Supervisor.Spec

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 c: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 Process.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)

Summary

Types

child_id()

Supported id values

modules()

Supported module values

restart()

Supported restart values

shutdown()

Supported shutdown values

spec()

The supervisor specification

strategy()

Supported strategies

worker()

Supported worker values

Functions

supervise(children, options)

Receives a list of children (workers or supervisors) to supervise and a set of options

supervisor(module, args, options \\ [])

Defines the given module as a supervisor which will be started with the given arguments

worker(module, args, options \\ [])

Defines the given module as a worker which will be started with the given arguments

Types

child_id()

child_id() :: term()

Supported id values

modules()

modules() :: :dynamic | [module()]

Supported module values

restart()

restart() :: :permanent | :transient | :temporary

Supported restart values

shutdown()

shutdown() :: :brutal_kill | :infinity | non_neg_integer()

Supported shutdown values

spec()

spec() :: {child_id(), start_fun :: {module(), atom(), [term()]}, restart(), shutdown(), worker(), modules()}

The supervisor specification

strategy()

strategy() ::
  :simple_one_for_one |
  :one_for_one |
  :one_for_all |
  :rest_for_one

Supported strategies

worker()

worker() :: :worker | :supervisor

Supported worker values

Functions

supervise(children, options)

supervise([spec()], [strategy: strategy(), max_restarts: non_neg_integer(), max_seconds: non_neg_integer()]) :: {:ok, tuple()}

Receives a list of children (workers or supervisors) to supervise and a set of options.

Returns a tuple containing the supervisor specification. This tuple can be used as the return value of the c:init/1 callback when implementing a module-based supervisor.

Examples

supervise(children, strategy: :one_for_one)

Options

• :strategy - the restart strategy option. It can be either :one_for_one, :rest_for_one, :one_for_all, or :simple_one_for_one. You can learn more about strategies in the Supervisor module docs.

• :max_restarts - the maximum amount of restarts allowed in a time frame. Defaults to 3.

• :max_seconds - the time frame in which :max_restarts applies. Defaults to 5.

The :strategy option is required and by default a maximum of 3 restarts is allowed within 5 seconds. Check the Supervisor module for a detailed description of the available strategies.

supervisor(module, args, options \\ [])

supervisor(module(), [term()], [restart: restart(), shutdown: shutdown(), id: term(), function: atom(), modules: modules()]) :: spec()

Defines the given module as a supervisor which will be started with the given arguments.

supervisor(ExUnit.Runner, [], restart: :permanent)

By default, the function start_link is invoked on the given module. Overall, the default values for the options are:

[id: module,
 function: :start_link,
 restart: :permanent,
 shutdown: :infinity,
 modules: [module]]

Check the documentation for the Supervisor.Spec module for more information on the options.

worker(module, args, options \\ [])

worker(module(), [term()], [restart: restart(), shutdown: shutdown(), id: term(), function: atom(), modules: modules()]) :: spec()

Defines the given module as a worker which will be started with the given arguments.

worker(ExUnit.Runner, [], restart: :permanent)

By default, the function start_link is invoked on the given module. Overall, the default values for the options are:

[id: module,
 function: :start_link,
 restart: :permanent,
 shutdown: 5000,
 modules: [module]]

Check the documentation for the Supervisor.Spec module for more information on the options.