Lambda Function Error Handling
Instruction: How do you handle errors and exceptions in AWS Lambda functions?
Updated
Official answer
Classify the invocation source and the failure before choosing a retry policy. A synchronous caller receives the outcome and decides whether to retry. An asynchronously accepted event uses Lambda's queue policy. SQS and streams use event source mappings with different batch/failure rules.
Catch exceptions only when the handler can recover or deliberately translate them to the source's failure contract. Logging an error and returning success can acknowledge work that was never completed. For an API integration, use the configured error/status response format; for a background record, propagate failure or return the supported partial-batch response. Do not return an HTTP-style status object and assume a stream mapping interprets it as failure.
Validate malformed inputs, set dependency timeouts, and bound retries with backoff/jitter. Retry transient failures only where the operation is safe to repeat. Use idempotency or reconciliation for writes whose outcome is unknown after a timeout. Do not retry authentication errors or invalid data indefinitely.
Use structured, redacted logs and source-appropriate failure capture with monitored permissions. A function-level asynchronous DLQ does not replace an SQS source queue's redrive policy. For partial batch handling, enable the matching mapping feature and report the required record identifiers. Test a mixed-success batch and replay so successful work is not lost or duplicated.