> ## Documentation Index
> Fetch the complete documentation index at: https://docs.sndbox.app/llms.txt
> Use this file to discover all available pages before exploring further.

# Schedules and event queue

> Persist schedule occurrences and deliver events with explicit retry, expiry, and replay behavior.

The durable scheduler is an independently deployable service. It calculates interval, calendar, and cron occurrences in an IANA timezone and persists an occurrence before a worker can claim it.

## Duplicate prevention

PostgreSQL advisory locking elects one scheduler transaction at a time. Row locks and the `(schedule_id, scheduled_for)` constraint prevent a restart or overlapping tick from emitting the same occurrence twice.

Schedules define daylight-saving, misfire, jitter, concurrency, and no-runner behavior. A spring-forward wall time that does not exist is not silently shifted. Bounded backfill is explicit, and an existing occurrence is not re-emitted.

## Queue delivery

Events use at-least-once delivery. A claim carries a short hashed visibility token. An expired claim returns to the queue and adds immutable attempt history. Retries use exponential backoff with deterministic jitter; the event enters dead-letter state after its attempt bound or three identical failures.

## Replay

Administrative replay is audited and creates a new queue row, correlation ID, and idempotency key while preserving the original logical event identity. Replay does not prove an external side effect is safe; node recovery policy still applies.

Payloads are referenced through encrypted storage rather than stored inline. Queue rows carry workspace, workflow revision, routing requirements, expiry, attempts, and correlation identity.