The sorted set
as a clock.

Time is a constraint the queue itself enforces. Reach for a delayed queue when work must run later, not now — a reminder, a retry after a cooldown, a digest at a fixed hour. Reach for a scheduler when the same work recurs on a cadence. Reach for priority when one class of work must jump ahead of another. Reach for backoff when a failure should be retried, but not immediately and not all at once. Each is a position on one axis: when a job becomes eligible, and in what order the eligible jobs are served.

EchoMQ answers all of these with one schedule set and one server clock. A scheduled job is parked on emq:{q}:schedule at its run-at millisecond — a visibility fence, not a second queue. A pump promotes the due range back to pending on the server's own clock, so the delay is measured on the same clock the reaper and the promote pump read, never the caller's.

Five teaching modules, then a workshop. Each module is a hub with three dives.

The hard part is not the commands; it is choosing the mechanism. Name the time constraint that matters most for a workload, and the mechanism follows.

The chapter closes with R4.06: schedule codemojex's notification and digest jobs. A game-result notice is enqueued on a fair lane and delivered under a rate limit; over budget, the worker re-enqueues the same job with EchoMQ.Jobs.enqueue_in/5 after the bucket's reported wait — deferred on the bus, durable, not dropped. A retry on a transient delivery failure rides capped exponential backoff with jitter, and a scheduled job that must survive a crash reaches the durable floor.

After the clock comes the durable log — the streams and events later chapters replay and archive.