A table of sequencers indexed by key, so that at any moment for each key there is at most one job running.
An 'a Sequencer_table.Make(Key).t is similar in concept to:
{ mutable state : 'a option
; jobs : 'a option Throttle.Sequencer.t
} Key.Table.t
It allows one to run jobs that are indexed by a key, while allowing jobs with distinct keys to run simultaneously, and ensuring that for any given key, at most one job with that key is running at a time. A sequencer table maintains optional state for each key, and supplies that state to each running job indexed by that key.
The implementation of a sequencer table is optimized for having a large number of keys with state, while only a few keys with active jobs at any given time. So, it is implemented with two tables, one of states and one of sequencers:
{ states : 'a Key.Table.t
; jobs : ('a option -> unit Deferred.t) Sequencer.t Key.Table.t
}
The implementation automatically adds a sequencer to the jobs table, if necessary,
when one adds a job, and automatically removes a sequencer from the jobs table
whenever the sequencer has no jobs to run.
The implementation does not automatically release state; one must call set_state t
~key None.