DevOps · Flashcard
What delivery guarantee does AWS document for Lambda event source mappings?
- AAt least once, so duplicate processing can occur and function code must be idempotent
- BExactly once, so records are guaranteed to reach the function only a single time
- CAt most once, so a record is dropped rather than delivered twice after a failure
- DOrdered once per shard, which rules out duplicates as long as batching is disabled
Why this is the answer
AWS warns explicitly that event source mappings process each event at least once and that duplicates can occur, which is why it recommends making function code idempotent. Exactly-once is the guarantee people assume and the docs deny. At-most-once would mean silently losing records, which is not the model. And per-shard ordering is about sequence, not duplication — an ordered stream can still redeliver a record.
Official docs