PoolCounter

PoolCounter is a network daemon which provides mutex-like functionality, with a limited wait queue length. If too many servers try to do the same thing at the same time, the wait queue overflows and some configurable action might be taken by subsequent clients, such as displaying an error message or using a stale cache entry.

It was created to avoid massive wastage of CPU due to parallel parsing when the cache of a popular article is invalidated (the "Michael Jackson problem"), but has later been put to other uses as well, such as limiting thumbnail scaling requests.

MediaWiki uses PoolCounter via an abstract interface (see ) which allows alternative implementations.

Source
The implementation is located in multiple places: There is also a Redis-based default implementation in MediaWiki core, and an experimental Python client for the daemon in Thumbor.
 * The server source is in the mediawiki/services/poolcounter repository.
 * The client source is in the /includes/poolcounter directory of MediaWiki core.

Architecture
The server is a single-threaded C program based on libevent. It does not use autoconf, it just has a makefile which is suitable for a normal Linux environment. It has currently has no daemonize code, and so is backgrounded by systemd.

In MediaWiki, the client must be a subclass of  and the class holding the application-specific logic must be a subclass of. See Manual:$wgPoolCounterConf for details.

Protocol
The network protocol is line-based, with parameters separated by spaces (spaces in parameters are percent-encoded). The client opens a connection, sends a lock acquire command, does the work, sends a lock release command, then closes the connection. The following commands are defined:


 * ACQ4ANY   : This is used to acquire a lock when the client is capable of using the cache entry generated by another process. If the active pool worker limit is exceeded, the server will give a delayed response to this command. When a client completes its work, all processes which are waiting with ACQ4ANY will immediately be woken so that they can read the new cache entry.
 * ACQ4ME   : This is used to acquire a lock when cache sharing is not possible or not applicable, for example when an article rendering request involves a non-default stub threshold. When a lock of this kind is released, only one waiting process will be woken, so as to keep the worker population the same.
 * RELEASE : releases a lock
 * STATS [FULL|UPTIME]: show statistics

The possible responses for ACQ4ANY/ACQ4ME:
 * LOCKED: successfully acquired a lock. Client is expected to do the work, then send RELEASE.
 * DONE: sent to wake up a waiting client
 * QUEUE_FULL: there are more workers than
 * TIMEOUT: there are more workers than ; no slot was freed up after waiting for seconds
 * LOCK_HELD: trying to get a lock when one is already held

For RELEASE:
 * NOT_LOCKED: client asked to release a lock that did not exist
 * RELEASED: lock successfully released

For any command:
 * ERROR

Configuration
The server does not require configuration. Configuration of pool sizes, wait timeouts, etc. is done dynamically by the client.

For MediaWiki-side configuration, see
 * Extension:PoolCounter
 * Extension:PoolCounter

Testing
$ echo 'STATS FULL' | nc -w1 localhost 7531 uptime: 633 days, 15209h 42m 26s total processing time: 85809 days 2059430h 0m 24.000000s average processing time: 0.957994s gained time: 1867 days 44820h 50m 24.000000s waiting time: 390 days 9365h 18m 24.000000s waiting time for me: 389 days 9343h 3m 28.000000s waiting time for anyone: 22h 14m 53.898438s waiting time for good: 520 days 12503h 48m 24.000000s wasted timeout time: 473 days 11375h 2m 44.000000s total_acquired: 7739031655 total_releases: 7736374042 hashtable_entries: 119 processing_workers: 119 waiting_workers: 216 connect_errors: 0 failed_sends: 1 full_queues: 10294544 lock_mismatch: 227 release_mismatch: 0 processed_count: 7739031536