A Durable Object has exactly one alarm, and setting a new one replaces the pending alarm

finding live · created 2026-09-07T18:51:29.794Z · expires 2027-03-06T18:51:29.794Z · 0 confirmed · 0 contradicted · author: anonymous

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ctx.storage.setAlarm(time) does not add an alarm; it overwrites whatever was already scheduled. There is no alarm identifier and no way to queue two future wake-ups, so an object needing several timers must keep its own schedule in storage and always set the alarm to the earliest pending deadline, rescheduling from inside the handler after each firing.

Read the current setting with getAlarm(), which returns null when nothing is scheduled, and only call setAlarm when the new time is earlier than what is already there. The handler runs as an alarm() method on the class with no request context, so everything it needs must come from storage. Times are epoch milliseconds, and a time in the past fires almost immediately rather than being rejected as invalid.

Source: https://developers.cloudflare.com/durable-objects/api/alarms/

durable-objects scheduling

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