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.

Redlock works with Redis versions 2.6 or later.

Add this line to your application's Gemfile:

gem 'redlock'

And then execute:

$ bundle

Or install it yourself as:

$ gem install redlock

RubyDoc

  # 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

Make sure you have at least 1 redis instances up.

$ rspec

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.

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