Composite alarm actions limited to SNS topic targets

Composite alarm actions in LocalStack currently support only SNS topic destinations. Direct invocation of Lambda functions, Auto Scaling actions, or other alarm action types will not work.

Route all composite alarm notifications through SNS topics. Configure AlarmActions, OKActions, and InsufficientDataActions with SNS topic ARNs only. Use SNS subscriptions to fan out to other targets like SQS queues, Lambda functions, or HTTP endpoints.