Convenience functions for defining a supervision specification.

Example

By using the functions in this module one can define a supervisor and start it 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)

In many situations, 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.

Explicit supervisors as above are required when there is a need to:

1. Partially change the supervision tree during hot-code swaps.

2. Define supervisors inside other supervisors.

3. Perform actions inside the supervision init/1 callback.

   For example, you may want to start an ETS table that is linked to the supervisor (i.e. if the supervision tree needs to be restarted, the ETS table must be restarted too).

Supervisor and worker options

In the example above, we defined workers and supervisors and each accepts the following options:

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

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

• :restart - defines when a terminated child process should be restarted

• :shutdown - defines how a child process should be terminated

• :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:

• :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 another exit reason than :normal, :shutdown or {:shutdown, term}

Shutdown values (:shutdown)

The following shutdown values are supported:

• :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.

• Finally, the value 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 (in milliseconds), the child process is unconditionally terminated using Process.exit(child, :kill).