonProcessingException;
+import com.fasterxml.jackson.databind.JsonNode;
+import com.fasterxml.jackson.databind.ObjectMapper;
+import com.fasterxml.jackson.databind.node.ObjectNode;
+import io.github.hectorvent.floci.core.common.AwsException;
+import io.github.hectorvent.floci.services.cloudformation.model.StackResource;
+import io.github.hectorvent.floci.services.cloudwatch.metrics.CloudWatchMetricsService;
+import io.github.hectorvent.floci.services.cloudwatch.metrics.model.CompositeAlarm;
+import jakarta.enterprise.context.ApplicationScoped;
+
+import java.util.List;
+import java.util.Map;
+import java.util.Set;
+import java.util.TreeSet;
+import java.util.stream.Collectors;
+
+@ApplicationScoped
+public class CloudWatchCompositeAlarmCfnProvisioner implements CfnResourceProvisioner {
+    private static final String TYPE = "AWS::CloudWatch::CompositeAlarm";
+    private static final String SNAPSHOT = "FlociCompositeAlarmSnapshot";
+    private static final String TAG_KEYS = "FlociCompositeAlarmTagKeys";
+    private static final String EXPLICIT_NAME = "FlociCompositeAlarmExplicitName";
+    private static final ObjectMapper MAPPER = new ObjectMapper();
+    private final CloudWatchMetricsService service;
+
+    public CloudWatchCompositeAlarmCfnProvisioner(CloudWatchMetricsService service) {
+        this.service = service;
+    }
+
+    @Override
+    public Set<String> resourceTypes() {
+        return Set.of(TYPE);
+    }
+
+    @Override
+    public void provision(StackResource resource, JsonNode props, ProvisionContext ctx) {
+        for (String unsupported : List.of("ActionsSuppressor", "ActionsSuppressorWaitPeriod",
+                "ActionsSuppressorExtensionPeriod")) {
+            if (props.has(unsupported)) {
+                throw new AwsException("ValidationError", "Composite action suppression is not supported", 400);
+            }
+        }
+        if (resource.getAttributes().containsKey(SNAPSHOT)) {
+            throw new AwsException("ResourceConflictException", "Alarm update cleanup is still pending", 400);
+        }
+        String explicit = ctx.resolveOptional(props, "AlarmName");
+        boolean removedName = explicit == null && "true".equals(resource.getAttributes().get(EXPLICIT_NAME));
+        String name = removedName ? ctx.generatePhysicalName(resource.getLogicalId(), 255, false)
+                : ctx.stablePhysicalName(explicit, resource.getLogicalId(), 255, false);
+        CompositeAlarm alarm = new CompositeAlarm();
+        alarm.setAlarmName(name);
+        alarm.setAlarmDescription(ctx.resolveOptional(props, "AlarmDescription"));
+        alarm.setAlarmRule(ctx.resolveOptional(props, "AlarmRule"));
+        alarm.setActionsEnabled(!"false".equals(ctx.resolveOptional(props, "ActionsEnabled")));
+        alarm.setTags(ctx.resolveTags(props, "Tags"));
+        actions(props, "AlarmActions", ctx, alarm.getAlarmActions());
+        actions(props, "OKActions", ctx, alarm.getOkActions());
+        actions(props, "InsufficientDataActions", ctx, alarm.getInsufficientDataActions());
+        Map<String, String> desiredTags = Map.copyOf(alarm.getTags());
+        Map<String, String> priorAttributes = Map.copyOf(resource.getAttributes());
+        boolean inPlace = ctx.reusesPriorEntity(name);
+        String updateSnapshot = null;
+        if (inPlace) {
+            ObjectNode snapshot = MAPPER.createObjectNode().put("region", ctx.region()).put("name", name);
+            List<CompositeAlarm> previous = service.describeCompositeAlarms(List.of(name), null, ctx.region());
+            if (!previous.isEmpty()) {
+                snapshot.set("alarm", MAPPER.valueToTree(previous.getFirst()));
+            }
+            snapshot.set("attributes", MAPPER.valueToTree(priorAttributes));
+            Set<String> changedKeys = new TreeSet<>(desiredTags.keySet());
+            changedKeys.addAll(tagKeys(resource.getAttributes().get(TAG_KEYS)));
+            snapshot.set("changedTagKeys", MAPPER.valueToTree(changedKeys));
+            updateSnapshot = snapshot.toString();
+        }
+        service.putCompositeAlarm(alarm, ctx.region(), inPlace);
+        if (inPlace) {
+            resource.getAttributes().put(SNAPSHOT, updateSnapshot);
+            reconcileTags(alarm.getAlarmArn(), tagKeys(priorAttributes.get(TAG_KEYS)), desiredTags, ctx.region());
+        }
+        resource.getAttributes().put(TAG_KEYS, MAPPER.valueToTree(desiredTags.keySet()).toString());
+        resource.setPhysicalId(name);
+        resource.getAttributes().put("Arn", alarm.getAlarmArn());
+        resource.getAttributes().put(EXPLICIT_NAME, Boolean.toString(explicit != null));
+        ReplacementCleanup.record(resource, ctx, priorAttributes);
+    }
+
+    private static void actions(JsonNode props, String field, ProvisionContext ctx, List<String> target) {
+        JsonNode values = ctx.engine().resolveNode(props.path(field));
+        if (values != null && values.isArray()) {
+            values.forEach(value -> target.add(value.asText()));
+        }
+    }
+
+    private static Set<String> tagKeys(String json) {
+        Set<String> keys = new TreeSet<>();
+        if (json != null) {
+            try {
+                MAPPER.readTree(json).forEach(value -> keys.add(value.asText()));
+            } catch (JsonProcessingException error) {
+                throw new IllegalStateException("Cannot read composite alarm tag ownership", error);
+            }
+        }
+        return keys;
+    }
+
+    private void reconcileTags(String arn, Set<String> owned, Map<String, String> desired, String region) {
+        List<String> removed = owned.stream().filter(key -> !desired.containsKey(key)).toList();
+        if (!removed.isEmpty()) {
+            service.untagResource(arn, removed, region);
+        }
+        if (!desired.isEmpty()) {
+            service.tagResource(arn, desired, region);
+        }
+    }
+
+    @Override
+    public void delete(String type, String physicalId, String region) {
+        service.deleteAlarms(List.of(physicalId), region);
+    }
+
+    @Override
+    public boolean retainsFailedUpdateState(StackResource resource) {
+        return resource.getAttributes().containsKey(SNAPSHOT);
+    }
+
+    @Override
+    public boolean rollbackUpdate(StackResource resource) {
+        if (ReplacementCleanup.rollback(resource, this::delete)) {
+            return true;
+        }
+        String raw = resource.getAttributes().get(SNAPSHOT);
+        if (raw == null) {
+            return false;
+        }
+        try {
+            JsonNode snapshot = MAPPER.readTree(raw);
+            String region = snapshot.path("region").asText();
+            if (snapshot.has("alarm")) {
+                CompositeAlarm original = MAPPER.treeToValue(snapshot.get("alarm"), CompositeAlarm.class);
+                Map<String, String> originalTags = Map.copyOf(original.getTags());
+                service.putCompositeAlarm(original, region);
+                Set<String> changed = tagKeys(snapshot.path("changedTagKeys").toString());
+                reconcileTags(original.getAlarmArn(), changed, originalTags.entrySet().stream()
+                        .filter(entry -> changed.contains(entry.getKey()))
+                        .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue)), region);
+            } else {
+                delete(TYPE, snapshot.path("name").asText(), region);
+            }
+            resource.getAttributes().clear();
+            snapshot.path("attributes").fields().forEachRemaining(entry ->
+                    resource.getAttributes().put(entry.getKey(), entry.getValue().asText()));
+        } catch (JsonProcessingException error) {
+            throw new IllegalStateException("Cannot read composite alarm rollback state", error);
+        }
+        return true;
+    }
+
+    @Override
+    public boolean hasReplacementUpdate(StackResource resource) {
+        return ReplacementCleanup.hasReplacement(resource);
+    }
+
+    @Override
+    public String updateCleanupPhysicalId(StackResource resource) {
+        return ReplacementCleanup.cleanupPhysicalId(resource);
+    }
+
+    @Override
+    public UpdateCleanupResult completeUpdate(StackResource resource) {
+        if (resource.getAttributes().containsKey(SNAPSHOT) && !ReplacementCleanup.hasReplacement(resource)) {
+            return new UpdateCleanupResult(true, true, null, 0, null);
+        }
+        return ReplacementCleanup.complete(resource, this::delete);
+    }
+
+    @Override
+    public void clearUpdate(StackResource resource) {
+        resource.getAttributes().remove(SNAPSHOT);
+        ReplacementCleanup.clear(resource);
+    }
+}

src/main/java/io/github/hectorvent/floci/services/cloudwatch/metrics/AlarmSnsNotifier.java

@@ -11,6 +11,7 @@ import org.jboss.logging.Logger;
 import java.time.Instant;
 import java.time.ZoneOffset;
 import java.time.format.DateTimeFormatter;
+import java.util.ArrayList;
 import java.util.LinkedHashMap;
 import java.util.List;
 import java.util.Locale;
@@ -63,9 +64,11 @@ public class AlarmSnsNotifier {
         this.objectMapper = objectMapper;
     }
 
-    public void notifyTransition(MetricAlarm alarm, String oldState) {
+    record Notification(String topicArn, String message, String subject, String region) {}
+
+    List<Notification> prepareTransition(MetricAlarm alarm, String oldState) {
         if (!alarm.isActionsEnabled() || alarm.getStateValue().equals(oldState)) {
-            return;
+            return List.of();
         }
         List<String> actions = switch (alarm.getStateValue()) {
             case "ALARM" -> alarm.getAlarmActions();
@@ -74,8 +77,9 @@ public class AlarmSnsNotifier {
             default -> List.of();
         };
         if (actions == null) {
-            return;
+            return List.of();
         }
+        List<Notification> notifications = new ArrayList<>();
         for (String action : actions) {
             if (!AwsArnUtils.isArnFor(action, "sns")) {
                 continue;
@@ -86,12 +90,25 @@ public class AlarmSnsNotifier {
                 if (subject.length() > 100) {
                     subject = subject.substring(0, 100);
                 }
-                snsService.publish(action, null, message(alarm, oldState), subject, topic.region());
+                notifications.add(new Notification(action, message(alarm, oldState), subject, topic.region()));
             } catch (Exception e) {
                 LOG.warnv(e, "Unable to publish alarm {0} state {1} to SNS action {2}",
                         alarm.getAlarmName(), alarm.getStateValue(), action);
             }
         }
+        return notifications;
+    }
+
+    void publish(List<Notification> notifications) {
+        for (Notification notification : notifications) {
+            try {
+                snsService.publish(notification.topicArn(), null, notification.message(),
+                        notification.subject(), notification.region());
+            } catch (Exception error) {
+                LOG.warnv(error, "Unable to publish alarm transition {0} to SNS action {1}",
+                        notification.subject(), notification.topicArn());
+            }
+        }
     }
 
     private String message(MetricAlarm alarm, String oldState) throws JsonProcessingException {

src/main/java/io/github/hectorvent/floci/services/cloudwatch/metrics/CloudWatchMetricsJsonHandler.java

@@ -5,9 +5,16 @@ import com.fasterxml.jackson.databind.ObjectMapper;
 import com.fasterxml.jackson.databind.node.ArrayNode;
 import com.fasterxml.jackson.databind.node.ObjectNode;
 import io.github.hectorvent.floci.core.common.AwsErrorResponse;
+import io.github.hectorvent.floci.core.common.AwsException;
 import io.github.hectorvent.floci.core.common.PaginatedResult;
 import io.github.hectorvent.floci.services.cloudwatch.dashboards.CloudWatchDashboardsService;
 import io.github.hectorvent.floci.services.cloudwatch.dashboards.model.Dashboard;
+import io.github.hectorvent.floci.services.cloudwatch.metrics.model.Alarm;
+import io.github.hectorvent.floci.services.cloudwatch.metrics.model.AlarmMetricDataQuery;
+import io.github.hectorvent.floci.services.cloudwatch.metrics.model.CompositeAlarm;
+import io.github.hectorvent.floci.services.cloudwatch.metrics.model.Dimension;
+import io.github.hectorvent.floci.services.cloudwatch.metrics.model.MetricAlarm;
+import io.github.hectorvent.floci.services.cloudwatch.metrics.model.MetricDatum;
 import io.github.hectorvent.floci.services.cloudwatch.metricstreams.CloudWatchMetricStreamsService;
 import io.github.hectorvent.floci.services.cloudwatch.metricstreams.model.MetricStream;
 import io.github.hectorvent.floci.services.cloudwatch.metricstreams.model.MetricStreamFilter;
@@ -57,6 +64,7 @@ public class CloudWatchMetricsJsonHandler {
             case "ListMetrics" -> handleListMetrics(request, region);
             case "GetMetricStatistics" -> handleGetMetricStatistics(request, region);
             case "PutMetricAlarm" -> handlePutMetricAlarm(request, region);
+            case "PutCompositeAlarm" -> handlePutCompositeAlarm(request, region);
             case "DescribeAlarms" -> handleDescribeAlarms(request, region);
             case "DeleteAlarms" -> handleDeleteAlarms(request, region);
             case "SetAlarmState" -> handleSetAlarmState(request, region);
@@ -156,8 +164,7 @@ public class CloudWatchMetricsJsonHandler {
 
     private Response handlePutMetricAlarm(JsonNode request, String region) {
         MetricAlarm alarm = new MetricAlarm();
-        alarm.setAlarmName(request.path("AlarmName").asText());
-        alarm.setAlarmDescription(request.path("AlarmDescription").asText(null));
+        parseAlarmFields(request, alarm);
         alarm.setMetricName(request.path("MetricName").asText(null));
         alarm.setNamespace(request.path("Namespace").asText(null));
         alarm.setStatistic(request.path("Statistic").asText(null));
@@ -175,9 +182,33 @@ public class CloudWatchMetricsJsonHandler {
         alarm.setThreshold(request.path("Threshold").asDouble(0));
         alarm.setComparisonOperator(request.path("ComparisonOperator").asText(null));
         alarm.setTreatMissingData(request.path("TreatMissingData").asText(null));
-        alarm.setActionsEnabled(request.path("ActionsEnabled").asBoolean(true));
         alarm.setDimensions(parseDimensionsJson(request.path("Dimensions")));
 
+        metricsService.putMetricAlarm(alarm, region);
+        return Response.ok(objectMapper.createObjectNode()).build();
+    }
+
+    private Response handlePutCompositeAlarm(JsonNode request, String region) {
+        rejectSuppressor(request);
+        CompositeAlarm alarm = new CompositeAlarm();
+        parseAlarmFields(request, alarm);
+        alarm.setAlarmRule(request.path("AlarmRule").asText(null));
+        metricsService.putCompositeAlarm(alarm, region);
+        return Response.ok(objectMapper.createObjectNode()).build();
+    }
+
+    private static void rejectSuppressor(JsonNode request) {
+        if (request.has("ActionsSuppressor") || request.has("ActionsSuppressorWaitPeriod")
+                || request.has("ActionsSuppressorExtensionPeriod")) {
+            throw new AwsException("ValidationError", "Composite action suppression is not supported", 400);
+        }
+    }
+
+    private void parseAlarmFields(JsonNode request, Alarm alarm) {
+        alarm.setAlarmName(request.path("AlarmName").asText());
+        alarm.setAlarmDescription(request.path("AlarmDescription").asText(null));
+        alarm.setActionsEnabled(request.path("ActionsEnabled").asBoolean(true));
+
         JsonNode alarmActions = request.path("AlarmActions");
         if (alarmActions.isArray()) {
             alarmActions.forEach(a -> alarm.getAlarmActions().add(a.asText()));
@@ -198,8 +229,6 @@ public class CloudWatchMetricsJsonHandler {
             alarm.setTags(tags);
         }
 
-        metricsService.putMetricAlarm(alarm, region);
-        return Response.ok(objectMapper.createObjectNode()).build();
     }
 
     private Response handleDescribeAlarms(JsonNode request, String region) {
@@ -210,21 +239,17 @@ public class CloudWatchMetricsJsonHandler {
         }
         String prefix = request.has("AlarmNamePrefix") ? request.path("AlarmNamePrefix").asText() : null;
 
-        List<MetricAlarm> alarms = metricsService.describeAlarms(alarmNames, prefix, region);
+        List<String> types = new ArrayList<>();
+        request.path("AlarmTypes").forEach(value -> types.add(value.asText()));
+        metricsService.validateAlarmTypes(types);
+        List<MetricAlarm> alarms = types.isEmpty() || types.contains("MetricAlarm")
+                ? metricsService.describeAlarms(alarmNames, prefix, region) : List.of();
 
         ObjectNode response = objectMapper.createObjectNode();
         ArrayNode arr = response.putArray("MetricAlarms");
         for (MetricAlarm a : alarms) {
             ObjectNode node = arr.addObject();
-            node.put("AlarmName", a.getAlarmName());
-            if (a.getAlarmArn() != null) node.put("AlarmArn", a.getAlarmArn());
-            if (a.getAlarmDescription() != null) node.put("AlarmDescription", a.getAlarmDescription());
-            ArrayNode alarmActions = node.putArray("AlarmActions");
-            a.getAlarmActions().forEach(alarmActions::add);
-            ArrayNode okActions = node.putArray("OKActions");
-            a.getOkActions().forEach(okActions::add);
-            ArrayNode insufficientDataActions = node.putArray("InsufficientDataActions");
-            a.getInsufficientDataActions().forEach(insufficientDataActions::add);
+            describeAlarmFields(node, a);
             if (!a.getMetrics().isEmpty()) {
                 node.set("Metrics", objectMapper.valueToTree(a.getMetrics()));
             } else {
@@ -257,17 +282,48 @@ public class CloudWatchMetricsJsonHandler {
             node.put("Threshold", a.getThreshold());
             if (a.getComparisonOperator() != null) node.put("ComparisonOperator", a.getComparisonOperator());
             if (a.getTreatMissingData() != null) node.put("TreatMissingData", a.getTreatMissingData());
-            node.put("ActionsEnabled", a.isActionsEnabled());
-            if (a.getStateValue() != null) node.put("StateValue", a.getStateValue());
-            if (a.getStateReason() != null) node.put("StateReason", a.getStateReason());
-            if (a.getStateReasonData() != null) node.put("StateReasonData", a.getStateReasonData());
-            node.put("StateUpdatedTimestamp", a.getStateUpdatedTimestamp());
+
+        }
+        ArrayNode composites = response.putArray("CompositeAlarms");
+        if (types.contains("CompositeAlarm")) {
+            for (CompositeAlarm alarm : metricsService.describeCompositeAlarms(alarmNames, prefix, region)) {
+                ObjectNode node = composites.addObject();
+                describeAlarmFields(node, alarm);
+                node.put("AlarmRule", alarm.getAlarmRule());
+            }
         }
-        response.putArray("CompositeAlarms");
         response.putArray("LogAlarms");
         return Response.ok(response).build();
     }
 
+    private void describeAlarmFields(ObjectNode node, Alarm a) {
+        node.put("AlarmName", a.getAlarmName());
+        if (a.getAlarmArn() != null) {
+            node.put("AlarmArn", a.getAlarmArn());
+        }
+        if (a.getAlarmDescription() != null) {
+            node.put("AlarmDescription", a.getAlarmDescription());
+        }
+        ArrayNode alarmActions = node.putArray("AlarmActions");
+        a.getAlarmActions().forEach(alarmActions::add);
+        ArrayNode okActions = node.putArray("OKActions");
+        a.getOkActions().forEach(okActions::add);
+        ArrayNode insufficientDataActions = node.putArray("InsufficientDataActions");
+        a.getInsufficientDataActions().forEach(insufficientDataActions::add);
+        node.put("ActionsEnabled", a.isActionsEnabled());
+        if (a.getStateValue() != null) {
+            node.put("StateValue", a.getStateValue());
+        }
+        if (a.getStateReason() != null) {
+            node.put("StateReason", a.getStateReason());
+        }
+        if (a.getStateReasonData() != null) {
+            node.put("StateReasonData", a.getStateReasonData());
+        }
+        node.put("StateUpdatedTimestamp", a.getStateUpdatedTimestamp());
+        node.put("AlarmConfigurationUpdatedTimestamp", a.getAlarmConfigurationUpdatedTimestamp());
+    }
+
     private Response handleDeleteAlarms(JsonNode request, String region) {
         List<String> alarmNames = new ArrayList<>();
         JsonNode namesNode = request.path("AlarmNames");

src/main/java/io/github/hectorvent/floci/services/cloudwatch/metrics/CloudWatchMetricsQueryHandler.java

@@ -1,5 +1,6 @@
 package io.github.hectorvent.floci.services.cloudwatch.metrics;
 
+import io.github.hectorvent.floci.core.common.AwsException;
 import io.github.hectorvent.floci.core.common.AwsNamespaces;
 import io.github.hectorvent.floci.core.common.AwsQueryResponse;
 import io.github.hectorvent.floci.core.common.PaginatedResult;
@@ -7,6 +8,12 @@ import io.github.hectorvent.floci.core.common.Pagination;
 import io.github.hectorvent.floci.core.common.XmlBuilder;
 import io.github.hectorvent.floci.services.cloudwatch.dashboards.CloudWatchDashboardsService;
 import io.github.hectorvent.floci.services.cloudwatch.dashboards.model.Dashboard;
+import io.github.hectorvent.floci.services.cloudwatch.metrics.model.Alarm;
+import io.github.hectorvent.floci.services.cloudwatch.metrics.model.AlarmMetricDataQuery;
+import io.github.hectorvent.floci.services.cloudwatch.metrics.model.CompositeAlarm;
+import io.github.hectorvent.floci.services.cloudwatch.metrics.model.Dimension;
+import io.github.hectorvent.floci.services.cloudwatch.metrics.model.MetricAlarm;
+import io.github.hectorvent.floci.services.cloudwatch.metrics.model.MetricDatum;
 import io.github.hectorvent.floci.services.cloudwatch.metricstreams.CloudWatchMetricStreamsService;
 import io.github.hectorvent.floci.services.cloudwatch.metricstreams.model.MetricStream;
 import io.github.hectorvent.floci.services.cloudwatch.metricstreams.model.MetricStreamFilter;
@@ -58,6 +65,7 @@ public class CloudWatchMetricsQueryHandler {
             case "GetMetricStatistics" -> handleGetMetricStatistics(params, region);
             case "GetMetricData" -> handleGetMetricData(params, region);
             case "PutMetricAlarm" -> handlePutMetricAlarm(params, region);
+            case "PutCompositeAlarm" -> handlePutCompositeAlarm(params, region);
             case "DescribeAlarms" -> handleDescribeAlarms(params, region);
             case "DeleteAlarms" -> handleDeleteAlarms(params, region);
             case "SetAlarmState" -> handleSetAlarmState(params, region);
@@ -159,7 +167,8 @@ public class CloudWatchMetricsQueryHandler {
             xml.elem("Unit", dp.unit()).end("member");
         }
         xml.end("Datapoints");
-        return Response.ok(AwsQueryResponse.envelope("GetMetricStatistics", AwsNamespaces.CW, xml.build())).build();
+        return Response.ok(AwsQueryResponse.envelope("GetMetricStatistics",
+                AwsNamespaces.CW, xml.build())).build();
     }
 
     private Response handleGetMetricData(MultivaluedMap<String, String> params, String region) {
@@ -243,6 +252,26 @@ public class CloudWatchMetricsQueryHandler {
         return Response.ok(AwsQueryResponse.envelopeNoResult("PutMetricAlarm", null)).build();
     }
 
+    private Response handlePutCompositeAlarm(MultivaluedMap<String, String> params, String region) {
+        if (params.containsKey("ActionsSuppressor") || params.containsKey("ActionsSuppressorWaitPeriod")
+                || params.containsKey("ActionsSuppressorExtensionPeriod")) {
+            throw new AwsException("ValidationError", "Composite action suppression is not supported", 400);
+        }
+        CompositeAlarm alarm = new CompositeAlarm();
+        parseAlarmFields(params, alarm);
+        alarm.setAlarmRule(params.getFirst("AlarmRule"));
+        metricsService.putCompositeAlarm(alarm, region);
+        return Response.ok(AwsQueryResponse.envelopeNoResult("PutCompositeAlarm", null)).build();
+    }
+
+    private static List<String> alarmMembers(MultivaluedMap<String, String> params, String key) {
+        List<String> values = new ArrayList<>();
+        for (int i = 1; params.getFirst(key + ".member." + i) != null; i++) {
+            values.add(params.getFirst(key + ".member." + i));
+        }
+        return values;
+    }
+
     private Response handleDescribeAlarms(MultivaluedMap<String, String> params, String region) {
         List<String> alarmNames = new ArrayList<>();
         for (int i = 1; ; i++) {
@@ -252,15 +281,24 @@ public class CloudWatchMetricsQueryHandler {
         }
         String prefix = params.getFirst("AlarmNamePrefix");
 
-        List<MetricAlarm> alarms = metricsService.describeAlarms(alarmNames, prefix, region);
+        List<String> types = alarmMembers(params, "AlarmTypes");
+        metricsService.validateAlarmTypes(types);
+        List<MetricAlarm> alarms = types.isEmpty() || types.contains("MetricAlarm")
+                ? metricsService.describeAlarms(alarmNames, prefix, region) : List.of();
 
         var xml = new XmlBuilder().start("MetricAlarms");
         for (MetricAlarm a : alarms) {
             toAlarmXml(xml, a);
         }
-        xml.end("MetricAlarms");
-        xml.start("CompositeAlarms").end("CompositeAlarms");
-        xml.start("LogAlarms").end("LogAlarms");
+        xml.end("MetricAlarms").start("CompositeAlarms");
+        if (types.contains("CompositeAlarm")) {
+            for (CompositeAlarm alarm : metricsService.describeCompositeAlarms(alarmNames, prefix, region)) {
+                xml.start("member");
+                describeAlarmFields(xml, alarm);
+                xml.elem("AlarmRule", alarm.getAlarmRule()).end("member");
+            }
+        }
+        xml.end("CompositeAlarms").start("LogAlarms").end("LogAlarms");
         return Response.ok(AwsQueryResponse.envelope("DescribeAlarms", null, xml.build())).build();
     }
 
@@ -682,10 +720,7 @@ public class CloudWatchMetricsQueryHandler {
 
     private MetricAlarm parseAlarm(MultivaluedMap<String, String> params) {
         MetricAlarm a = new MetricAlarm();
-        a.setAlarmName(params.getFirst("AlarmName"));
-        a.setAlarmDescription(params.getFirst("AlarmDescription"));
-        String actionsEnabled = params.getFirst("ActionsEnabled");
-        a.setActionsEnabled(actionsEnabled == null || Boolean.parseBoolean(actionsEnabled));
+        parseAlarmFields(params, a);
         a.setMetricName(params.getFirst("MetricName"));
         a.setNamespace(params.getFirst("Namespace"));
         a.setStatistic(params.getFirst("Statistic"));
@@ -710,35 +745,24 @@ public class CloudWatchMetricsQueryHandler {
         }
         a.setDimensions(dims);
 
-        // Actions
-        for (int i = 1; ; i++) {
-            String act = params.getFirst("OKActions.member." + i);
-            if (act == null) break;
-            a.getOkActions().add(act);
-        }
-        for (int i = 1; ; i++) {
-            String act = params.getFirst("AlarmActions.member." + i);
-            if (act == null) break;
-            a.getAlarmActions().add(act);
-        }
-        for (int i = 1; ; i++) {
-            String act = params.getFirst("InsufficientDataActions.member." + i);
-            if (act == null) break;
-            a.getInsufficientDataActions().add(act);
-        }
-
-        // Tags
-        Map<String, String> tags = new LinkedHashMap<>();
-        for (int i = 1; ; i++) {
-            String key = params.getFirst("Tags.member." + i + ".Key");
-            if (key == null) break;
-            tags.put(key, params.getFirst("Tags.member." + i + ".Value"));
-        }
-        a.setTags(tags);
-
         return a;
     }
 
+    private void parseAlarmFields(MultivaluedMap<String, String> params, Alarm alarm) {
+        alarm.setAlarmName(params.getFirst("AlarmName"));
+        alarm.setAlarmDescription(params.getFirst("AlarmDescription"));
+        String actionsEnabled = params.getFirst("ActionsEnabled");
+        alarm.setActionsEnabled(actionsEnabled == null || Boolean.parseBoolean(actionsEnabled));
+        alarm.setAlarmActions(alarmMembers(params, "AlarmActions"));
+        alarm.setOkActions(alarmMembers(params, "OKActions"));
+        alarm.setInsufficientDataActions(alarmMembers(params, "InsufficientDataActions"));
+        Map<String, String> tags = new LinkedHashMap<>();
+        for (int i = 1; params.getFirst("Tags.member." + i + ".Key") != null; i++) {
+            tags.put(params.getFirst("Tags.member." + i + ".Key"), params.getFirst("Tags.member." + i + ".Value"));
+        }
+        alarm.setTags(tags);
+    }
+
     private List<AlarmMetricDataQuery> parseAlarmMetricQueries(MultivaluedMap<String, String> params) {
         List<AlarmMetricDataQuery> queries = new ArrayList<>();
         for (int index = 1; ; index++) {
@@ -784,22 +808,8 @@ public class CloudWatchMetricsQueryHandler {
     }
 
     private void toAlarmXml(XmlBuilder xml, MetricAlarm a) {
-        xml.start("member")
-                .elem("AlarmName", a.getAlarmName())
-                .elem("AlarmArn", a.getAlarmArn())
-                .elem("AlarmDescription", a.getAlarmDescription());
-        xml.start("AlarmActions");
-        a.getAlarmActions().forEach(act -> xml.elem("member", act));
-        xml.end("AlarmActions")
-                .elem("AlarmConfigurationUpdatedTimestamp", Instant.ofEpochSecond(a.getAlarmConfigurationUpdatedTimestamp()).toString())
-                .elem("ActionsEnabled", String.valueOf(a.isActionsEnabled()));
-
-        xml.start("OKActions");
-        a.getOkActions().forEach(act -> xml.elem("member", act));
-        xml.end("OKActions");
-        xml.start("InsufficientDataActions");
-        a.getInsufficientDataActions().forEach(act -> xml.elem("member", act));
-        xml.end("InsufficientDataActions");
+        xml.start("member");
+        describeAlarmFields(xml, a);
         if (!a.getMetrics().isEmpty()) {
             toAlarmMetricsXml(xml, a.getMetrics());
         } else {
@@ -815,11 +825,7 @@ public class CloudWatchMetricsQueryHandler {
                     .elem("Unit", a.getUnit());
         }
 
-        xml.elem("StateValue", a.getStateValue())
-                .elem("StateReason", a.getStateReason())
-                .elem("StateReasonData", a.getStateReasonData())
-                .elem("StateUpdatedTimestamp", Instant.ofEpochSecond(a.getStateUpdatedTimestamp()).toString())
-                .elem("EvaluationPeriods", String.valueOf(a.getEvaluationPeriods()))
+        xml.elem("EvaluationPeriods", String.valueOf(a.getEvaluationPeriods()))
                 .elem("Threshold", String.valueOf(a.getThreshold()))
                 .elem("ComparisonOperator", a.getComparisonOperator())
                 .elem("TreatMissingData", a.getTreatMissingData());
@@ -831,6 +837,28 @@ public class CloudWatchMetricsQueryHandler {
         xml.end("member");
     }
 
+    private void describeAlarmFields(XmlBuilder xml, Alarm a) {
+        xml.elem("AlarmName", a.getAlarmName())
+                .elem("AlarmArn", a.getAlarmArn())
+                .elem("AlarmDescription", a.getAlarmDescription());
+        xml.start("AlarmActions");
+        a.getAlarmActions().forEach(act -> xml.elem("member", act));
+        xml.end("AlarmActions")
+                .elem("AlarmConfigurationUpdatedTimestamp", Instant.ofEpochSecond(a.getAlarmConfigurationUpdatedTimestamp()).toString())
+                .elem("ActionsEnabled", String.valueOf(a.isActionsEnabled()));
+
+        xml.start("OKActions");
+        a.getOkActions().forEach(act -> xml.elem("member", act));
+        xml.end("OKActions");
+        xml.start("InsufficientDataActions");
+        a.getInsufficientDataActions().forEach(act -> xml.elem("member", act));
+        xml.end("InsufficientDataActions");
+        xml.elem("StateValue", a.getStateValue())
+                .elem("StateReason", a.getStateReason())
+                .elem("StateReasonData", a.getStateReasonData())
+                .elem("StateUpdatedTimestamp", Instant.ofEpochSecond(a.getStateUpdatedTimestamp()).toString());
+    }
+
     private void toAlarmMetricsXml(XmlBuilder xml, List<AlarmMetricDataQuery> metrics) {
         xml.start("Metrics");
         for (AlarmMetricDataQuery query : metrics) {

src/main/java/io/github/hectorvent/floci/services/cloudwatch/metrics/CloudWatchMetricsService.java

@@ -1,10 +1,15 @@
 package io.github.hectorvent.floci.services.cloudwatch.metrics;
 
+import com.fasterxml.jackson.core.type.TypeReference;
+import io.github.hectorvent.floci.core.common.AwsArnUtils;
 import io.github.hectorvent.floci.core.common.AwsException;
 import io.github.hectorvent.floci.core.common.RegionResolver;
+import io.github.hectorvent.floci.core.common.SmithyValidationErrors;
 import io.github.hectorvent.floci.core.storage.AccountAwareStorageBackend;
 import io.github.hectorvent.floci.core.storage.StorageBackend;
 import io.github.hectorvent.floci.core.storage.StorageFactory;
+import io.github.hectorvent.floci.services.cloudwatch.metrics.model.Alarm;
+import io.github.hectorvent.floci.services.cloudwatch.metrics.model.CompositeAlarm;
 import io.github.hectorvent.floci.services.cloudwatch.metrics.model.Dimension;
 import io.github.hectorvent.floci.services.cloudwatch.metrics.model.MetricAlarm;
 import io.github.hectorvent.floci.services.cloudwatch.metrics.model.MetricDatum;
@@ -14,11 +19,16 @@ import jakarta.inject.Inject;
 import org.jboss.logging.Logger;
 
 import java.time.Instant;
+import java.util.ArrayDeque;
 import java.util.ArrayList;
 import java.util.Comparator;
+import java.util.HashMap;
+import java.util.HashSet;
+import java.util.Iterator;
 import java.util.LinkedHashMap;
 import java.util.List;
 import java.util.Map;
+import java.util.Set;
 import java.util.UUID;
 import java.util.stream.Collectors;
 
@@ -27,8 +37,10 @@ public class CloudWatchMetricsService {
 
     private static final Logger LOG = Logger.getLogger(CloudWatchMetricsService.class);
 
+    private static final List<String> ALARM_TYPES = List.of("CompositeAlarm", "MetricAlarm", "LogAlarm");
+
     private final StorageBackend<String, MetricDatum> metricStore;
-    private final StorageBackend<String, MetricAlarm> alarmStore;
+    private final StorageBackend<String, Alarm> alarmStore;
     private final RegionResolver regionResolver;
     private final AlarmSnsNotifier alarmSnsNotifier;
 
@@ -42,19 +54,19 @@ public class CloudWatchMetricsService {
         this.metricStore = storageFactory.create("cloudwatchmetrics", "cwmetrics.json",
                 new TypeReference<Map<String, MetricDatum>>() {});
         this.alarmStore = storageFactory.create("cloudwatchmetrics", "cwalarms.json",
-                new TypeReference<Map<String, MetricAlarm>>() {});
+                new TypeReference<Map<String, Alarm>>() {});
         this.regionResolver = regionResolver;
         this.alarmSnsNotifier = alarmSnsNotifier;
     }
 
     CloudWatchMetricsService(StorageBackend<String, MetricDatum> metricStore,
-                             StorageBackend<String, MetricAlarm> alarmStore,
+                             StorageBackend<String, Alarm> alarmStore,
                              RegionResolver regionResolver) {
         this(metricStore, alarmStore, regionResolver, null);
     }
 
     CloudWatchMetricsService(StorageBackend<String, MetricDatum> metricStore,
-                             StorageBackend<String, MetricAlarm> alarmStore,
+                             StorageBackend<String, Alarm> alarmStore,
                              RegionResolver regionResolver, AlarmSnsNotifier alarmSnsNotifier) {
         this.metricStore = metricStore;
         this.alarmStore = alarmStore;
@@ -296,45 +308,236 @@ public class CloudWatchMetricsService {
         };
     }
 
-    public void putMetricAlarm(MetricAlarm alarm, String region) {
+    public synchronized void putMetricAlarm(MetricAlarm alarm, String region) {
+        putAlarm(alarm, region);
+        evaluateComposites(region, Set.of(alarm.getAlarmName()), null);
+    }
+
+    public void putCompositeAlarm(CompositeAlarm alarm, String region) {
+        putCompositeAlarm(alarm, region, true);
+    }
+
+    public synchronized void putCompositeAlarm(CompositeAlarm alarm, String region, boolean replaceExisting) {
+        if (!replaceExisting && alarmStore.get(region + "::" + alarm.getAlarmName()).isPresent()) {
+            throw new AwsException("AlreadyExists", "Alarm already exists: " + alarm.getAlarmName(), 400);
+        }
+        Set<String> references = alarmReferences(alarm, region);
+        if (references.size() > 100) {
+            throw new AwsException("LimitExceeded", "AlarmRule references more than 100 child alarms", 400);
+        }
+        List<CompositeAlarm> existing = describeCompositeAlarms(null, null, region);
+        for (String reference : references) {
+            long parents = existing.stream().filter(parent -> !parent.getAlarmName().equals(alarm.getAlarmName()))
+                    .filter(parent -> alarmReferences(parent, region).contains(reference))
+                    .count();
+            if (parents >= 150) {
+                throw new AwsException("LimitExceeded", "An alarm can be referenced by at most 150 composite alarms", 400);
+            }
+        }
+        boolean created = alarmStore.get(region + "::" + alarm.getAlarmName()).isEmpty();
+        putAlarm(alarm, region);
+        if (created) {
+            evaluateComposites(region, Set.of(), alarm.getAlarmName());
+        }
+    }
+
+    private Set<String> alarmReferences(CompositeAlarm alarm, String region) {
+        return CompositeAlarmRule.parse(alarm.getAlarmRule()).references().stream()
+                .map(reference -> AwsArnUtils.isArn(reference) ? reference
+                        : regionResolver.buildArn("cloudwatch", region, "alarm:" + reference))
+                .collect(Collectors.toSet());
+    }
+
+    private void putAlarm(Alarm alarm, String region) {
+        String name = alarm.getAlarmName();
+        if (name == null || name.isBlank() || name.length() > 255) {
+            throw SmithyValidationErrors.validationError(name == null ? SmithyValidationErrors.notNull("alarmName")
+                    : "Value '" + name + "' at 'alarmName' failed to satisfy constraint: Member must have length "
+                            + (name.length() > 255 ? "less than or equal to 255" : "greater than or equal to 1"));
+        }
+        Alarm previous = alarmStore.get(region + "::" + name).orElse(null);
+        if (previous != null) {
+            if (!previous.getClass().equals(alarm.getClass())) {
+                throw new AwsException("ValidationError", "An alarm of another type already has this name", 400);
+            }
+            if (alarm instanceof CompositeAlarm) {
+                alarm.setStateValue(previous.getStateValue());
+                alarm.setStateReason(previous.getStateReason());
+                alarm.setStateReasonData(previous.getStateReasonData());
+                alarm.setStateUpdatedTimestamp(previous.getStateUpdatedTimestamp());
+                alarm.setTags(new HashMap<>(previous.getTags()));
+            }
+        }
         if (alarm.getAlarmArn() == null) {
-            alarm.setAlarmArn(regionResolver.buildArn("cloudwatch", region, "alarm:" + alarm.getAlarmName()));
+            alarm.setAlarmArn(regionResolver.buildArn("cloudwatch", region, "alarm:" + name));
         }
         alarm.setRegion(region);
         alarm.setAlarmConfigurationUpdatedTimestamp(Instant.now().getEpochSecond());
-        alarmStore.put(region + "::" + alarm.getAlarmName(), alarm);
-        LOG.infov("PutMetricAlarm: {0} in {1}", alarm.getAlarmName(), region);
+        alarmStore.put(region + "::" + name, alarm);
+        LOG.infov("Put {0}: {1} in {2}", alarm.getClass().getSimpleName(), name, region);
     }
 
-    /** Every stored alarm, across all regions. Used by the background {@link AlarmEvaluator}
-     * tick, which has no per-request region to scope a lookup to. */
+    /** Metric alarms evaluated by the existing metric evaluator. */
     public List<MetricAlarm> allAlarms() {
-        return alarmStore.scan(k -> true);
+        return alarmStore.scan(k -> true).stream().filter(MetricAlarm.class::isInstance)
+                .map(MetricAlarm.class::cast).toList();
     }
 
-    public List<MetricAlarm> describeAlarms(List<String> alarmNames, String alarmNamePrefix, String region) {
-        String prefix = region + "::";
-        List<MetricAlarm> all = alarmStore.scan(k -> k.startsWith(prefix));
-
-        if (alarmNames != null && !alarmNames.isEmpty()) {
-            return all.stream().filter(a -> alarmNames.contains(a.getAlarmName())).toList();
-        }
-        if (alarmNamePrefix != null && !alarmNamePrefix.isBlank()) {
-            return all.stream().filter(a -> a.getAlarmName().startsWith(alarmNamePrefix)).toList();
-        }
-        return all;
+    public List<MetricAlarm> describeAlarms(List<String> names, String prefix, String region) {
+        return matchingAlarms(names, prefix, region).stream().filter(MetricAlarm.class::isInstance)
+                .map(MetricAlarm.class::cast).toList();
     }
 
-    public void deleteAlarms(List<String> alarmNames, String region) {
+    public void validateAlarmTypes(List<String> types) {
+        if (!ALARM_TYPES.containsAll(types)) {
+            throw SmithyValidationErrors.validationError(
+                    SmithyValidationErrors.notAllInEnum(types, "alarmTypes", ALARM_TYPES));
+        }
+    }
+
+    public List<CompositeAlarm> describeCompositeAlarms(List<String> names, String prefix, String region) {
+        return matchingAlarms(names, prefix, region).stream().filter(CompositeAlarm.class::isInstance)
+                .map(CompositeAlarm.class::cast).toList();
+    }
+
+    private List<Alarm> matchingAlarms(List<String> names, String prefix, String region) {
+        return alarmStore.scan(k -> k.startsWith(region + "::")).stream()
+                .filter(a -> names == null || names.isEmpty() || names.contains(a.getAlarmName()))
+                .filter(a -> prefix == null || a.getAlarmName().startsWith(prefix)).toList();
+    }
+
+    private Map<String, Alarm> alarmIndex(String region) {
+        Map<String, Alarm> alarms = new HashMap<>();
+        for (Alarm alarm : alarmStore.scan(k -> k.startsWith(region + "::"))) {
+            alarms.put(alarm.getAlarmName(), alarm);
+            alarms.put(alarm.getAlarmArn(), alarm);
+        }
+        return alarms;
+    }
+
+    private void evaluateComposites(String region, Set<String> changedAlarmNames, String newAlarm) {
+        if (changedAlarmNames.isEmpty() && newAlarm == null) {
+            return;
+        }
+        Map<String, Alarm> alarms = alarmIndex(region);
+        Set<String> changed = new HashSet<>(changedAlarmNames);
+        for (String name : changedAlarmNames) {
+            changed.add(regionResolver.buildArn("cloudwatch", region, "alarm:" + name));
+        }
+        Map<String, CompositeAlarmRule.Rule> candidates = new HashMap<>();
+        for (Alarm alarm : new HashSet<>(alarms.values())) {
+            if (alarm instanceof CompositeAlarm composite && !changedAlarmNames.contains(alarm.getAlarmName())) {
+                candidates.put(alarm.getAlarmName(), CompositeAlarmRule.parse(composite.getAlarmRule()));
+            }
+        }
+        Set<String> affected = new HashSet<>(changed);
+        Map<String, CompositeAlarmRule.Rule> pending = new HashMap<>();
+        boolean discovered;
+        do {
+            discovered = false;
+            Iterator<Map.Entry<String, CompositeAlarmRule.Rule>> iterator = candidates.entrySet().iterator();
+            while (iterator.hasNext()) {
+                Map.Entry<String, CompositeAlarmRule.Rule> next = iterator.next();
+                if (next.getKey().equals(newAlarm) || next.getValue().references().stream().anyMatch(affected::contains)) {
+                    pending.put(next.getKey(), next.getValue());
+                    affected.add(next.getKey());
+                    affected.add(alarms.get(next.getKey()).getAlarmArn());
+                    iterator.remove();
+                    discovered = true;
+                }
+            }
+        } while (discovered);
+        Set<String> cycles = pending.keySet().stream()
+                .filter(name -> isCyclic(name, alarms)).collect(Collectors.toSet());
+        boolean progress;
+        do {
+            progress = false;
+            Iterator<Map.Entry<String, CompositeAlarmRule.Rule>> iterator = pending.entrySet().iterator();
+            while (iterator.hasNext()) {
+                Map.Entry<String, CompositeAlarmRule.Rule> next = iterator.next();
+                boolean blocked = next.getValue().references().stream().map(alarms::get)
+                        .anyMatch(child -> child != null && pending.containsKey(child.getAlarmName()));
+                if (blocked || cycles.contains(next.getKey())) {
+                    continue;
+                }
+                if (!next.getKey().equals(newAlarm)
+                        && next.getValue().references().stream().noneMatch(changed::contains)) {
+                    iterator.remove();
+                    progress = true;
+                    continue;
+                }
+                Alarm alarm = alarms.get(next.getKey());
+                boolean triggered = next.getValue().evaluate(reference -> {
+                    Alarm child = alarms.get(reference);
+                    return child == null ? null : child.getStateValue();
+                });
+                String state = triggered ? "ALARM" : "OK";
+                if (!state.equals(alarm.getStateValue())) {
+                    alarm.setStateValue(state);
+                    alarm.setStateReason("AlarmRule evaluated to " + state);
+                    alarm.setStateReasonData(null);
+                    alarm.setStateUpdatedTimestamp(Instant.now().getEpochSecond());
+                    alarmStore.put(region + "::" + alarm.getAlarmName(), alarm);
+                    changed.add(alarm.getAlarmName());
+                    changed.add(alarm.getAlarmArn());
+                }
+                iterator.remove();
+                progress = true;
+            }
+        } while (progress && !pending.isEmpty());
+        if (!pending.isEmpty()) {
+            LOG.debugv("Composite alarm evaluation stopped at cyclic dependencies: {0}", pending.keySet());
+        }
+    }
+
+    public synchronized void deleteAlarms(List<String> alarmNames, String region) {
+        Map<String, Alarm> alarms = alarmIndex(region);
+        long composites = alarmNames.stream().map(alarms::get).filter(CompositeAlarm.class::isInstance).count();
+        if (composites > 1) {
+            throw new AwsException("ValidationError", "DeleteAlarms accepts at most one composite alarm", 400);
+        }
         for (String name : alarmNames) {
+            if (isCyclic(name, alarms)) {
+                throw new AwsException("ValidationError", "Break the composite alarm cycle before deleting " + name, 400);
+            }
+        }
+        Set<String> deletedNames = new HashSet<>();
+        for (String name : alarmNames) {
+            if (alarmStore.get(region + "::" + name).isPresent()) {
+                deletedNames.add(name);
+            }
             alarmStore.delete(region + "::" + name);
         }
+        evaluateComposites(region, deletedNames, null);
         LOG.infov("Deleted alarms: {0} in {1}", alarmNames, region);
     }
 
+    private boolean isCyclic(String name, Map<String, Alarm> alarms) {
+        Set<String> visited = new HashSet<>();
+        ArrayDeque<String> pending = new ArrayDeque<>();
+        pending.add(name);
+        while (!pending.isEmpty()) {
+            Alarm next = alarms.get(pending.remove());
+            if (next instanceof CompositeAlarm composite && visited.add(next.getAlarmName())) {
+                for (String reference : CompositeAlarmRule.parse(composite.getAlarmRule()).references()) {
+                    Alarm child = alarms.get(reference);
+                    if (child != null && child.getAlarmName().equals(name)) {
+                        return true;
+                    }
+                    if (child != null) {
+                        pending.add(child.getAlarmName());
+                    }
+                }
+            }
+        }
+        return false;
+    }
+
     public void setAlarmState(String alarmName, String stateValue, String stateReason, String stateReasonData, String region) {
+        List<AlarmSnsNotifier.Notification> notifications = List.of();
+        synchronized (this) {
             String key = region + "::" + alarmName;
-        MetricAlarm alarm = alarmStore.get(key)
+            Alarm alarm = alarmStore.get(key)
                     .orElseThrow(() -> new AwsException("ResourceNotFound", "Alarm not found: " + alarmName, 404));
 
             String oldState = alarm.getStateValue();
@@ -344,9 +547,14 @@ public class CloudWatchMetricsService {
             alarm.setStateUpdatedTimestamp(Instant.now().getEpochSecond());
 
             alarmStore.put(key, alarm);
+            evaluateComposites(region, Set.of(alarmName), null);
             LOG.infov("SetAlarmState: {0} -> {1}", alarmName, stateValue);
+            if (alarmSnsNotifier != null && alarm instanceof MetricAlarm metricAlarm) {
+                notifications = alarmSnsNotifier.prepareTransition(metricAlarm, oldState);
+            }
+        }
         if (alarmSnsNotifier != null) {
-            alarmSnsNotifier.notifyTransition(alarm, oldState);
+            alarmSnsNotifier.publish(notifications);
         }
     }
 
@@ -364,7 +572,7 @@ public class CloudWatchMetricsService {
      * {@code insight-rule/} ARN being the one AWS documents; reporting that no resource
      * matches the ARN is true of those, where "alarm not found" would not be.
      */
-    private MetricAlarm requireAlarm(String resourceArn, String region) {
+    private Alarm requireAlarm(String resourceArn, String region) {
         return alarmStore.scan(k -> k.startsWith(region + "::"))
                 .stream()
                 .filter(a -> a.getAlarmArn() != null && a.getAlarmArn().equals(resourceArn))
@@ -373,18 +581,18 @@ public class CloudWatchMetricsService {
                         "No CloudWatch resource matches the ARN " + resourceArn + ".", 404));
     }
 
-    public Map<String, String> listTagsForResource(String resourceArn, String region) {
-        return requireAlarm(resourceArn, region).getTags();
+    public synchronized Map<String, String> listTagsForResource(String resourceArn, String region) {
+        return new HashMap<>(requireAlarm(resourceArn, region).getTags());
     }
 
-    public void tagResource(String resourceArn, Map<String, String> tags, String region) {
-        MetricAlarm alarm = requireAlarm(resourceArn, region);
+    public synchronized void tagResource(String resourceArn, Map<String, String> tags, String region) {
+        Alarm alarm = requireAlarm(resourceArn, region);
         alarm.getTags().putAll(tags);
         alarmStore.put(region + "::" + alarm.getAlarmName(), alarm);
     }
 
-    public void untagResource(String resourceArn, List<String> tagKeys, String region) {
-        MetricAlarm alarm = requireAlarm(resourceArn, region);
+    public synchronized void untagResource(String resourceArn, List<String> tagKeys, String region) {
+        Alarm alarm = requireAlarm(resourceArn, region);
         tagKeys.forEach(alarm.getTags()::remove);
         alarmStore.put(region + "::" + alarm.getAlarmName(), alarm);
     }

src/main/java/io/github/hectorvent/floci/services/cloudwatch/metrics/CompositeAlarmRule.java

@@ -0,0 +1,184 @@
+package io.github.hectorvent.floci.services.cloudwatch.metrics;
+
+import io.github.hectorvent.floci.core.common.AwsException;
+import io.github.hectorvent.floci.core.common.SmithyValidationErrors;
+
+import java.util.HashSet;
+import java.util.Set;
+import java.util.function.Function;
+
+/** CloudWatch's boolean alarm language, parsed completely before any alarm is stored. */
+final class CompositeAlarmRule {
+    private final String input;
+    private final Set<String> references = new HashSet<>();
+    private int offset;
+    private int elements;
+
+    private CompositeAlarmRule(String input) {
+        this.input = input;
+    }
+
+    static Rule parse(String input) {
+        if (input == null || input.isBlank() || input.length() > 10240) {
+            throw SmithyValidationErrors.validationError(input == null ? SmithyValidationErrors.notNull("alarmRule")
+                    : "Value '" + input + "' at 'alarmRule' failed to satisfy constraint: Member must have length "
+                            + (input.length() > 10240 ? "less than or equal to 10240" : "greater than or equal to 1"));
+        }
+        CompositeAlarmRule parser = new CompositeAlarmRule(input);
+        Expression expression = parser.or();
+        parser.whitespace();
+        if (parser.offset != input.length()) {
+            throw invalid("Invalid AlarmRule");
+        }
+        return new Rule(expression, Set.copyOf(parser.references));
+    }
+
+    private Expression or() {
+        Expression left = and();
+        while (word("OR")) {
+            Expression prior = left;
+            Expression right = and();
+            left = states -> prior.evaluate(states) || right.evaluate(states);
+        }
+        return left;
+    }
+
+    private Expression and() {
+        Expression left = atom();
+        while (word("AND")) {
+            Expression prior = left;
+            Expression right = atom();
+            left = states -> prior.evaluate(states) && right.evaluate(states);
+        }
+        return left;
+    }
+
+    private Expression atom() {
+        boolean negate = false;
+        while (word("NOT")) {
+            negate = !negate;
+        }
+        Expression operand = primary();
+        return negate ? states -> !operand.evaluate(states) : operand;
+    }
+
+    private Expression primary() {
+        element();
+        if (character('(')) {
+            Expression nested = or();
+            require(')');
+            element();
+            return nested;
+        }
+        if (word("TRUE")) {
+            return states -> true;
+        }
+        if (word("FALSE")) {
+            return states -> false;
+        }
+        String state;
+        if (word("ALARM")) {
+            state = "ALARM";
+        } else if (word("OK")) {
+            state = "OK";
+        } else if (word("INSUFFICIENT_DATA")) {
+            state = "INSUFFICIENT_DATA";
+        } else {
+            throw invalid("Expected an alarm state, boolean, NOT, or parenthesis");
+        }
+        require('(');
+        whitespace();
+        String name;
+        if (character('"')) {
+            StringBuilder quoted = new StringBuilder();
+            boolean closed = false;
+            while (offset < input.length()) {
+                char next = input.charAt(offset++);
+                if (next == '"') {
+                    closed = true;
+                    break;
+                }
+                if (next == '\\') {
+                    if (offset == input.length()) {
+                        throw invalid("Unterminated quoted alarm name");
+                    }
+                    next = input.charAt(offset++);
+                    if (next != '\\' && next != '"') {
+                        throw invalid("Invalid quoted alarm name escape");
+                    }
+                }
+                quoted.append(next);
+            }
+            if (!closed) {
+                throw invalid("Unterminated quoted alarm name");
+            }
+            name = quoted.toString();
+        } else {
+            int start = offset;
+            while (offset < input.length() && input.charAt(offset) != ')') {
+                offset++;
+            }
+            name = input.substring(start, offset).trim();
+        }
+        if (name.isEmpty()) {
+            throw invalid("An alarm name or ARN is required");
+        }
+        require(')');
+        references.add(name);
+        return states -> state.equals(states.apply(name));
+    }
+
+    private void element() {
+        if (++elements > 500) {
+            throw invalid("AlarmRule exceeds 500 child alarms, booleans, and parentheses");
+        }
+    }
+
+    private boolean word(String word) {
+        whitespace();
+        if (!input.startsWith(word, offset)) {
+            return false;
+        }
+        int end = offset + word.length();
+        if (end < input.length() && (Character.isLetterOrDigit(input.charAt(end)) || input.charAt(end) == '_')) {
+            return false;
+        }
+        offset = end;
+        return true;
+    }
+
+    private boolean character(char expected) {
+        whitespace();
+        if (offset < input.length() && input.charAt(offset) == expected) {
+            offset++;
+            return true;
+        }
+        return false;
+    }
+
+    private void require(char expected) {
+        if (!character(expected)) {
+            throw invalid("Expected '" + expected + "' in AlarmRule");
+        }
+    }
+
+    private void whitespace() {
+        while (offset < input.length() && Character.isWhitespace(input.charAt(offset))) {
+            offset++;
+        }
+    }
+
+    private static AwsException invalid(String message) {
+        return new AwsException("ValidationError", message, 400);
+    }
+
+    record Rule(Expression expression, Set<String> references) {
+        boolean evaluate(Function<String, String> states) {
+            return expression.evaluate(states);
+        }
+    }
+
+    interface Expression {
+        boolean evaluate(Function<String, String> states);
+    }
+}

src/main/java/io/github/hectorvent/floci/services/cloudwatch/metrics/model/Alarm.java

@@ -0,0 +1,87 @@
+package io.github.hectorvent.floci.services.cloudwatch.metrics.model;
+
+import com.fasterxml.jackson.annotation.JsonIgnoreProperties;
+import com.fasterxml.jackson.annotation.JsonSubTypes;
+import com.fasterxml.jackson.annotation.JsonTypeInfo;
+import io.quarkus.runtime.annotations.RegisterForReflection;
+
+import java.time.Instant;
+import java.util.ArrayList;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+
+@RegisterForReflection
+@JsonIgnoreProperties(ignoreUnknown = true)
+@JsonTypeInfo(use = JsonTypeInfo.Id.NAME, include = JsonTypeInfo.As.EXISTING_PROPERTY,
+        property = "alarmType", defaultImpl = MetricAlarm.class)
+@JsonSubTypes({@JsonSubTypes.Type(value = MetricAlarm.class, name = "MetricAlarm"),
+        @JsonSubTypes.Type(value = CompositeAlarm.class, name = "CompositeAlarm")})
+public abstract class Alarm {
+    private String alarmName;
+    private String alarmArn;
+    private String alarmDescription;
+    private long alarmConfigurationUpdatedTimestamp;
+    private boolean actionsEnabled;
+    private List<String> okActions = new ArrayList<>();
+    private List<String> alarmActions = new ArrayList<>();
+    private List<String> insufficientDataActions = new ArrayList<>();
+    private String stateValue = "INSUFFICIENT_DATA";
+    private String stateReason = "Unchecked";
+    private String stateReasonData;
+    private long stateUpdatedTimestamp;
+    private Map<String, String> tags = new HashMap<>();
+    private String region;
+
+
+    protected Alarm() {
+        long now = Instant.now().getEpochSecond();
+        this.alarmConfigurationUpdatedTimestamp = now;
+        this.stateUpdatedTimestamp = now;
+    }
+
+    public abstract String getAlarmType();
+
+    public String getAlarmName() { return alarmName; }
+    public void setAlarmName(String alarmName) { this.alarmName = alarmName; }
+
+    public String getAlarmArn() { return alarmArn; }
+    public void setAlarmArn(String alarmArn) { this.alarmArn = alarmArn; }
+
+    public String getAlarmDescription() { return alarmDescription; }
+    public void setAlarmDescription(String alarmDescription) { this.alarmDescription = alarmDescription; }
+
+    public long getAlarmConfigurationUpdatedTimestamp() { return alarmConfigurationUpdatedTimestamp; }
+    public void setAlarmConfigurationUpdatedTimestamp(long timestamp) { this.alarmConfigurationUpdatedTimestamp = timestamp; }
+
+    public boolean isActionsEnabled() { return actionsEnabled; }
+    public void setActionsEnabled(boolean actionsEnabled) { this.actionsEnabled = actionsEnabled; }
+
+    public List<String> getOkActions() { return okActions; }
+    public void setOkActions(List<String> okActions) { this.okActions = okActions; }
+
+    public List<String> getAlarmActions() { return alarmActions; }
+    public void setAlarmActions(List<String> alarmActions) { this.alarmActions = alarmActions; }
+
+    public List<String> getInsufficientDataActions() { return insufficientDataActions; }
+    public void setInsufficientDataActions(List<String> insufficientDataActions) { this.insufficientDataActions = insufficientDataActions; }
+
+    public String getStateValue() { return stateValue; }
+    public void setStateValue(String stateValue) { this.stateValue = stateValue; }
+
+    public String getStateReason() { return stateReason; }
+    public void setStateReason(String stateReason) { this.stateReason = stateReason; }
+
+    public String getStateReasonData() { return stateReasonData; }
+    public void setStateReasonData(String stateReasonData) { this.stateReasonData = stateReasonData; }
+
+    public long getStateUpdatedTimestamp() { return stateUpdatedTimestamp; }
+    public void setStateUpdatedTimestamp(long stateUpdatedTimestamp) { this.stateUpdatedTimestamp = stateUpdatedTimestamp; }
+
+    public Map<String, String> getTags() { return tags; }
+    public void setTags(Map<String, String> tags) { this.tags = tags; }
+
+
+    public String getRegion() { return region; }
+    public void setRegion(String region) { this.region = region; }
+}

src/main/java/io/github/hectorvent/floci/services/cloudwatch/metrics/model/CompositeAlarm.java

@@ -0,0 +1,14 @@
+package io.github.hectorvent.floci.services.cloudwatch.metrics.model;
+
+import io.quarkus.runtime.annotations.RegisterForReflection;
+
+@RegisterForReflection
+public class CompositeAlarm extends Alarm {
+    private String alarmRule;
+
+    @Override
+    public String getAlarmType() { return "CompositeAlarm"; }
+
+    public String getAlarmRule() { return alarmRule; }
+    public void setAlarmRule(String alarmRule) { this.alarmRule = alarmRule; }
+}

src/main/java/io/github/hectorvent/floci/services/cloudwatch/metrics/model/MetricAlarm.java

@@ -3,27 +3,15 @@ package io.github.hectorvent.floci.services.cloudwatch.metrics.model;
 import com.fasterxml.jackson.annotation.JsonIgnoreProperties;
 import io.quarkus.runtime.annotations.RegisterForReflection;
 
-import java.time.Instant;
 import java.util.ArrayList;
-import java.util.HashMap;
 import java.util.List;
-import java.util.Map;
 
 @RegisterForReflection
 @JsonIgnoreProperties(ignoreUnknown = true)
-public class MetricAlarm {
-    private String alarmName;
-    private String alarmArn;
-    private String alarmDescription;
-    private long alarmConfigurationUpdatedTimestamp;
-    private boolean actionsEnabled;
-    private List<String> okActions = new ArrayList<>();
-    private List<String> alarmActions = new ArrayList<>();
-    private List<String> insufficientDataActions = new ArrayList<>();
-    private String stateValue = "INSUFFICIENT_DATA";
-    private String stateReason = "Unchecked";
-    private String stateReasonData;
-    private long stateUpdatedTimestamp;
+public class MetricAlarm extends Alarm {
+    @Override
+    public String getAlarmType() { return "MetricAlarm"; }
+
     private String metricName;
     private String namespace;
     private String statistic;
@@ -45,50 +33,6 @@ public class MetricAlarm {
     private String comparisonOperator;
     private String treatMissingData = "missing";
     private String evaluateLowSampleCountPercentile;
-    private Map<String, String> tags = new HashMap<>();
-    private String region;
-
-    public MetricAlarm() {
-        long now = Instant.now().getEpochSecond();
-        this.alarmConfigurationUpdatedTimestamp = now;
-        this.stateUpdatedTimestamp = now;
-    }
-
-    public String getAlarmName() { return alarmName; }
-    public void setAlarmName(String alarmName) { this.alarmName = alarmName; }
-
-    public String getAlarmArn() { return alarmArn; }
-    public void setAlarmArn(String alarmArn) { this.alarmArn = alarmArn; }
-
-    public String getAlarmDescription() { return alarmDescription; }
-    public void setAlarmDescription(String alarmDescription) { this.alarmDescription = alarmDescription; }
-
-    public long getAlarmConfigurationUpdatedTimestamp() { return alarmConfigurationUpdatedTimestamp; }
-    public void setAlarmConfigurationUpdatedTimestamp(long timestamp) { this.alarmConfigurationUpdatedTimestamp = timestamp; }
-
-    public boolean isActionsEnabled() { return actionsEnabled; }
-    public void setActionsEnabled(boolean actionsEnabled) { this.actionsEnabled = actionsEnabled; }
-
-    public List<String> getOkActions() { return okActions; }
-    public void setOkActions(List<String> okActions) { this.okActions = okActions; }
-
-    public List<String> getAlarmActions() { return alarmActions; }
-    public void setAlarmActions(List<String> alarmActions) { this.alarmActions = alarmActions; }
-
-    public List<String> getInsufficientDataActions() { return insufficientDataActions; }
-    public void setInsufficientDataActions(List<String> insufficientDataActions) { this.insufficientDataActions = insufficientDataActions; }
-
-    public String getStateValue() { return stateValue; }
-    public void setStateValue(String stateValue) { this.stateValue = stateValue; }
-
-    public String getStateReason() { return stateReason; }
-    public void setStateReason(String stateReason) { this.stateReason = stateReason; }
-
-    public String getStateReasonData() { return stateReasonData; }
-    public void setStateReasonData(String stateReasonData) { this.stateReasonData = stateReasonData; }
-
-    public long getStateUpdatedTimestamp() { return stateUpdatedTimestamp; }
-    public void setStateUpdatedTimestamp(long stateUpdatedTimestamp) { this.stateUpdatedTimestamp = stateUpdatedTimestamp; }
 
     public String getMetricName() { return metricName; }
     public void setMetricName(String metricName) { this.metricName = metricName; }
@@ -131,11 +75,4 @@ public class MetricAlarm {
     public String getEvaluateLowSampleCountPercentile() { return evaluateLowSampleCountPercentile; }
     public void setEvaluateLowSampleCountPercentile(String percentile) { this.evaluateLowSampleCountPercentile = percentile; }
 
-    public Map<String, String> getTags() { return tags; }
-    public void setTags(Map<String, String> tags) { this.tags = tags; }
-
-    /** Not part of the AWS shape; set by {@code putMetricAlarm} so a background evaluator
-     * that only holds the alarm object still knows which region's metric data to query. */
-    public String getRegion() { return region; }
-    public void setRegion(String region) { this.region = region; }
 }

src/test/java/io/github/hectorvent/floci/services/cloudformation/CfnProvisionerFixture.java

@@ -45,6 +45,7 @@ import io.github.hectorvent.floci.services.cloudformation.provisioners.AppSyncCf
 import io.github.hectorvent.floci.services.cloudformation.provisioners.CdkMetadataCfnProvisioner;
 import io.github.hectorvent.floci.services.cloudformation.provisioners.CloudTrailCfnProvisioner;
 import io.github.hectorvent.floci.services.cloudformation.provisioners.CloudWatchCfnProvisioner;
+import io.github.hectorvent.floci.services.cloudformation.provisioners.CloudWatchCompositeAlarmCfnProvisioner;
 import io.github.hectorvent.floci.services.cloudformation.provisioners.CloudWatchDashboardCfnProvisioner;
 import io.github.hectorvent.floci.services.cloudformation.provisioners.CognitoCfnProvisioner;
 import io.github.hectorvent.floci.services.cloudformation.provisioners.CognitoUserPoolGroupCfnProvisioner;
@@ -300,6 +301,7 @@ final class CfnProvisionerFixture {
             }
             if (cloudWatchMetricsService != null) {
                 discovered.add(new CloudWatchCfnProvisioner(cloudWatchMetricsService));
+                discovered.add(new CloudWatchCompositeAlarmCfnProvisioner(cloudWatchMetricsService));
             }
             if (cloudWatchDashboardsService != null) {
                 discovered.add(new CloudWatchDashboardCfnProvisioner(cloudWatchDashboardsService));

src/test/java/io/github/hectorvent/floci/services/cloudformation/CloudFormationCompositeAlarmIntegrationTest.java

@@ -0,0 +1,190 @@
+package io.github.hectorvent.floci.services.cloudformation;
+
+import com.fasterxml.jackson.core.JsonProcessingException;
+import com.fasterxml.jackson.databind.ObjectMapper;
+import io.github.hectorvent.floci.testing.CfnStackWaits;
+import io.github.hectorvent.floci.testing.PartitionCleanup;
+import io.github.hectorvent.floci.testing.RestAssuredJsonUtils;
+import io.quarkus.test.common.http.TestHTTPResource;
+import io.quarkus.test.junit.QuarkusTest;
+import io.restassured.response.Response;
+import org.junit.jupiter.api.BeforeAll;
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.extension.RegisterExtension;
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.ValueSource;
+
+import java.net.URI;
+import java.util.LinkedHashMap;
+import java.util.List;
+import java.util.Map;
+import java.util.stream.Collectors;
+
+import static io.restassured.RestAssured.given;
+import static org.junit.jupiter.api.Assertions.*;
+
+@QuarkusTest
+class CloudFormationCompositeAlarmIntegrationTest {
+    private static final ObjectMapper MAPPER = new ObjectMapper();
+
+    @TestHTTPResource
+    URI baseUri;
+
+    @RegisterExtension
+    final PartitionCleanup cleanup = new PartitionCleanup();
+
+    @BeforeAll
+    static void configureRestAssured() {
+        RestAssuredJsonUtils.configureAwsContentTypes();
+    }
+
+    @ParameterizedTest
+    @ValueSource(booleans = {false, true})
+    void failedUpdateRestoresRuleTagsAndIdentity(boolean rename) throws Exception {
+        String stack = unique("composite-rollback");
+        String name = stack + "-alarm";
+        String next = rename ? name + "-new" : name;
+        deploy("CreateStack", stack, template(name, "TRUE", Map.of("team", "old", "drop", "restore"), false, false));
+        assertEquals("CREATE_COMPLETE", CfnStackWaits.awaitTerminal(stack).status());
+        String arn = alarm(name).get("AlarmArn").toString();
+        Response described = given().contentType("application/x-www-form-urlencoded")
+                .formParam("Action", "DescribeStacks").formParam("StackName", stack).post("/");
+        assertEquals(name, described.xmlPath().getString(
+                "DescribeStacksResponse.DescribeStacksResult.Stacks.member.Outputs.member.find { it.OutputKey == 'Name' }.OutputValue"));
+        assertEquals(arn, described.xmlPath().getString(
+                "DescribeStacksResponse.DescribeStacksResult.Stacks.member.Outputs.member.find { it.OutputKey == 'Arn' }.OutputValue"));
+        cw("TagResource", Map.of("ResourceARN", arn, "Tags", List.of(Map.of("Key", "external", "Value", "keep"))))
+                .then().statusCode(200);
+        deploy("UpdateStack", stack, template(next, "FALSE", Map.of("team", "new", "added", "remove"), true, false));
+        assertEquals("UPDATE_ROLLBACK_COMPLETE", CfnStackWaits.awaitTerminal(stack).status());
+        assertEquals("TRUE", alarm(name).get("AlarmRule"));
+        List<Map<String, String>> tags = cw("ListTagsForResource", Map.of("ResourceARN", arn))
+                .then().statusCode(200).extract().jsonPath().getList("Tags");
+        assertEquals(Map.of("team", "old", "drop", "restore", "external", "keep"),
+                tags.stream().collect(Collectors.toMap(t -> t.get("Key"), t -> t.get("Value"))));
+        if (rename) {
+            assertMissing(next);
+        }
+        deleteStack(stack);
+        assertMissing(name);
+    }
+
+    @ParameterizedTest
+    @ValueSource(booleans = {false, true})
+    void replacementHonorsRetentionAndDeletesItsCurrentBackingAlarm(boolean retain) throws Exception {
+        String stack = unique("composite-replace");
+        String original = stack + "-old";
+        String next = stack + "-new";
+        deploy("CreateStack", stack, template(original, "TRUE", Map.of(), false, retain));
+        assertEquals("CREATE_COMPLETE", CfnStackWaits.awaitTerminal(stack).status());
+        deploy("UpdateStack", stack, template(next, "FALSE", Map.of(), false, retain));
+        assertEquals("UPDATE_COMPLETE", CfnStackWaits.awaitTerminal(stack).status());
+        assertEquals("FALSE", alarm(next).get("AlarmRule"));
+        if (retain) {
+            assertEquals("TRUE", alarm(original).get("AlarmRule"));
+        } else {
+            assertMissing(original);
+        }
+        deleteStack(stack);
+        assertMissing(next);
+        if (retain) {
+            assertEquals("TRUE", alarm(original).get("AlarmRule"));
+            cw("DeleteAlarms", Map.of("AlarmNames", List.of(original))).then().statusCode(200);
+        }
+    }
+
+    @Test
+    void committedInPlaceUpdatesAllowAnotherUpdateAndReplacement() throws Exception {
+        String stack = unique("composite-sequence");
+        String name = stack + "-alarm";
+        String replacement = name + "-new";
+        deploy("CreateStack", stack, template(name, "TRUE", Map.of("team", "initial"), false, false));
+        assertEquals("CREATE_COMPLETE", CfnStackWaits.awaitTerminal(stack).status());
+        for (String rule : List.of("FALSE", "TRUE")) {
+            deploy("UpdateStack", stack, template(name, rule, Map.of("team", rule), false, false));
+            assertEquals("UPDATE_COMPLETE", CfnStackWaits.awaitTerminal(stack).status());
+            assertEquals(rule, alarm(name).get("AlarmRule"));
+        }
+        deploy("UpdateStack", stack, template(replacement, "FALSE", Map.of(), false, false));
+        assertEquals("UPDATE_COMPLETE", CfnStackWaits.awaitTerminal(stack).status());
+        assertMissing(name);
+        assertEquals("FALSE", alarm(replacement).get("AlarmRule"));
+        deleteStack(stack);
+        assertMissing(replacement);
+    }
+
+    @Test
+    void createCollisionCannotOverwriteOrDeleteAnUnmanagedAlarm() throws Exception {
+        String stack = unique("composite-foreign");
+        String name = stack + "-alarm";
+        cw("PutCompositeAlarm", Map.of("AlarmName", name, "AlarmRule", "TRUE")).then().statusCode(200);
+        deploy("CreateStack", stack, template(name, "FALSE", Map.of(), false, false));
+        assertEquals("ROLLBACK_COMPLETE", CfnStackWaits.awaitTerminal(stack).status());
+        assertEquals("TRUE", alarm(name).get("AlarmRule"));
+        deleteStack(stack);
+        assertEquals("TRUE", alarm(name).get("AlarmRule"));
+        cw("DeleteAlarms", Map.of("AlarmNames", List.of(name))).then().statusCode(200);
+    }
+
+    private String template(String name, String rule, Map<String, String> tags, boolean fail, boolean retain)
+            throws JsonProcessingException {
+        cleanup.register(() -> cw("DeleteAlarms", Map.of("AlarmNames", List.of(name))).then().statusCode(200));
+        Map<String, Object> resources = new LinkedHashMap<>();
+        resources.put("Alarm", Map.of("Type", "AWS::CloudWatch::CompositeAlarm",
+                "UpdateReplacePolicy", retain ? "Retain" : "Delete",
+                "Properties", Map.of("AlarmName", name, "AlarmRule", rule,
+                        "Tags", tags.entrySet().stream().map(e -> Map.of("Key", e.getKey(), "Value", e.getValue())).toList())));
+        if (fail) {
+            resources.put("Failure", Map.of("Type", "AWS::SecretsManager::Secret", "DependsOn", "Alarm",
+                    "Properties", Map.of("SecretString", "explicit", "GenerateSecretString", Map.of("PasswordLength", 32))));
+        }
+        return MAPPER.writeValueAsString(Map.of("Resources", resources, "Outputs", Map.of(
+                "Name", Map.of("Value", Map.of("Ref", "Alarm")),
+                "Arn", Map.of("Value", Map.of("Fn::GetAtt", List.of("Alarm", "Arn"))))));
+    }
+
+    private void deploy(String action, String stack, String template) {
+        if ("CreateStack".equals(action)) {
+            cleanup.register(() -> given().port(baseUri.getPort())
+                    .contentType("application/x-www-form-urlencoded")
+                    .formParam("Action", "DeleteStack").formParam("StackName", stack)
+                    .post("/").then().statusCode(200));
+        }
+        given().contentType("application/x-www-form-urlencoded").formParam("Action", action)
+                .formParam("StackName", stack).formParam("TemplateBody", template).post("/").then().statusCode(200);
+    }
+
+    private Response cw(String operation, Map<String, ?> body) {
+        if ("PutCompositeAlarm".equals(operation)) {
+            String name = body.get("AlarmName").toString();
+            cleanup.register(() -> cw("DeleteAlarms", Map.of("AlarmNames", List.of(name))).then().statusCode(200));
+        }
+        return given().port(baseUri.getPort()).contentType("application/x-amz-json-1.0")
+                .header("X-Amz-Target", "GraniteServiceVersion20100801." + operation)
+                .body(MAPPER.valueToTree(body).toString()).post("/");
+    }
+
+    private Map<String, Object> alarm(String name) throws JsonProcessingException {
+        List<Map<String, Object>> alarms = cw("DescribeAlarms", Map.of("AlarmNames", List.of(name),
+                "AlarmTypes", List.of("CompositeAlarm"))).then().statusCode(200)
+                .extract().jsonPath().getList("CompositeAlarms");
+        assertEquals(1, alarms.size(), name);
+        return alarms.getFirst();
+    }
+
+    private void assertMissing(String name) throws JsonProcessingException {
+        assertEquals(List.of(), cw("DescribeAlarms", Map.of("AlarmNames", List.of(name),
+                "AlarmTypes", List.of("CompositeAlarm"))).then().statusCode(200)
+                .extract().jsonPath().getList("CompositeAlarms"));
+    }
+
+    private void deleteStack(String stack) {
+        given().contentType("application/x-www-form-urlencoded").formParam("Action", "DeleteStack")
+                .formParam("StackName", stack).post("/").then().statusCode(200);
+        CfnStackWaits.awaitStackDeleted(stack);
+    }
+
+    private static String unique(String prefix) {
+        return prefix + "-" + Long.toString(System.nanoTime(), 36);
+    }
+}

src/test/java/io/github/hectorvent/floci/services/cloudformation/provisioners/CloudWatchCompositeAlarmCfnProvisionerTest.java

@@ -0,0 +1,89 @@
+package io.github.hectorvent.floci.services.cloudformation.provisioners;
+
+import com.fasterxml.jackson.databind.JsonNode;
+import com.fasterxml.jackson.databind.ObjectMapper;
+import io.github.hectorvent.floci.core.common.AwsException;
+import io.github.hectorvent.floci.services.cloudformation.CloudFormationTemplateEngine;
+import io.github.hectorvent.floci.services.cloudformation.model.StackResource;
+import io.github.hectorvent.floci.services.cloudwatch.metrics.CloudWatchMetricsService;
+import org.junit.jupiter.api.Test;
+
+import java.util.List;
+import java.util.Map;
+import java.util.Set;
+
+import static org.junit.jupiter.api.Assertions.*;
+import static org.mockito.ArgumentMatchers.*;
+import static org.mockito.Mockito.*;
+
+class CloudWatchCompositeAlarmCfnProvisionerTest {
+    @Test
+    void rejectedBackingUpdateDoesNotArmRollbackToDeleteAnUnmanagedReplacement() throws Exception {
+        CloudWatchMetricsService service = mock(CloudWatchMetricsService.class);
+        CloudWatchCompositeAlarmCfnProvisioner provisioner = new CloudWatchCompositeAlarmCfnProvisioner(service);
+        CloudFormationTemplateEngine engine = mock(CloudFormationTemplateEngine.class);
+        when(engine.resolve(any())).thenAnswer(call -> {
+            JsonNode node = call.getArgument(0);
+            return node == null || node.isMissingNode() ? null : node.asText();
+        });
+        when(engine.resolveNode(any())).thenAnswer(call -> call.getArgument(0));
+        StackResource resource = new StackResource();
+        resource.setLogicalId("Alarm");
+        resource.setResourceType("AWS::CloudWatch::CompositeAlarm");
+        resource.setPhysicalId("kept");
+        resource.getAttributes().put("Arn", "prior-arn");
+        AwsException collision = new AwsException("ValidationError", "A metric alarm now has this name", 400);
+        doThrow(collision).when(service).putCompositeAlarm(any(), eq("us-east-1"), eq(true));
+        ProvisionContext context = new ProvisionContext(engine, "us-east-1", "000000000000", "stack", "kept");
+        JsonNode props = new ObjectMapper().readTree("{\"AlarmName\":\"kept\",\"AlarmRule\":\"TRUE\"}");
+        assertSame(collision, assertThrows(AwsException.class, () -> provisioner.provision(resource, props, context)));
+        assertFalse(provisioner.retainsFailedUpdateState(resource));
+        assertFalse(provisioner.rollbackUpdate(resource));
+        assertEquals("kept", resource.getPhysicalId());
+        assertEquals(Map.of("Arn", "prior-arn"), resource.getAttributes());
+        verify(service, never()).deleteAlarms(any(), anyString());
+    }
+
+    @Test
+    void inPlaceUpdateRetriesPreviouslyDisplacedAlarmCleanup() throws Exception {
+        CloudWatchMetricsService service = mock(CloudWatchMetricsService.class);
+        CloudWatchCompositeAlarmCfnProvisioner provisioner = new CloudWatchCompositeAlarmCfnProvisioner(service);
+        CloudFormationTemplateEngine engine = mock(CloudFormationTemplateEngine.class);
+        when(engine.resolve(any())).thenAnswer(call -> {
+            JsonNode node = call.getArgument(0);
+            return node == null || node.isMissingNode() ? null : node.asText();
+        });
+        when(engine.resolveNode(any())).thenAnswer(call -> call.getArgument(0));
+        StackResource resource = new StackResource();
+        resource.setLogicalId("Alarm");
+        resource.setResourceType("AWS::CloudWatch::CompositeAlarm");
+        resource.setPhysicalId("current");
+        ReplacementCleanup.record(resource,
+                new ProvisionContext(engine, "us-east-1", "000000000000", "stack", "displaced"), Map.of());
+        doThrow(new IllegalStateException("delete unavailable"))
+                .doThrow(new IllegalStateException("delete still unavailable")).doNothing()
+                .when(service).deleteAlarms(List.of("displaced"), "us-east-1");
+        assertFalse(provisioner.completeUpdate(resource).complete());
+        provisioner.clearUpdate(resource);
+        assertEquals(Set.of("displaced"), ReplacementCleanup.owedPhysicalIds(resource));
+
+        ProvisionContext context = new ProvisionContext(engine, "us-east-1", "000000000000", "stack", "current");
+        JsonNode props = new ObjectMapper().readTree("{\"AlarmName\":\"current\",\"AlarmRule\":\"TRUE\"}");
+        provisioner.provision(resource, props, context);
+        assertTrue(provisioner.retainsFailedUpdateState(resource));
+        UpdateCleanupResult retry = provisioner.completeUpdate(resource);
+        assertFalse(retry.complete());
+        assertEquals(2, retry.attempts());
+        assertEquals("displaced", retry.previousPhysicalId());
+        assertEquals("delete still unavailable", retry.failureReason());
+        assertEquals(Set.of("displaced"), ReplacementCleanup.owedPhysicalIds(resource));
+        assertTrue(provisioner.completeUpdate(resource).complete());
+        provisioner.clearUpdate(resource);
+        assertFalse(provisioner.retainsFailedUpdateState(resource));
+        assertTrue(ReplacementCleanup.owedPhysicalIds(resource).isEmpty());
+        assertEquals("current", resource.getPhysicalId());
+        verify(service, times(3)).deleteAlarms(List.of("displaced"), "us-east-1");
+        verify(service, never()).deleteAlarms(List.of("current"), "us-east-1");
+    }
+
+}

src/test/java/io/github/hectorvent/floci/services/cloudwatch/metrics/AlarmSnsNotifierTest.java

@@ -4,6 +4,7 @@ import com.fasterxml.jackson.databind.JsonNode;
 import com.fasterxml.jackson.databind.ObjectMapper;
 import io.github.hectorvent.floci.core.common.RegionResolver;
 import io.github.hectorvent.floci.core.storage.InMemoryStorage;
+import io.github.hectorvent.floci.services.cloudwatch.metrics.model.CompositeAlarm;
 import io.github.hectorvent.floci.services.cloudwatch.metrics.model.MetricAlarm;
 import io.github.hectorvent.floci.services.sns.SnsService;
 import org.junit.jupiter.api.BeforeEach;
@@ -12,6 +13,11 @@ import org.mockito.ArgumentCaptor;
 
 import java.time.Instant;
 import java.util.List;
+import java.util.concurrent.CountDownLatch;
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Executors;
+import java.util.concurrent.Future;
+import java.util.concurrent.TimeUnit;
 
 import static org.junit.jupiter.api.Assertions.*;
 import static org.mockito.ArgumentMatchers.*;
@@ -124,4 +130,41 @@ class AlarmSnsNotifierTest {
         service.setAlarmState("test-alarm", "ALARM", "breached", null, REGION);
         verify(sns).publish(eq(topic), isNull(), contains("arn:aws-cn:cloudwatch:"), anyString(), eq("cn-north-1"));
     }
+
+    @Test
+    void blockedNotificationDoesNotHoldTheAlarmMutationLock() throws Exception {
+        CountDownLatch publishing = new CountDownLatch(1);
+        CountDownLatch release = new CountDownLatch(1);
+        doAnswer(call -> {
+            if (call.<String>getArgument(3).startsWith("ALARM:")) {
+                publishing.countDown();
+                assertTrue(release.await(5, TimeUnit.SECONDS));
+            }
+            return "message-id";
+        }).when(sns).publish(eq(TOPIC), isNull(), anyString(), anyString(), eq(REGION));
+
+        try (ExecutorService executor = Executors.newFixedThreadPool(2)) {
+            Future<?> transition = executor.submit(() ->
+                    service.setAlarmState("test-alarm", "ALARM", "breached", null, REGION));
+            try {
+                assertTrue(publishing.await(2, TimeUnit.SECONDS));
+                Future<?> unrelated = executor.submit(() -> {
+                    CompositeAlarm composite = new CompositeAlarm();
+                    composite.setAlarmName("unrelated");
+                    composite.setAlarmRule("FALSE");
+                    service.putCompositeAlarm(composite, REGION);
+                    service.setAlarmState("test-alarm", "OK", "recovered", null, REGION);
+                });
+                unrelated.get(2, TimeUnit.SECONDS);
+                assertEquals("OK", alarm.getStateValue());
+            } finally {
+                release.countDown();
+            }
+            transition.get(2, TimeUnit.SECONDS);
+        }
+        ArgumentCaptor<String> messages = ArgumentCaptor.forClass(String.class);
+        verify(sns, times(2)).publish(eq(TOPIC), isNull(), messages.capture(), anyString(), eq(REGION));
+        assertEquals("ALARM", mapper.readTree(messages.getAllValues().getFirst()).path("NewStateValue").asText());
+        assertEquals("OK", mapper.readTree(messages.getAllValues().getLast()).path("NewStateValue").asText());
+    }
 }

src/test/java/io/github/hectorvent/floci/services/cloudwatch/metrics/CloudWatchAlarmQueryHandlerTest.java

@@ -0,0 +1,45 @@
+package io.github.hectorvent.floci.services.cloudwatch.metrics;
+
+import io.github.hectorvent.floci.core.common.RegionResolver;
+import io.github.hectorvent.floci.core.storage.InMemoryStorage;
+import io.github.hectorvent.floci.services.cloudwatch.metrics.model.Alarm;
+import jakarta.ws.rs.core.MultivaluedHashMap;
+import jakarta.ws.rs.core.MultivaluedMap;
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.ValueSource;
+
+import java.util.List;
+import java.util.Map;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+class CloudWatchAlarmQueryHandlerTest {
+    @ParameterizedTest
+    @ValueSource(strings = {"PutMetricAlarm", "PutCompositeAlarm"})
+    void sharedFieldsReachBothAlarmTypes(String operation) {
+        String region = "us-east-1";
+        CloudWatchMetricsService service = new CloudWatchMetricsService(new InMemoryStorage<>(),
+                new InMemoryStorage<>(), new RegionResolver(region, "123456789012"));
+        CloudWatchMetricsQueryHandler handler = new CloudWatchMetricsQueryHandler(service, null, null);
+        MultivaluedMap<String, String> request = new MultivaluedHashMap<>();
+        request.putSingle("AlarmName", "shared");
+        request.putSingle("AlarmRule", "FALSE");
+        request.putSingle("AlarmDescription", "shared fields");
+        request.putSingle("ActionsEnabled", "false");
+        request.putSingle("AlarmActions.member.1", "alarm-action");
+        request.putSingle("OKActions.member.1", "ok-action");
+        request.putSingle("InsufficientDataActions.member.1", "missing-action");
+        request.putSingle("Tags.member.1.Key", "team");
+        request.putSingle("Tags.member.1.Value", "metrics");
+        assertEquals(200, handler.handle(operation, request, region).getStatus());
+        Alarm alarm = "PutMetricAlarm".equals(operation)
+                ? service.describeAlarms(List.of("shared"), null, region).getFirst()
+                : service.describeCompositeAlarms(List.of("shared"), null, region).getFirst();
+        assertEquals("shared fields", alarm.getAlarmDescription());
+        assertFalse(alarm.isActionsEnabled());
+        assertEquals(List.of("alarm-action"), alarm.getAlarmActions());
+        assertEquals(List.of("ok-action"), alarm.getOkActions());
+        assertEquals(List.of("missing-action"), alarm.getInsufficientDataActions());
+        assertEquals(Map.of("team", "metrics"), alarm.getTags());
+    }
+}

src/test/java/io/github/hectorvent/floci/services/cloudwatch/metrics/CloudWatchMetricMathAlarmTest.java

@@ -10,7 +10,7 @@ import io.github.hectorvent.floci.core.storage.InMemoryStorage;
 import io.github.hectorvent.floci.core.storage.PersistentStorage;
 import io.github.hectorvent.floci.core.storage.StorageBackend;
 import io.github.hectorvent.floci.services.cloudwatch.dashboards.CloudWatchDashboardsService;
-import io.github.hectorvent.floci.services.cloudwatch.metrics.model.MetricAlarm;
+import io.github.hectorvent.floci.services.cloudwatch.metrics.model.Alarm;
 import jakarta.ws.rs.core.MultivaluedHashMap;
 import jakarta.ws.rs.core.MultivaluedMap;
 import org.junit.jupiter.api.BeforeEach;
@@ -43,7 +43,7 @@ class CloudWatchMetricMathAlarmTest {
         initialize(new InMemoryStorage<>());
     }
 
-    private void initialize(StorageBackend<String, MetricAlarm> alarmStore) {
+    private void initialize(StorageBackend<String, Alarm> alarmStore) {
         RegionResolver resolver = new RegionResolver(REGION, "000000000000");
         CloudWatchMetricsService service = new CloudWatchMetricsService(new InMemoryStorage<>(), alarmStore, resolver);
         CloudWatchDashboardsService dashboards = new CloudWatchDashboardsService(new InMemoryStorage<>(), resolver);
@@ -64,12 +64,12 @@ class CloudWatchMetricMathAlarmTest {
     void metricMathQueriesSurvivePersistentStorageReload(boolean queryProtocol, @TempDir Path directory)
             throws Exception {
         Path storageFile = directory.resolve("alarms.json");
-        TypeReference<Map<String, MetricAlarm>> type = new TypeReference<>() { };
+        TypeReference<Map<String, Alarm>> type = new TypeReference<>() { };
         initialize(new PersistentStorage<>(storageFile, type));
         ObjectNode request = metricMathRequest();
         put(request, queryProtocol);
 
-        PersistentStorage<String, MetricAlarm> reloaded = new PersistentStorage<>(storageFile, type);
+        PersistentStorage<String, Alarm> reloaded = new PersistentStorage<>(storageFile, type);
         reloaded.load();
         assertEquals(1, reloaded.keys().size(), "the stored alarm must deserialize after a restart");
         initialize(reloaded);

src/test/java/io/github/hectorvent/floci/services/cloudwatch/metrics/CompositeAlarmIntegrationTest.java

@@ -0,0 +1,160 @@
+package io.github.hectorvent.floci.services.cloudwatch.metrics;
+
+import com.fasterxml.jackson.core.JsonProcessingException;
+import com.fasterxml.jackson.databind.ObjectMapper;
+import io.github.hectorvent.floci.testing.PartitionCleanup;
+import io.github.hectorvent.floci.testing.RestAssuredJsonUtils;
+import io.quarkus.test.common.http.TestHTTPResource;
+import io.quarkus.test.junit.QuarkusTest;
+import io.restassured.response.Response;
+import org.junit.jupiter.api.BeforeAll;
+import org.junit.jupiter.api.extension.RegisterExtension;
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.ValueSource;
+
+import java.net.URI;
+import java.util.LinkedHashMap;
+import java.util.List;
+import java.util.Map;
+
+import static io.restassured.RestAssured.given;
+import static org.junit.jupiter.api.Assertions.*;
+
+@QuarkusTest
+class CompositeAlarmIntegrationTest {
+    private static final ObjectMapper MAPPER = new ObjectMapper();
+    private static final String SCOPE =
+            "AWS4-HMAC-SHA256 Credential=test/20261010/cn-north-1/monitoring/aws4_request";
+
+    @TestHTTPResource
+    URI baseUri;
+
+    @RegisterExtension
+    final PartitionCleanup cleanup = new PartitionCleanup();
+
+    @BeforeAll
+    static void configureRestAssured() {
+        RestAssuredJsonUtils.configureAwsContentTypes();
+    }
+
+    @ParameterizedTest
+    @ValueSource(booleans = {false, true})
+    void bothProtocolsCreateDescribeEvaluateAndDeleteRealAlarms(boolean json) throws Exception {
+        String prefix = "wire-composite-" + Long.toString(System.nanoTime(), 36);
+        String source = prefix + "-source";
+        String aggregate = prefix + "-aggregate";
+        call(json, "PutCompositeAlarm", Map.of("AlarmName", source, "AlarmRule", "TRUE")).then().statusCode(200);
+        call(json, "PutCompositeAlarm", Map.of("AlarmName", aggregate, "AlarmRule", "ALARM(\"" + source + "\")",
+                "ActionsEnabled", false, "AlarmDescription", "combined")).then().statusCode(200);
+        Response result = call(json, "DescribeAlarms", Map.of("AlarmNames", List.of(aggregate),
+                "AlarmTypes", List.of("CompositeAlarm")));
+        result.then().statusCode(200);
+        if (json) {
+            assertEquals(List.of(), result.jsonPath().getList("LogAlarms"));
+            assertEquals(1, result.jsonPath().getList("CompositeAlarms").size());
+            assertEquals("ALARM", result.jsonPath().getString("CompositeAlarms[0].StateValue"));
+            assertEquals("arn:aws-cn:cloudwatch:cn-north-1:000000000000:alarm:" + aggregate,
+                    result.jsonPath().getString("CompositeAlarms[0].AlarmArn"));
+            assertFalse(result.jsonPath().getBoolean("CompositeAlarms[0].ActionsEnabled"));
+        } else {
+            assertTrue(result.asString().contains("<LogAlarms>"));
+            assertEquals("ALARM", result.xmlPath().getString(
+                    "DescribeAlarmsResponse.DescribeAlarmsResult.CompositeAlarms.member.StateValue"));
+            assertTrue(result.asString().contains("arn:aws-cn:cloudwatch:cn-north-1:000000000000:alarm:" + aggregate));
+            assertEquals("false", result.xmlPath().getString(
+                    "DescribeAlarmsResponse.DescribeAlarmsResult.CompositeAlarms.member.ActionsEnabled"));
+        }
+        assertFalse(call(json, "DescribeAlarms", Map.of("AlarmNames", List.of(aggregate)))
+                .then().statusCode(200).extract().asString().contains(aggregate));
+        call(json, "SetAlarmState", Map.of("AlarmName", source, "StateValue", "OK",
+                "StateReason", "recovery")).then().statusCode(200);
+        Response recovered = call(json, "DescribeAlarms", Map.of("AlarmNames", List.of(aggregate),
+                "AlarmTypes", List.of("CompositeAlarm")));
+        assertEquals("OK", json ? recovered.jsonPath().getString("CompositeAlarms[0].StateValue")
+                : recovered.xmlPath().getString(
+                        "DescribeAlarmsResponse.DescribeAlarmsResult.CompositeAlarms.member.StateValue"));
+        for (String name : List.of(aggregate, source)) {
+            call(json, "DeleteAlarms", Map.of("AlarmNames", List.of(name))).then().statusCode(200);
+        }
+        Response empty = call(json, "DescribeAlarms", Map.of("AlarmNamePrefix", prefix,
+                "AlarmTypes", List.of("CompositeAlarm")));
+        assertFalse(empty.asString().contains(aggregate));
+        assertFalse(empty.asString().contains(source));
+    }
+
+    @ParameterizedTest
+    @ValueSource(booleans = {false, true})
+    void invalidRuleAndUnsupportedSuppressionCannotCreateAnAlarm(boolean json) throws Exception {
+        String name = "invalid-composite-" + Long.toString(System.nanoTime(), 36);
+        call(json, "PutCompositeAlarm", Map.of("AlarmName", name, "AlarmRule", "TRUE AND"))
+                .then().statusCode(400);
+        call(json, "PutCompositeAlarm", Map.of("AlarmName", name, "AlarmRule", "TRUE",
+                "ActionsSuppressor", "other")).then().statusCode(400);
+        assertFalse(call(json, "DescribeAlarms", Map.of("AlarmNames", List.of(name),
+                "AlarmTypes", List.of("CompositeAlarm"))).asString().contains(name));
+    }
+
+    @ParameterizedTest
+    @ValueSource(booleans = {false, true})
+    void alarmTypeValidationAcceptsLogAlarmsAndUsesSmithyErrors(boolean json) throws Exception {
+        call(json, "DescribeAlarms", Map.of("AlarmTypes", List.of("LogAlarm"))).then().statusCode(200);
+        Response error = call(json, "DescribeAlarms", Map.of("AlarmTypes", List.of("Unknown")));
+        error.then().statusCode(400);
+        assertEquals("1 validation error detected: Value '[Unknown]' at 'alarmTypes' failed to satisfy constraint: "
+                + "Member must satisfy constraint: [Member must satisfy enum value set: "
+                + "[CompositeAlarm, MetricAlarm, LogAlarm]]", errorMessage(json, error));
+    }
+
+    @ParameterizedTest
+    @ValueSource(booleans = {false, true})
+    void lengthErrorsUseSmithyMessages(boolean json) throws Exception {
+        for (String field : List.of("AlarmName", "AlarmRule")) {
+            int maximum = "AlarmName".equals(field) ? 255 : 10240;
+            String path = "AlarmName".equals(field) ? "alarmName" : "alarmRule";
+            for (String value : List.of("", "x".repeat(maximum + 1))) {
+                Map<String, String> request = new LinkedHashMap<>(Map.of("AlarmName", "length-validation", "AlarmRule", "TRUE"));
+                request.put(field, value);
+                Response error = call(json, "PutCompositeAlarm", request);
+                error.then().statusCode(400);
+                assertEquals("1 validation error detected: Value '" + value + "' at '" + path
+                        + "' failed to satisfy constraint: Member must have length "
+                        + (value.isEmpty() ? "greater than or equal to 1" : "less than or equal to " + maximum),
+                        errorMessage(json, error));
+            }
+        }
+    }
+
+    private String errorMessage(boolean json, Response response) {
+        return json ? response.jsonPath().getString("message") : response.xmlPath().getString("ErrorResponse.Error.Message");
+    }
+
+    private Response call(boolean json, String operation, Map<String, ?> body)
+            throws JsonProcessingException {
+        if ("PutCompositeAlarm".equals(operation)) {
+            String name = body.get("AlarmName").toString();
+            cleanup.register(() -> given().port(baseUri.getPort()).contentType("application/x-amz-json-1.0")
+                    .header("Authorization", SCOPE)
+                    .header("X-Amz-Target", "GraniteServiceVersion20100801.DeleteAlarms")
+                    .body(MAPPER.valueToTree(Map.of("AlarmNames", List.of(name))).toString())
+                    .post("/").then().statusCode(200));
+        }
+        if (json) {
+            return given().contentType("application/x-amz-json-1.0").header("Authorization", SCOPE)
+                    .header("X-Amz-Target", "GraniteServiceVersion20100801." + operation)
+                    .body(MAPPER.writeValueAsString(body)).post("/");
+        }
+        Map<String, Object> form = new LinkedHashMap<>();
+        form.put("Action", operation);
+        body.forEach((key, value) -> {
+            if (value instanceof List<?> values) {
+                for (int index = 0; index < values.size(); index++) {
+                    form.put(key + ".member." + (index + 1), values.get(index));
+                }
+            } else {
+                form.put(key, value);
+            }
+        });
+        return given().contentType("application/x-www-form-urlencoded").header("Authorization", SCOPE)
+                .formParams(form).post("/");
+    }
+}

src/test/java/io/github/hectorvent/floci/services/cloudwatch/metrics/CompositeAlarmServiceTest.java

@@ -0,0 +1,274 @@
+package io.github.hectorvent.floci.services.cloudwatch.metrics;
+
+import com.fasterxml.jackson.core.type.TypeReference;
+import com.fasterxml.jackson.databind.ObjectMapper;
+import io.github.hectorvent.floci.core.common.AwsException;
+import io.github.hectorvent.floci.core.common.RegionResolver;
+import io.github.hectorvent.floci.core.storage.InMemoryStorage;
+import io.github.hectorvent.floci.services.cloudwatch.metrics.model.Alarm;
+import io.github.hectorvent.floci.services.cloudwatch.metrics.model.CompositeAlarm;
+import io.github.hectorvent.floci.services.cloudwatch.metrics.model.MetricAlarm;
+import io.github.hectorvent.floci.testing.ThreadStates;
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.ValueSource;
+
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+import java.util.concurrent.CountDownLatch;
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Executors;
+import java.util.concurrent.Future;
+import java.util.concurrent.TimeUnit;
+import java.util.concurrent.atomic.AtomicReference;
+import java.util.stream.Collectors;
+import java.util.stream.IntStream;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+class CompositeAlarmServiceTest {
+    private static final String REGION = "cn-north-1";
+    private final CloudWatchMetricsService service = new CloudWatchMetricsService(
+            new InMemoryStorage<>(), new InMemoryStorage<>(), new RegionResolver(REGION, "123456789012"));
+
+    @Test
+    void metricTransitionsPropagateThroughNamesArnsAndNestedComposites() {
+        MetricAlarm metric = new MetricAlarm();
+        metric.setAlarmName("latency");
+        service.putMetricAlarm(metric, REGION);
+        put("combined", "ALARM(\"" + metric.getAlarmArn() + "\") AND NOT ALARM(\"maintenance\")");
+        put("outer", "ALARM(combined) OR (TRUE AND FALSE)");
+        assertEquals("OK", state("outer"));
+        service.setAlarmState("latency", "ALARM", "high latency", null, REGION);
+        assertEquals("ALARM", state("combined"));
+        assertEquals("ALARM", state("outer"));
+        service.setAlarmState("latency", "OK", "recovered", null, REGION);
+        assertEquals("OK", state("outer"));
+        assertEquals(List.of(metric), service.describeAlarms(null, null, REGION));
+        assertTrue(service.describeCompositeAlarms(null, null, "us-east-1").isEmpty());
+        assertTrue(service.describeCompositeAlarms(List.of("combined"), null, REGION)
+                .getFirst().getAlarmArn().startsWith("arn:aws-cn:cloudwatch:"));
+    }
+
+    @Test
+    void updatesPreserveStateAndTagsUntilTheNextDependencyChange() {
+        put("source", "FALSE");
+        CompositeAlarm alarm = put("aggregate", "ALARM(source)");
+        service.tagResource(alarm.getAlarmArn(), Map.of("owner", "retained"), REGION);
+        service.setAlarmState("aggregate", "ALARM", "manual", null, REGION);
+        CompositeAlarm replacement = put("aggregate", "OK(source)");
+        assertEquals("ALARM", replacement.getStateValue());
+        assertEquals(Map.of("owner", "retained"), replacement.getTags());
+        service.setAlarmState("source", "ALARM", "dependency changed", null, REGION);
+        assertEquals("OK", state("aggregate"));
+    }
+
+    @Test
+    void unrelatedWritesPreserveManualCompositeStatesAndParentStates() {
+        put("source", "FALSE");
+        put("aggregate", "ALARM(source)");
+        put("parent", "ALARM(aggregate)");
+        service.setAlarmState("aggregate", "ALARM", "manual", null, REGION);
+        assertEquals("ALARM", state("parent"));
+        put("unrelated", "TRUE");
+        assertEquals("ALARM", state("aggregate"));
+        assertEquals("ALARM", state("parent"));
+        MetricAlarm metric = new MetricAlarm();
+        metric.setAlarmName("unrelated-metric");
+        service.putMetricAlarm(metric, REGION);
+        assertEquals("ALARM", state("aggregate"));
+        service.setAlarmState("unrelated-metric", "ALARM", "unrelated", null, REGION);
+        assertEquals("ALARM", state("aggregate"));
+        service.deleteAlarms(List.of("unrelated"), REGION);
+        assertEquals("ALARM", state("aggregate"));
+        assertEquals("ALARM", state("parent"));
+        service.setAlarmState("source", "ALARM", "child changed", null, REGION);
+        service.setAlarmState("source", "OK", "child recovered", null, REGION);
+        assertEquals("OK", state("aggregate"));
+        assertEquals("OK", state("parent"));
+    }
+
+    @Test
+    void unchangedChildStateDoesNotResetManuallySetParent() {
+        put("source", "FALSE");
+        put("child", "ALARM(source) AND FALSE");
+        put("parent", "ALARM(child)");
+        service.setAlarmState("parent", "ALARM", "manual", null, REGION);
+        service.setAlarmState("source", "ALARM", "child condition remains false", null, REGION);
+        assertEquals("OK", state("child"));
+        assertEquals("ALARM", state("parent"));
+    }
+
+    @Test
+    void creationAndDeletionReevaluateOnlyReferencingComposites() {
+        MetricAlarm metric = new MetricAlarm();
+        metric.setAlarmName("late");
+        put("watcher", "INSUFFICIENT_DATA(late)");
+        put("outer", "ALARM(watcher)");
+        assertEquals("OK", state("watcher"));
+        service.putMetricAlarm(metric, REGION);
+        assertEquals("ALARM", state("watcher"));
+        assertEquals("ALARM", state("outer"));
+        service.deleteAlarms(List.of("late"), REGION);
+        assertEquals("OK", state("watcher"));
+        assertEquals("OK", state("outer"));
+    }
+
+    @Test
+    void tagReadIsAnIndependentSnapshotOfTheOwnedAlarm() {
+        CompositeAlarm alarm = put("tag-snapshot", "TRUE");
+        service.tagResource(alarm.getAlarmArn(), Map.of("team", "old"), REGION);
+        Map<String, String> read = service.listTagsForResource(alarm.getAlarmArn(), REGION);
+        service.tagResource(alarm.getAlarmArn(), Map.of("team", "new"), REGION);
+        assertEquals(Map.of("team", "old"), read);
+        read.put("injected", "unowned");
+        assertEquals(Map.of("team", "new"), service.listTagsForResource(alarm.getAlarmArn(), REGION));
+    }
+
+    @Test
+    void cyclesStopEvaluationEvenBehindShortCircuitAndMustBeBrokenBeforeDelete() {
+        put("a", "FALSE AND ALARM(b)");
+        put("b", "ALARM(a)");
+        service.setAlarmState("b", "ALARM", "manual", null, REGION);
+        assertThrows(AwsException.class, () -> service.deleteAlarms(List.of("a"), REGION));
+        assertEquals(2, service.describeCompositeAlarms(null, null, REGION).size());
+        put("a", "FALSE");
+        service.deleteAlarms(List.of("b"), REGION);
+        assertEquals("OK", state("a"));
+        service.deleteAlarms(List.of("a"), REGION);
+        assertTrue(service.describeCompositeAlarms(null, null, REGION).isEmpty());
+    }
+
+    @Test
+    void manuallyChangingOneMemberDoesNotResumeACyclicDependency() {
+        put("a", "ALARM(b)");
+        put("b", "ALARM(a)");
+        assertEquals("OK", state("a"));
+        service.setAlarmState("b", "ALARM", "manual", null, REGION);
+        assertEquals("OK", state("a"));
+        put("b", "TRUE");
+        service.setAlarmState("b", "ALARM", "cycle broken", null, REGION);
+        assertEquals("ALARM", state("a"));
+    }
+
+    @Test
+    void parentQuotaTreatsAChildNameAndItsArnAsTheSameAlarm() {
+        CompositeAlarm child = put("child", "FALSE");
+        for (int index = 0; index < 150; index++) {
+            put("parent-" + index, "ALARM(child)");
+        }
+        assertThrows(AwsException.class, () -> put("overflow", "ALARM(\"" + child.getAlarmArn() + "\")"));
+        service.deleteAlarms(List.of("parent-0"), REGION);
+        assertEquals("OK", put("replacement", "ALARM(\"" + child.getAlarmArn() + "\")").getStateValue());
+    }
+
+    @ParameterizedTest
+    @ValueSource(strings = {"", "ALARM()", "TRUE junk", "TRUE AND", "ALARM(\"unterminated)",
+            "(TRUE", "ALARM(x) XOR FALSE", "ALARM(\"x\\z\")"})
+    void invalidRulesDoNotReplaceAnExistingAlarm(String invalid) {
+        put("saved", "TRUE");
+        assertThrows(AwsException.class, () -> put("saved", invalid));
+        assertEquals("TRUE", service.describeCompositeAlarms(List.of("saved"), null, REGION).getFirst().getAlarmRule());
+        assertEquals("ALARM", state("saved"));
+    }
+
+    @Test
+    void quotasAndAtomicCreatePreventExcessReferencesAndForeignNameReplacement() {
+        String many = IntStream.range(0, 101).mapToObj(i -> "ALARM(a" + i + ")")
+                .collect(Collectors.joining(" OR "));
+        assertThrows(AwsException.class, () -> put("too-many", many));
+        put("owned", "TRUE");
+        CompositeAlarm foreign = new CompositeAlarm();
+        foreign.setAlarmName("owned");
+        foreign.setAlarmRule("FALSE");
+        assertThrows(AwsException.class, () -> service.putCompositeAlarm(foreign, REGION, false));
+        assertEquals("ALARM", state("owned"));
+        assertThrows(AwsException.class, () -> put("deep", "(".repeat(250) + "TRUE" + ")".repeat(250)));
+        assertEquals("ALARM", put("many-not", "NOT ".repeat(502) + "TRUE").getStateValue());
+        assertThrows(AwsException.class, () -> put("elements", "FALSE OR ".repeat(500) + "TRUE"));
+    }
+
+    @Test
+    void mixedPersistenceLoadsLegacyMetricsAndRoundTripsComposites() throws Exception {
+        ObjectMapper mapper = new ObjectMapper();
+        TypeReference<Map<String, Alarm>> type = new TypeReference<>() {};
+        Map<String, Alarm> legacy = mapper.readValue(
+                "{\"legacy\":{\"alarmName\":\"old\",\"metricName\":\"CPU\",\"stateValue\":\"OK\"}}", type);
+        assertInstanceOf(MetricAlarm.class, legacy.get("legacy"));
+        CompositeAlarm alarm = put("new", "FALSE");
+        Map<String, Alarm> restored = mapper.readValue(mapper.writeValueAsBytes(Map.of("new", alarm)), type);
+        assertEquals("FALSE", assertInstanceOf(CompositeAlarm.class, restored.get("new")).getAlarmRule());
+        assertEquals("OK", restored.get("new").getStateValue());
+    }
+
+    @ParameterizedTest
+    @ValueSource(booleans = {false, true})
+    void concurrentTagChangesPreserveTheUpdatedDefinition(boolean remove) throws Exception {
+        CountDownLatch updating = new CountDownLatch(1);
+        CountDownLatch release = new CountDownLatch(1);
+        InMemoryStorage<String, Alarm> storage = new InMemoryStorage<>() {
+            @Override
+            public void put(String key, Alarm value) {
+                if (value instanceof CompositeAlarm composite && "TRUE".equals(composite.getAlarmRule())) {
+                    updating.countDown();
+                    try {
+                        assertTrue(release.await(5, TimeUnit.SECONDS));
+                    } catch (InterruptedException error) {
+                        Thread.currentThread().interrupt();
+                        throw new AssertionError(error);
+                    }
+                }
+                super.put(key, value);
+            }
+        };
+        CloudWatchMetricsService concurrent = new CloudWatchMetricsService(new InMemoryStorage<>(), storage,
+                new RegionResolver(REGION, "123456789012"));
+        CompositeAlarm original = new CompositeAlarm();
+        original.setAlarmName("concurrent");
+        original.setAlarmRule("FALSE");
+        original.setTags(new HashMap<>(Map.of("team", "old")));
+        concurrent.putCompositeAlarm(original, REGION);
+        CompositeAlarm replacement = new CompositeAlarm();
+        replacement.setAlarmName("concurrent");
+        replacement.setAlarmRule("TRUE");
+        AtomicReference<Thread> tagThread = new AtomicReference<>();
+        try (ExecutorService executor = Executors.newFixedThreadPool(2)) {
+            Future<?> update = executor.submit(() -> concurrent.putCompositeAlarm(replacement, REGION));
+            Future<?> tags;
+            try {
+                assertTrue(updating.await(2, TimeUnit.SECONDS));
+                tags = executor.submit(() -> {
+                    tagThread.set(Thread.currentThread());
+                    if (remove) {
+                        concurrent.untagResource(original.getAlarmArn(), List.of("team"), REGION);
+                    } else {
+                        concurrent.tagResource(original.getAlarmArn(), Map.of("team", "new"), REGION);
+                    }
+                });
+                ThreadStates.awaitBlockedOrDone(tagThread, tags);
+                assertFalse(tags.isDone(), "tag mutation must share the definition update lock");
+            } finally {
+                release.countDown();
+            }
+            update.get(2, TimeUnit.SECONDS);
+            tags.get(2, TimeUnit.SECONDS);
+        }
+        CompositeAlarm result = concurrent.describeCompositeAlarms(List.of("concurrent"), null, REGION).getFirst();
+        assertEquals("TRUE", result.getAlarmRule());
+        assertEquals(remove ? Map.of() : Map.of("team", "new"), result.getTags());
+    }
+
+    private CompositeAlarm put(String name, String rule) {
+        CompositeAlarm alarm = new CompositeAlarm();
+        alarm.setAlarmName(name);
+        alarm.setAlarmRule(rule);
+        alarm.setActionsEnabled(true);
+        service.putCompositeAlarm(alarm, REGION);
+        return alarm;
+    }
+
+    private String state(String name) {
+        return service.describeCompositeAlarms(List.of(name), null, REGION).getFirst().getStateValue();
+    }
+}

src/test/resources/cloudformation/supported-resource-types.tsv

@@ -41,6 +41,7 @@ AWS::CloudFront::OriginRequestPolicy	CloudFrontCfnProvisioner
 AWS::CloudFront::ResponseHeadersPolicy	CloudFrontCfnProvisioner
 AWS::CloudTrail::Trail	CloudTrailCfnProvisioner
 AWS::CloudWatch::Alarm	CloudWatchCfnProvisioner
+AWS::CloudWatch::CompositeAlarm	CloudWatchCompositeAlarmCfnProvisioner
 AWS::CloudWatch::Dashboard	CloudWatchDashboardCfnProvisioner
 AWS::CodeBuild::Project	CodeBuildCfnProvisioner
 AWS::CodePipeline::CustomActionType	CodePipelineCfnProvisioner

tools/docs/cfn_resource_types.yaml

@@ -169,6 +169,9 @@ notes:
 # Trailing prose appended to a row after its type list, for behaviour that belongs to the service
 # rather than to one type.
 row_notes:
+  AWS::CloudWatch: >-
+    Composite alarms have backing definitions with name Ref and Arn GetAtt, rollback, and
+    replacement cleanup. Composite actions are stored only; suppression properties are rejected.
   AWS::Lambda: >-
     Inline `ZipFile` packages include the `cfn-response` (Node.js) / `cfnresponse` (Python) module
     AWS injects for that code path, so Solutions-style custom-resource handlers work.