NewDeadLetter return an instance of DeadLetter storage.

NewTimer return an instance of delay queue.

Poll from multiple queues using blocking method; OR pop a job from one queue using non-blocking method.

Because the DeadLetter is not like Timer which is a singleton, DeadLetters are transient objects like Queue.

SetLogger will set the logger for engine.

Setup set the essential config of redis engine.

DeadLetter is where dead job will be buried, the job can be respawned into ready queue.

Engine that connects all the dots including: - store jobs to timer set or ready queue - deliver jobs to clients - manage dead letters.

Pool stores all the jobs' data.

Queue is the "ready queue" that has all the jobs that can be consumed right now.

Timer is the other way of saying "delay queue".