Lambda Function Times Out Before API Gateway Integration Completes: Diagnosing Mismatched Timeout Settings

Goal: Configure an AWS Lambda function invoked synchronously through Amazon API Gateway so that the function completes before the integration timeout, thereby avoiding 504 Gateway Timeout errors while keeping compute waste low.

Constraints: The Lambda runtime enforces a hard timeout at the configured value, issuing a SIGTERM that allows a brief cleanup window before forced termination; API Gateway applies its own integration timeout that can cancel the request independently of the function’s execution. The function must perform any side‑effects idempotently because client cancellation does not stop the Lambda runtime.

Uncertainty: Determining the optimal Lambda timeout relative to the API Gateway timeout, accounting for the SIGTERM window, and balancing the risk of premature termination against wasted compute on timed‑out invocations.

What Lambda timeout value should be set relative to the API Gateway integration timeout to prevent 504 errors? How large should the SIGTERM cleanup window be compared to the function timeout to allow graceful shutdown? What trade‑offs arise when lowering the Lambda timeout to reduce wasted compute versus increasing the chance of incomplete work?