Describe how a user request triggers an AWS Step Function, including the flow from the user request to the Step Function execution.

An end‑to‑end request that triggers a Step Function typically starts with an API call from the client. The request is received by Amazon API Gateway, which validates the caller and forwards the payload to a Lambda function (or directly to the Step Function if using the 'StartExecution' API). The Lambda function performs any pre‑processing, such as authentication, input validation, or enrichment, and then calls the Step Functions API to start a state machine execution. The Step Function orchestrates a series of tasks—each task can be a Lambda, ECS task, or other AWS service—by following the JSON‑defined state machine. Each state can pass data to the next, and the Step Function handles retries, timeouts, and error handling. Once the state machine completes, the final state can trigger a callback to API Gateway or write results to a database. This flow ensures that the request is processed asynchronously, decouples the client from long‑running logic, and provides built‑in monitoring via CloudWatch. The key triggers are the API Gateway event, the Lambda invocation, and the Step Functions 'StartExecution' call.