ConcurrencyLimiter.php

TLDR

The ConcurrencyLimiter class is responsible for acquiring and releasing locks on concurrent tasks using Redis. It allows only a certain number of tasks to run concurrently, and if the limit is reached, it blocks until a slot becomes available.

Methods

block

Attempt to acquire the lock for the given number of seconds. If the lock is acquired, it executes a callback function and releases the lock afterwards. If the lock is not acquired within the specified timeout, a LimiterTimeoutException is thrown.

acquire

Attempt to acquire a lock. It generates a list of slot names based on the limiter name and the maximum number of locks. It executes a Lua script that checks if any of the slots are available. If a slot is available, the Redis key for that slot is set to the unique identifier for the lock, and the slot name is returned. If no slot is available, null is returned.

release

Release a lock by executing a Lua script that checks if the lock still exists with the given unique identifier. If the lock exists, it is deleted.

Classes

There are no classes in this file.

<?php

namespace Illuminate\Redis\Limiters;

use Illuminate\Contracts\Redis\LimiterTimeoutException;
use Illuminate\Support\Sleep;
use Illuminate\Support\Str;
use Throwable;

class ConcurrencyLimiter
{
    /**
     * The Redis factory implementation.
     *
     * @var \Illuminate\Redis\Connections\Connection
     */
    protected $redis;

    /**
     * The name of the limiter.
     *
     * @var string
     */
    protected $name;

    /**
     * The allowed number of concurrent tasks.
     *
     * @var int
     */
    protected $maxLocks;

    /**
     * The number of seconds a slot should be maintained.
     *
     * @var int
     */
    protected $releaseAfter;

    /**
     * Create a new concurrency limiter instance.
     *
     * @param  \Illuminate\Redis\Connections\Connection  $redis
     * @param  string  $name
     * @param  int  $maxLocks
     * @param  int  $releaseAfter
     * @return void
     */
    public function __construct($redis, $name, $maxLocks, $releaseAfter)
    {
        $this->name = $name;
        $this->redis = $redis;
        $this->maxLocks = $maxLocks;
        $this->releaseAfter = $releaseAfter;
    }

    /**
     * Attempt to acquire the lock for the given number of seconds.
     *
     * @param  int  $timeout
     * @param  callable|null  $callback
     * @param  int  $sleep
     * @return mixed
     *
     * @throws \Illuminate\Contracts\Redis\LimiterTimeoutException
     * @throws \Throwable
     */
    public function block($timeout, $callback = null, $sleep = 250)
    {
        $starting = time();

        $id = Str::random(20);

        while (! $slot = $this->acquire($id)) {
            if (time() - $timeout >= $starting) {
                throw new LimiterTimeoutException;
            }

            Sleep::usleep($sleep * 1000);
        }

        if (is_callable($callback)) {
            try {
                return tap($callback(), function () use ($slot, $id) {
                    $this->release($slot, $id);
                });
            } catch (Throwable $exception) {
                $this->release($slot, $id);

                throw $exception;
            }
        }

        return true;
    }

    /**
     * Attempt to acquire the lock.
     *
     * @param  string  $id  A unique identifier for this lock
     * @return mixed
     */
    protected function acquire($id)
    {
        $slots = array_map(function ($i) {
            return $this->name.$i;
        }, range(1, $this->maxLocks));

        return $this->redis->eval(...array_merge(
            [$this->lockScript(), count($slots)],
            array_merge($slots, [$this->name, $this->releaseAfter, $id])
        ));
    }

    /**
     * Get the Lua script for acquiring a lock.
     *
     * KEYS    - The keys that represent available slots
     * ARGV[1] - The limiter name
     * ARGV[2] - The number of seconds the slot should be reserved
     * ARGV[3] - The unique identifier for this lock
     *
     * @return string
     */
    protected function lockScript()
    {
        return <<<'LUA'
for index, value in pairs(redis.call('mget', unpack(KEYS))) do
    if not value then
        redis.call('set', KEYS[index], ARGV[3], "EX", ARGV[2])
        return ARGV[1]..index
    end
end
LUA;
    }

    /**
     * Release the lock.
     *
     * @param  string  $key
     * @param  string  $id
     * @return void
     */
    protected function release($key, $id)
    {
        $this->redis->eval($this->releaseScript(), 1, $key, $id);
    }

    /**
     * Get the Lua script to atomically release a lock.
     *
     * KEYS[1] - The name of the lock
     * ARGV[1] - The unique identifier for this lock
     *
     * @return string
     */
    protected function releaseScript()
    {
        return <<<'LUA'
if redis.call('get', KEYS[1]) == ARGV[1]
then
    return redis.call('del', KEYS[1])
else
    return 0
end
LUA;
    }
}