An SQS-triggered fee-posting Lambda can be invoked more than once f…

Question

An SQS-triggered fee-posting Lambda can be invoked more than once for the same message because of at-least-once delivery. Double posts would corrupt municipal ledgers. What must the handler do?

Accepted Answer

Make the handler idempotent using conditional writes or deduplication tokens so repeated deliveries do not create duplicate ledger posts. — SQS can knock twice—treat fee posting like a stamped voucher, not a blank check. Conditional writes or dedupe tokens keep a second delivery from double-charging the ledger. Hoping for exactly-once delivery is wishful thinking.

Answer

Assume SQS exactly-once delivery and write blindly on every invoke.

Answer

Disable the queue's visibility timeout so duplicates never occur.

Answer

Delete the DynamoDB table after each successful post to prevent duplicates.

An SQS-triggered fee-posting Lambda can be invoked more than once for the same message because of at-least-once delivery. Double posts would corrupt municipal ledgers. What must the handler do?

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Full Explanation