Execution guarantees - Amazon Step Functions
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Execution guarantees

Standard Workflows Asynchronous Express Workflows Synchronous Express Workflows
Exactly-once workflow execution At-least-once workflow execution At-most-once workflow execution
Execution state internally persists between state transitions. Execution state doesn't persist between state transitions. Execution state doesn't persist between state transitions.
Automatically returns an idempotent response on starting an execution with the same name as a currently-running workflow. The new workflow doesn't start and an exception is thrown once the currently-running workflow is complete. Idempotency is not automatically managed. Starting multiple workflows with the same name results in concurrent executions. Can result in loss of internal workflow state if state machine logic is not idempotent. Idempotency is not automatically managed. Step Functions waits once an execution starts and returns the state machine's result on completion. Workflows don't restart if an exception occurs.
Execution history data removed after 90 days. Workflow names can be reused after removal of out-of-date execution data. Execution history is not captured by Step Functions. Logging must be enabled through Amazon CloudWatch Logs. Execution history is not captured by Step Functions. Logging must be enabled through Amazon CloudWatch Logs.