Locker

A quote from https://github.com/wooga/locker:

locker is a distributed de-centralized consistent in-memory key-value store written in Erlang. An entry expires after a certain amount of time, unless the lease is extended. This makes it a good practical option for locks, mutexes and leader election in a distributed system.

Lockeris a Elixir wrapper for the locker Erlang library that provides some useful libraries that should make using locker a bit easier.

The Locker module implements an OTP application that runs locker. In addition to that is also provides a wrapper for the locker API, just for convenience.

Idea of Locker is to provide an abstraction for globally registering GenServer or :gen_fsm processes to :locker key-value store in a cluster of Erlang servers. These are implemented in Locker.Server and Locker.Fsm modules. In the simplest case, all you have to do is replace use GenServer with use Locker.Server and your process is automatically registered to global :locker cluster.

Installing

You can install Locker by adding it as a dependency to your project's mix.exs file:

defp deps do
  [
    {:elixir_locker, "~> 0.1.4"}
  ]
end

Also, remember to add :elixir_locker to your :applications list if you wish that the Locker application is started automatically.

Examples

Globally registered processes

You can use Locker as a global process registry. The Locker.Registry module handles the API for registering processes in a way that GenServer or :gen_fsm can use it directly. Here is an example of an GenServer process that will be registered to Locker on startup:

defmodule MyServer do
  use Locker.Server, lease_length: 60000
  
  def init(_) do
    {:ok, %{}}
  end
  
  def handle_info(_info, state) do
    {:noreply, state}
  end
  
end

You can start your process with name "my_server_process" the way you would start any GenServer process. For example, without supervision:

iex> MyServer.start([], name: "my_server_process")
{:ok, #PID<0.166.0>}

Once the process has been started, you can query the process id using Locker.Registry like this:

iex> Locker.Registry.whereis_name("my_server_process")
#PID<0.166.0>

If Locker cluster has been properly configured, you can query the process name from any node on your cluster. What Locker.Server does is that it updates the lease on the registered process name within the given :lease_length. If the lease expires, the process cannot be found from the registry anymore. Also, Locker.Server releases the registered name on terminate/2.

Locking resources

It should not come as a surprise that you can use Locker to lock resources globally. For this, you can use Locker module that provides a direct mapping to the :locker Erlang module. Naturally, you can also use :locker directly. Here is how you can create a lock that expires in one minute and wait that the lock gets released.

iex(1)> Locker.lock("my_lock", self, 60000)
{:ok, 1, 1, 1}
iex(2)> Locker.wait_for_release("my_lock", 2 * 60000)
{:ok, :released}

So what we did here was that we registered a lock called "my_lock" and set the value to our own process id and timeout to 60000 milliseconds, i.e. one minute. Then we called wait_for_release that will block until the lock is released.