Mosquito::Queue

A named Queue.

Named Queues exist and have 4 ordered lists: waiting, pending, scheduled, and dead.

• The Waiting list is for jobs which need to be executed as soon as possible.
• The Pending list is for jobs which are currently being executed.
• The Scheduled list is indexed by execution time and holds jobs which need to be executed at a later time.
• The Dead list is for jobs which have been retried too many times and are no longer viable.

A job_run is represented in a queue by its id.

A job_run flows through the queues in this manner:

 Time=0: JobRun does not exist yet, lists are empty

   Waiting  Pending  Scheduled    Dead

 ---------------------------------
 Time=1: JobRun is enqueued

   Waiting  Pending  Scheduled    Dead
    JobRun#1

 ---------------------------------
 Time=2: JobRun begins. JobRun is moved to pending and executed

   Waiting  Pending  Scheduled    Dead
             JobRun#1

 ---------------------------------
 Time=3: JobRuns are Enqueued.

   Waiting  Pending  Scheduled    Dead
    JobRun#2   JobRun#1
    JobRun#3

 ---------------------------------
 Time=4: JobRun succeeds, next job_run begins.

   Waiting  Pending  Scheduled    Dead
    JobRun#3   JobRun#2

 ---------------------------------
 Time=5: JobRun fails and is scheduled for later, next job_run begins.

   Waiting  Pending  Scheduled     Dead
             JobRun#3  t=7:JobRun#2

 ---------------------------------
 Time=6: JobRun succeeds. Nothing is executing.

   Waiting  Pending  Scheduled     Dead
                     t=7:JobRun#2

 ---------------------------------
 Time=7: Scheduled job_run is due and is moved to waiting. Nothing is executing.

   Waiting  Pending  Scheduled     Dead
    JobRun#2

 ---------------------------------
 Time=8: JobRun begins executing (for the second time).

   Waiting  Pending  Scheduled     Dead
             JobRun#2

 ---------------------------------
 Time=9: JobRun finished successfully. No more job_runs present.

   Waiting  Pending  Scheduled     Dead