Idempotence

An operation is idempotent if executing it multiple times produces the same result as executing it once.

Techniques

• Message deduplication — track processed message IDs; discard duplicates before processing
• Conditional writes — use unique keys or version checks so repeated writes are no-ops (e.g. INSERT ... ON CONFLICT DO NOTHING)
• Exactly-once semantics — the messaging system itself guarantees no duplicates (requires producer + broker + consumer coordination)
• Dead-letter queue (DLQ) — messages that repeatedly fail are moved aside instead of retried indefinitely