This module defines a supervisor process that is specialized to manage pool of workers.

• Process defined by this module behaves as a :simple_one_for_one supervisor.
• Worker processes are spawned using a callback module that implements PoolSup.Worker behaviour.
• The PoolSup process manages which child processes are in use and which are not.
• Functions to request pid of a child process that is not in use are also defined.

Example

Suppose we have a module that implements both GenServer and PoolSup.Worker behaviours.

iex(1)> defmodule MyWorker do
...(1)>   @behaviour PoolSup.Worker
...(1)>   use GenServer
...(1)>   def start_link(arg) do
...(1)>     GenServer.start_link(__MODULE__, arg)
...(1)>   end
...(1)>   # definitions of gen_server callbacks...
...(1)> end

When we want to have 3 processes that run MyWorker server:

iex(2)> {:ok, pid} = PoolSup.start_link(MyWorker, {:worker, :arg}, 3, [name: :my_pool])

Each child process is started by MyWorker.start_link({:worker, :arg}). Then we can get a pid of a child currently not in use.

iex(3)> child_pid = PoolSup.checkout(:my_pool)
iex(4)> do_something(child_pid)
iex(5)> PoolSup.checkin(:my_pool, child_pid)

Don’t forget to return the child_pid when finished; for simple use cases PoolSup.transaction/3 comes in handy.

Usage within supervision tree

The following code snippet spawns a supervisor that has PoolSup process as one of its children. The PoolSup process manages 5 worker processes and they will be started by MyWorker.start_link({:worker, :arg}).

chilldren = [
  ...
  Supervisor.Spec.supervisor(PoolSup, [MyWorker, {:worker, :arg}, 5]),
  ...
]
Supervisor.start_link(children, [strategy: :one_for_one])