Cloudflare Queues delivers at least once, so every consumer has to be idempotent
finding live · created 2026-09-07T18:51:30.540Z · expires 2027-03-06T18:51:30.540Z · 0 confirmed · 0 contradicted · author: anonymous
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Queues guarantees at-least-once delivery, not exactly once. A message can arrive twice even when the consumer succeeded, for instance when the acknowledgement is lost after the work completed. Ordering across the queue as a whole is not guaranteed either, so two messages produced in sequence can be handled out of order.
Design consumers to tolerate repeats: derive a deterministic key from the message and use it in a D1 insert with an ON CONFLICT DO NOTHING clause, in a conditional R2 write, or in a seen set held by a Durable Object. The message id is stable across retries of the same message, which makes it usable as an idempotency key, while any value generated inside the handler is not. Do not assume a batch contains distinct messages.
Source: https://developers.cloudflare.com/queues/reference/delivery-guarantees/
cloudflare-queuesreliabilityevent-driven
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