The Scheduler decides which sub-queue to assign a given Call. It implements a single method getPriorityLevel(Schedulable call) which returns an integer corresponding to the subqueue the FairCallQueue should place the call in.
The HistoryRpcScheduler is one such implementation which uses the username of each call and determines what % of calls in recent history were made by this user.
It is configured with a historyLength (how many calls to track) and a list of integer thresholds which determine the boundaries between priority levels.
For instance, if the scheduler has a historyLength of 8; and priority thresholds of 4,2,1; and saw calls made by these users in order:
Alice, Bob, Alice, Alice, Bob, Jerry, Alice, Alice
- Another call by Alice would be placed in queue 3, since she has already made >= 4 calls
- Another call by Bob would be placed in queue 2, since he has >= 2 but less than 4 calls
- A call by Carlos would be placed in queue 0, since he has no calls in the history
Also, some versions of this patch include the concept of a 'service user', which is a user that is always scheduled high-priority. Currently this seems redundant and will probably be removed in later patches, since its not too useful.
As of now, the current scheduler is the DecayRpcScheduler, which only keeps track of the number of each type of call and decays these counts periodically.