Programming in Erlang and Elixir usually allows for no locking since the VM essentially handles it for you when communicating between processes. However, what about the situation when you have thousands of processes attempting to interact with a single resource such as a process? Usually they will overload the process and explode the message queue. ETS is the Swiss Army knife of the Erlang VM and can be applied to this problem. By using :ets.update_counter and :write_concurrency we can achieve a fast low contention semaphore on ETS.

Add it to mix.exs

defp deps do
  [{:semaphore, "~> 1.1"}]
end

Then just use it like a semaphore in any other language.

if Semaphore.acquire(:test, 1) do
  IO.puts "acquired"
  Semaphore.release(:test)
end

case Semaphore.call(:test, 1, fn -> :ok end) do
  :ok ->
    IO.puts "success"
  {:error, :max} ->
    IO.puts "too many callers"
end

Semaphore is released under the MIT License. Check LICENSE file for more information.