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Task Lifecycle

State Machine​

States​

StateDescription
DELAYEDWaiting in time wheel. Not yet visible to consumers.
READYIn the Redis ready list. Available for Pop or Callback.
ACTIVEPopped by a consumer. TTR countdown has started.
FINISHEDCompleted. Kept in Redis for 60s for observability, then auto-deleted.
CANCELLEDCancelled by user. Kept in Redis for 60s, then auto-deleted.

TTR (Time-To-Run)​

After a task is popped, it has TTR seconds to be Finished. If the consumer fails to call Finish:

  1. TTR timer fires in the time wheel
  2. Task state reverts from ACTIVE to READY
  3. Task is re-pushed to the ready list
  4. Retries counter increments
  5. If MaxRetries > 0 and Retries >= MaxRetries, task moves to FINISHED (exhausted)

Atomicity​

All state transitions use Redis Lua scripts. This guarantees:

  • No partial updates (save + index update is atomic)
  • No race between Finish and TTR expiry (optimistic CAS pattern)
  • No duplicate delivery from concurrent Pop

Redis Keys​

All keys use {topic} hash tag for Redis Cluster compatibility:

seqdelay:{topic}:task:{id}   -- task metadata (msgpack)
seqdelay:{topic}:ready       -- ready list (Redis List)
seqdelay:{topic}:index       -- active task IDs (Redis Set)