RedisGetlock

Distributed locking using redis. Unlike other implementations available, this gem ensures releasing orphaned lock shortly.

How It Works

This gem basically works as http://redis.io/commands/set describes for redis >= 2.6.12, and http://redis.io/commands/setnx describes for redis < 2.6.12.

Simple ruby codes which http://redis.io/commands/set describes are as follows:

loop do
  break if redis.set(key, 'anystring', {nx: true, ex: expire})
  sleep 1
end
puts 'get lock'
begin
  # do a job
ensure
  redis.del(key) # unlock
end

The problem here is the value of expire. The expiration time expire is necessary so that a lock will eventually be released even if a process is crashed or killed by SIGKILL before deleting the key. However, how long should we set if we are uncertain how long a job takes?

This gem takes a following approach to resolve this problem.

  1. Expiration time is set to 2 (default) seconds
  2. Extend the lock in each 1 (default) second interval invoking another thread

This way ensures to release orphaned lock in 2 seconds. We are released from caring of the value of expire!!

Simple ruby codes to explain how this gem works are as follows:

loop do
  break if redis.set(key, 'anystring', {nx: true, ex: 2})
  sleep 1
end
puts 'get lock'
thr = Thread.new do
  loop do
    redis.expire(key, 2)
    sleep 1
  end
end
begin
  # do a job
ensure
  thr.terminate
  redis.del(key) # unlock
end

Installation

Add this line to your application's Gemfile:

gem 'redis_getlock'

And then execute:

$ bundle

Or install it yourself as:

$ gem install redis_getlock

Usage

Similarly with ruby standard library mutex, following methods are available:

  • lock
    • Attempts to grab the lock and waits if it isn’t available. Returns true if successfully acquired a lock
  • locked?
    • Returns true if this lock is currently held by some (including myself).
  • synchronize {}
    • Obtains a lock, runs the block, and releases the lock when the block completes. Raises RedisGetlock::LockError error when failed to acquire a lock.
  • unlock
    • Releases the lock. Returns true if successfully released a lock
  • self_locked?
    • Returns true if this lock is currently held by myself.
  • try_lock
    • Attempts to grab the lock and returns immediately without waits. Returns true if successfully acquired a lock

Options of RedisGetlock.new are:

  • redis
    • Provide a redis instance
  • key
    • Key name for a distributed lock
  • timeout
    • The timeout of trying to get the lock. A negative value means infinite timeout (default: -1)
  • logger
    • Provide a logger for RedisGetlock (for debug)
  • expire
    • The expiration seconds of the lock. The default is 2 second. Users usually do not need to care of this because the expiration is automatically extended in a invoked thread.
  • interval
    • Interval to extend lock expiration. Must be expire > interval. The default is 1 second.

Example

require 'redis'
require 'redis_getlock'

redis = Redis.new # Redis.new(options)
mutex = RedisGetlock.new(redis: redis, key: 'lock_key')

mutex.lock
begin
  puts 'get lock'
ensure
  mutex.unlock
end

mutex.synchronize do
  puts 'get lock'
end

Contributing

Bug reports and pull requests are welcome on GitHub at https://github.com/sonots/redis_getlock. This project is intended to be a safe, welcoming space for collaboration, and contributors are expected to adhere to the Contributor Covenant code of conduct.

License

The gem is available as open source under the terms of the MIT License.

ChangeLog

CHANGELOG.md