Redlock - A ruby distributed lock using redis.

Distributed locks are a very useful primitive in many environments where different processes require to operate with shared resources in a mutually exclusive way.

There are a number of libraries and blog posts describing how to implement a DLM (Distributed Lock Manager) with Redis, but every library uses a different approach, and many use a simple approach with lower guarantees compared to what can be achieved with slightly more complex designs.

This is an implementation of a proposed distributed lock algorithm with Redis. It started as a fork from antirez implementation.

Compatibility

Redlock works with Redis versions 2.6 or later.

Installation

Add this line to your application's Gemfile:

gem 'redlock'

And then execute:

$ bundle

Or install it yourself as:

$ gem install redlock

Documentation

RubyDoc

Usage example

  # Locking
  lock_manager = Redlock::Client.new([ "redis://127.0.0.1:7777", "redis://127.0.0.1:7778", "redis://127.0.0.1:7779" ])
  first_try_lock_info = lock_manager.lock("resource_key", 2000)
  second_try_lock_info = lock_manager.lock("resource_key", 2000)

  # it prints lock info {validity: 1987, resource: "resource_key", value: "generated_uuid4"}
  p first_try_lock_info
  # it prints false
  p second_try_lock_info

  # Unlocking
  lock_manager.unlock(first_try_lock_info)
  second_try_lock_info = lock_manager.lock("resource_key", 2000)

  # now it prints lock info
  p second_try_lock_info

Redlock works seamlessly with redis sentinel, which is supported in redis 3.2+. It also allows clients to set any other arbitrary options on the Redis connection, e.g. password, driver, and more.

servers = [ 'redis://localhost:6379', Redis.new(:url => 'redis://someotherhost:6379') ]
redlock = Redlock::Client.new(servers)

There's also a block version that automatically unlocks the lock:

lock_manager.lock("resource_key", 2000) do |locked|
  if locked
    # critical code
  else
    # error handling
  end
end

There's also a bang version that only executes the block if the lock is successfully acquired, returning the block's value as a result, or raising an exception otherwise:

begin
  block_result = lock_manager.lock!("resource_key", 2000) do
    # critical code
  end
rescue Redlock::LockError
  # error handling
end

To extend the life of the lock:

begin
  block_result = lock_manager.lock!("resource_key", 2000) do |lock_info|
    # critical code
    lock_manager.lock("resource key", 3000, extend: lock_info)
    # more critical code
  end
rescue Redlock::LockError
  # error handling
end

The above code will also acquire the lock if the previous lock has expired and the lock is currently free. Keep in mind that this means the lock could have been acquired by someone else in the meantime. To only extend the life of the lock if currently locked by yourself, use extend_life parameter:

begin
  block_result = lock_manager.lock!("resource_key", 2000) do |lock_info|
    # critical code
    lock_manager.lock("resource key", 3000, extend: lock_info, extend_life: true)
    # more critical code, only if lock was still hold
  end
rescue Redlock::LockError
  # error handling
end

It's possible to customize the retry logic providing the following options:

  lock_manager = Redlock::Client.new(
                  servers, {
                  retry_count:   3,
                  retry_delay:   200, # milliseconds
                  retry_jitter:  50,  # milliseconds
                  retry_timeout: 0.1  # seconds
                 })

For more information you can check documentation

Run tests

Make sure you have at least 1 redis instances up.

$ rspec

Disclaimer

This code implements an algorithm which is currently a proposal, it was not formally analyzed. Make sure to understand how it works before using it in your production environments. You can see discussion about this approach at reddit.

Contributing

1. Fork it
2. Create your feature branch (git checkout -b my-new-feature)
3. Commit your changes (git commit -am 'Add some feature')
4. Push to the branch (git push origin my-new-feature)
5. Create a new Pull Request