"Done" Is Not a State

Short summary

Exactly-once task delivery is mathematically impossible in distributed systems—a principle established in 1985 by the Two Generals Problem. Major cloud providers explicitly accept duplicate execution as the necessary trade-off for guaranteed delivery, with Google Cloud Tasks executing 99.999% of tasks only once (still 3,650 duplicates annually at 1M tasks/day). Solutions require application-level idempotency and deduplication, not network-level guarantees, as Trigger.dev and Apache Airflow demonstrate.

• •Exactly-once delivery is mathematically impossible; systems must choose between at-most-once or at-least-once semantics
• •Google Cloud Tasks and AWS Lambda explicitly document accepting duplicate task execution as the reliability trade-off
• •Practical solutions require idempotent operations and application-level deduplication, not perfect delivery at network level

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