Convenience functions for defining supervisor specifications.

Example

By using the functions in this module one can specify the children to be used under a supervisor, started with Supervisor.start_link/2:

import Supervisor.Spec

children = [
  worker(MyWorker, [arg1, arg2, arg3]),
  supervisor(MySupervisor, [arg1])
]

Supervisor.start_link(children, strategy: :one_for_one)

Sometimes, it may be handy to define supervisors backed by a module:

defmodule MySupervisor do
  use Supervisor

  def start_link(arg) do
    Supervisor.start_link(__MODULE__, arg)
  end

  def init(arg) do
    children = [
      worker(MyWorker, [arg], restart: :temporary)
    ]

    supervise(children, strategy: :simple_one_for_one)
  end
end

Notice in this case we don’t have to explicitly import Supervisor.Spec as use Supervisor automatically does so. Defining a module-based supervisor can be useful, for example, to perform initialization tasks in the init/1 callback.

Supervisor and worker options

In the example above, we defined specs for workers and supervisors. These specs (both for workers as well as supervisors) accept the following options:

• :id - a name used to identify the child specification internally by the supervisor; defaults to the given module name for the child worker/supervisor

• :function - the function to invoke on the child to start it

• :restart - an atom that defines when a terminated child process should be restarted (see the “Restart values” section below)

• :shutdown - an atom that defines how a child process should be terminated (see the “Shutdown values” section below)

• :modules - it should be a list with one element [module], where module is the name of the callback module only if the child process is a Supervisor or GenServer; if the child process is a GenEvent, :modules should be :dynamic

Restart values (:restart)

The following restart values are supported in the :restart option:

• :permanent - the child process is always restarted

• :temporary - the child process is never restarted (not even when the supervisor’s strategy is :rest_for_one or :one_for_all)

• :transient - the child process is restarted only if it terminates abnormally, i.e., with an exit reason other than :normal, :shutdown or {:shutdown, term}

Shutdown values (:shutdown)

The following shutdown values are supported in the :shutdown option:

• :brutal_kill - the child process is unconditionally terminated using exit(child, :kill)

• :infinity - if the child process is a supervisor, this is a mechanism to give the subtree enough time to shutdown; it can also be used with workers with care

• any integer - the value of :shutdown can also be any integer meaning that the supervisor tells the child process to terminate by calling Process.exit(child, :shutdown) and then waits for an exit signal back. If no exit signal is received within the specified time (the value of this option, in milliseconds), the child process is unconditionally terminated using Process.exit(child, :kill)