Semaphore semantics to restrict how many workers may concurrently perform a task. More...

Inherited by MediaWiki\PoolCounter\PoolCounterClient, PoolCounterNull, and PoolCounterRedis.

Public Member Functions
	__construct (array $conf, string $type, string $key)

	acquireForAnyone ( $timeout=null)
	I want to do this task, but if anyone else does it instead, it's also fine for me.

	acquireForMe ( $timeout=null)
	I want to do this task and I need to do it myself.

	getKey ()

	isFastStaleEnabled ()
	Is fast stale mode (T250248) enabled? This may be overridden by the PoolCounterWork subclass.

	release ()
	I have successfully finished my task.

Public Attributes
const	DONE = 3

const	ERROR = -1

const	LOCK_HELD = -5

const	LOCKED = 1

const	NOT_LOCKED = -2

const	QUEUE_FULL = -3

const	RELEASED = 2

const	TIMEOUT = -4

Protected Member Functions
	hashKeyIntoSlots ( $type, $key, $slots)
	Given a key (any string) and the number of lots, returns a slot key (a prefix with a suffix integer from the [0..($slots-1)] range).

	onAcquire ()
	Update any lock tracking information when the lock is acquired.

	onRelease ()
	Update any lock tracking information when the lock is released.

	precheckAcquire ()
	Checks that the lock request is sensible.

Protected Attributes
string	$key
	All workers with the same key share the lock.

int	$maxqueue
	If this number of workers are already working/waiting, fail instead of wait.

int	$slots = 0
	Maximum number of workers working on this task type, regardless of key.

int	$timeout
	Maximum time in seconds to wait for the lock.

int	$workers
	Maximum number of workers working on tasks with the same key simultaneously.

Detailed Description

Semaphore semantics to restrict how many workers may concurrently perform a task.

When you have many workers (threads/servers) in service, and a cached item expensive to produce expires, you may get several workers computing the same expensive item at the same time.

Given enough incoming requests and the item expiring quickly (non-cacheable, or lots of edits or other invalidation events) that single task can end up unfairly using most (or all) of the CPUs of the server cluster. This is also known as "Michael Jackson effect", as this scenario happened on the English Wikipedia in 2009 on the day Michael Jackson died. See also https://wikitech.wikimedia.org/wiki/Michael_Jackson_effect.

PoolCounter was created to provide semaphore semantics to restrict the number of workers that may be concurrently performing a given task. Only one key can be locked by any PoolCounter instance of a process, except for keys that start with "nowait:". However, only non-blocking requests (timeout=0) may be used with a "nowait:" key.

By default PoolCounterNull is used, which provides no locking. Install the poolcounterd service from https://gerrit.wikimedia.org/g/mediawiki/services/poolcounter to enable this feature.

Since: 1.16

Stability: stable: to extend

Definition at line 49 of file PoolCounter.php.

Constructor & Destructor Documentation

◆ __construct()

PoolCounter::__construct	(	array	$conf,
		string	$type,
		string	$key )

Parameters

array	$conf
string	$type	The class of actions to limit concurrency for (task type)
string	$key

Definition at line 96 of file PoolCounter.php.

References $key, and hashKeyIntoSlots().

Member Function Documentation

◆ acquireForAnyone()

PoolCounter::acquireForAnyone ( $timeout = null )

abstract

I want to do this task, but if anyone else does it instead, it's also fine for me.

I will read its cached data.

Parameters

int | null $timeout Wait timeout, or null to use value passed to the constructor

Returns: Status Value is one of Locked/Done/Error

Reimplemented in MediaWiki\PoolCounter\PoolCounterClient, PoolCounterNull, and PoolCounterRedis.

◆ acquireForMe()

PoolCounter::acquireForMe ( $timeout = null )

abstract

I want to do this task and I need to do it myself.

Parameters

int | null $timeout Wait timeout, or null to use value passed to the constructor

Returns: Status Value is one of Locked/Error

Reimplemented in MediaWiki\PoolCounter\PoolCounterClient, PoolCounterNull, and PoolCounterRedis.

◆ getKey()

PoolCounter::getKey ( )

Returns: string

Definition at line 116 of file PoolCounter.php.

References $key.

◆ hashKeyIntoSlots()

PoolCounter::hashKeyIntoSlots	(	$type,
		$key,
		$slots )

protected

Given a key (any string) and the number of lots, returns a slot key (a prefix with a suffix integer from the [0..($slots-1)] range).

This is used for a global limit on the number of instances of a given type that can acquire a lock. The hashing is deterministic so that PoolCounter::$workers is always an upper limit of how many instances with the same key can acquire a lock.

Parameters

string	$type	The class of actions to limit concurrency for (task type)
string	$key	PoolCounter instance key (any string)
int	$slots	The number of slots (max allowed value is 65536)

Returns: string Slot key with the type and slot number

Definition at line 202 of file PoolCounter.php.

References $key, $slots, and $type.

Referenced by __construct().

◆ isFastStaleEnabled()

PoolCounter::isFastStaleEnabled ( )

Is fast stale mode (T250248) enabled? This may be overridden by the PoolCounterWork subclass.

Returns: bool

Definition at line 212 of file PoolCounter.php.

◆ onAcquire()

PoolCounter::onAcquire ( )

finalprotected

Update any lock tracking information when the lock is acquired.

Since: 1.25

Definition at line 178 of file PoolCounter.php.

Referenced by PoolCounterRedis\waitForSlotOrNotif().

◆ onRelease()

PoolCounter::onRelease ( )

finalprotected

Update any lock tracking information when the lock is released.

Since: 1.25

Definition at line 186 of file PoolCounter.php.

Referenced by PoolCounterRedis\release(), and PoolCounterRedis\waitForSlotOrNotif().

◆ precheckAcquire()

PoolCounter::precheckAcquire ( )

finalprotected

Checks that the lock request is sensible.

Returns: Status good for sensible requests, fatal for the not so sensible

Since: 1.25

Definition at line 153 of file PoolCounter.php.

Referenced by PoolCounterRedis\acquireForAnyone(), and PoolCounterRedis\acquireForMe().

◆ release()

PoolCounter::release ( )

abstract

I have successfully finished my task.

Lets another one grab the lock, and returns the workers waiting on acquireForAnyone()

Returns: Status Value is one of Released/NotLocked/Error

Reimplemented in MediaWiki\PoolCounter\PoolCounterClient, PoolCounterNull, and PoolCounterRedis.

Member Data Documentation

◆ $key

string PoolCounter::$key

protected

All workers with the same key share the lock.

Definition at line 62 of file PoolCounter.php.

Referenced by PoolCounterRedis\__construct(), __construct(), getKey(), and hashKeyIntoSlots().

◆ $maxqueue

int PoolCounter::$maxqueue

protected

If this number of workers are already working/waiting, fail instead of wait.

Definition at line 73 of file PoolCounter.php.

◆ $slots

int PoolCounter::$slots = 0

protected

Maximum number of workers working on this task type, regardless of key.

0 means unlimited. Max allowed value is 65536. The way the slot limit is enforced is overzealous - this option should be used with caution.

Definition at line 71 of file PoolCounter.php.

Referenced by hashKeyIntoSlots().

◆ $timeout

int PoolCounter::$timeout

protected

Maximum time in seconds to wait for the lock.

Definition at line 75 of file PoolCounter.php.

Referenced by PoolCounterRedis\acquireForAnyone(), PoolCounterRedis\acquireForMe(), and PoolCounterRedis\waitForSlotOrNotif().

◆ $workers

int PoolCounter::$workers

protected

Maximum number of workers working on tasks with the same key simultaneously.

Definition at line 64 of file PoolCounter.php.

◆ DONE

const PoolCounter::DONE = 3

Definition at line 53 of file PoolCounter.php.

Referenced by PoolCounterWork\execute(), and PoolCounterRedis\waitForSlotOrNotif().

◆ ERROR

const PoolCounter::ERROR = -1

Definition at line 55 of file PoolCounter.php.

Referenced by PoolCounterWork\execute().

◆ LOCK_HELD

const PoolCounter::LOCK_HELD = -5

Definition at line 59 of file PoolCounter.php.

Referenced by PoolCounterWork\execute(), and PoolCounterRedis\waitForSlotOrNotif().

◆ LOCKED

const PoolCounter::LOCKED = 1

Definition at line 51 of file PoolCounter.php.

Referenced by PoolCounterNull\acquireForAnyone(), PoolCounterNull\acquireForMe(), PoolCounterWork\execute(), and PoolCounterRedis\waitForSlotOrNotif().

◆ NOT_LOCKED

const PoolCounter::NOT_LOCKED = -2

Definition at line 56 of file PoolCounter.php.

Referenced by PoolCounterRedis\release().

◆ QUEUE_FULL

const PoolCounter::QUEUE_FULL = -3

Definition at line 57 of file PoolCounter.php.

Referenced by PoolCounterWork\execute(), and PoolCounterRedis\waitForSlotOrNotif().

◆ RELEASED

const PoolCounter::RELEASED = 2

Definition at line 52 of file PoolCounter.php.

Referenced by PoolCounterNull\release(), and PoolCounterRedis\release().

◆ TIMEOUT

const PoolCounter::TIMEOUT = -4

Definition at line 58 of file PoolCounter.php.

Referenced by PoolCounterWork\execute(), and PoolCounterRedis\waitForSlotOrNotif().

The documentation for this class was generated from the following file:

includes/poolcounter/PoolCounter.php

Public Member Functions

Public Attributes

Protected Member Functions

Protected Attributes

Detailed Description

Constructor & Destructor Documentation

◆ __construct()

Member Function Documentation

◆ acquireForAnyone()

◆ acquireForMe()

◆ getKey()

◆ hashKeyIntoSlots()

◆ isFastStaleEnabled()

◆ onAcquire()

◆ onRelease()

◆ precheckAcquire()

◆ release()

Member Data Documentation

◆ $key

◆ $maxqueue

◆ $slots

◆ $timeout

◆ $workers

◆ DONE

◆ ERROR

◆ LOCK_HELD

◆ LOCKED

◆ NOT_LOCKED

◆ QUEUE_FULL

◆ RELEASED

◆ TIMEOUT