Supervisor behaviour

A behaviour module for implementing supervision functionality.

A supervisor is a process which supervises other processes, which we refer to as child processes. Supervisors are used to build a hierarchical process structure called a supervision tree. Supervision trees are a nice way to structure fault-tolerant applications.

A supervisor implemented using this module has a standard set of interface functions and includes functionality for tracing and error reporting. It also fits into a supervision tree.

Examples

In order to define a supervisor, we need to first define a child process that is going to be supervised. In order to do so, we will define a GenServer that represents a stack:

defmodule Stack do
  use GenServer

  def start_link(state, opts \\ []) do
    GenServer.start_link(__MODULE__, state, opts)
  end

  def handle_call(:pop, _from, [h | t]) do
    {:reply, h, t}
  end

  def handle_cast({:push, h}, t) do
    {:noreply, [h | t]}
  end
end

We can now define our supervisor and start it as follows:

# Import helpers for defining supervisors
import Supervisor.Spec

# Supervise the Stack server which will be started with
# two arguments. The initial stack, [:hello], and a
# keyword list containing the GenServer options that
# set the registered name of the server to MyStack.
children = [
  worker(Stack, [[:hello], [name: MyStack]])
]

# Start the supervisor with our child
{:ok, pid} = Supervisor.start_link(children, strategy: :one_for_one)

# There is one child worker started
Supervisor.count_children(pid)
#=> %{active: 1, specs: 1, supervisors: 0, workers: 1}

Notice that when starting the GenServer, we are registering it with name MyStack, which allows us to call it directly and get what is on the stack:

GenServer.call(MyStack, :pop)
#=> :hello

GenServer.cast(MyStack, {:push, :world})
#=> :ok

GenServer.call(MyStack, :pop)
#=> :world

However, there is a bug in our stack server. If we call :pop and the stack is empty, it is going to crash because no clause matches:

GenServer.call(MyStack, :pop)
** (exit) exited in: GenServer.call(MyStack, :pop, 5000)

Luckily, since the server is being supervised by a supervisor, the supervisor will automatically start a new one, with the initial stack of [:hello]:

GenServer.call(MyStack, :pop)
#=> :hello

Supervisors support different strategies; in the example above, we have chosen :one_for_one. Furthermore, each supervisor can have many workers and supervisors as children, each of them with their specific configuration, shutdown values, and restart strategies.

The rest of this documentation will cover supervision strategies; also read the documentation for the Supervisor.Spec module to learn about the specification for workers and supervisors.

Module-based supervisors

In the example above, a supervisor was started by passing the supervision structure to start_link/2. However, supervisors can also be created by explicitly defining a supervision module:

defmodule MyApp.Supervisor do
  # Automatically imports Supervisor.Spec
  use Supervisor

  def start_link do
    Supervisor.start_link(__MODULE__, [])
  end

  def init([]) do
    children = [
      worker(Stack, [[:hello]])
    ]

    # supervise/2 is imported from Supervisor.Spec
    supervise(children, strategy: :one_for_one)
  end
end

You may want to use a module-based supervisor if:

You need to perform some particular action on supervisor initialization, like setting up an ETS table.
You want to perform partial hot-code swapping of the tree. For example, if you add or remove children, the module-based supervision will add and remove the new children directly, while dynamic supervision requires the whole tree to be restarted in order to perform such swaps.

Strategies

Supervisors support different supervision strategies (through the :strategy option, as seen above):

:one_for_one - if a child process terminates, only that process is restarted.
:one_for_all - if a child process terminates, all other child processes are terminated and then all child processes (including the terminated one) are restarted.
:rest_for_one - if a child process terminates, the “rest” of the child processes, i.e., the child processes after the terminated one in start order, are terminated. Then the terminated child process and the rest of the child processes are restarted.
:simple_one_for_one - similar to :one_for_one but suits better when dynamically attaching children. This strategy requires the supervisor specification to contain only one child. Many functions in this module behave slightly differently when this strategy is used.

Simple one for one

The :simple_one_for_one supervisor is useful when you want to dynamically start and stop supervised children. For example, imagine you want to dynamically create multiple stacks. We can do so by defining a :simple_one_for_one supervisor:

# Import helpers for defining supervisors
import Supervisor.Spec

# This time, we don't pass any argument because
# the argument will be given when we start the child
children = [
  worker(Stack, [], restart: :transient)
]

# Start the supervisor with our one child as a template
{:ok, sup_pid} = Supervisor.start_link(children, strategy: :simple_one_for_one)

# No child worker is active yet until start_child is called
Supervisor.count_children(sup_pid)
#=> %{active: 0, specs: 1, supervisors: 0, workers: 0}

There are a couple differences here:

the simple one for one specification can define only one child which works as a template for when we call start_child/2
we have defined the child to have a restart strategy of :transient. This means that, if the child process exits due to a :normal, :shutdown, or {:shutdown, term} reason, it won’t be restarted. This is useful as it allows our workers to politely shutdown and be removed from the :simple_one_for_one supervisor, without being restarted. You can find more information about restart strategies in the documentation for the Supervisor.Spec module

With the supervisor defined, let’s dynamically start stacks:

{:ok, pid} = Supervisor.start_child(sup_pid, [[:hello, :world], []])
GenServer.call(pid, :pop) #=> :hello
GenServer.call(pid, :pop) #=> :world

{:ok, pid} = Supervisor.start_child(sup_pid, [[:something, :else], []])
GenServer.call(pid, :pop) #=> :something
GenServer.call(pid, :pop) #=> :else

Supervisor.count_children(sup_pid)
#=> %{active: 2, specs: 1, supervisors: 0, workers: 2}

Exit reasons

From the example above, you may have noticed that the :transient restart strategy for the worker does not restart the child in case it exits with reason :normal, :shutdown or {:shutdown, term}.

So one may ask: which exit reason should I choose when exiting my worker? There are three options:

:normal - in such cases, the exit won’t be logged, there is no restart in transient mode, and linked processes do not exit
:shutdown or {:shutdown, term} - in such cases, the exit won’t be logged, there is no restart in transient mode, and linked processes exit with the same reason unless they’re trapping exits
any other term - in such cases, the exit will be logged, there are restarts in transient mode, and linked processes exit with the same reason unless they’re trapping exits

Name registration

A supervisor is bound to the same name registration rules as a GenServer. Read more about these rules in the documentation for GenServer.

Summary

Types

child()
name(): The Supervisor name
on_start(): Return values of start_link functions
on_start_child(): Return values of start_child functions
options(): Options used by the start* functions
supervisor(): The supervisor reference

Functions

count_children(supervisor): Returns a map containing count values for the given supervisor
delete_child(supervisor, child_id): Deletes the child specification identified by child_id
restart_child(supervisor, child_id): Restarts a child process identified by child_id
start_child(supervisor, child_spec_or_args): Dynamically adds a child specification to supervisor and starts that child
start_link(children, options): Starts a supervisor with the given children
start_link(module, arg, options \\ []): Starts a supervisor process with the given module and arg
stop(supervisor, reason \\ :normal, timeout \\ :infinity): Stops the given supervisor with the given reason
terminate_child(supervisor, pid_or_child_id): Terminates the given children, identified by PID or child id
which_children(supervisor): Returns a list with information about all children of the given supervisor

Callbacks

init(args): Callback invoked to start the supervisor and during hot code upgrades

Types

child()

child() :: pid() | :undefined

name()

name() :: atom() | {:global, term()} | {:via, module(), term()}

The Supervisor name

on_start()

on_start() ::
  {:ok, pid()} |
  :ignore |
  {:error, {:already_started, pid()} | {:shutdown, term()} | term()}

Return values of start_link functions

on_start_child()

on_start_child() ::
  {:ok, child()} |
  {:ok, child(), info :: term()} |
  {:error, {:already_started, child()} | :already_present | term()}

Return values of start_child functions

options()

options() :: [name: name(), strategy: Supervisor.Spec.strategy(), max_restarts: non_neg_integer(), max_seconds: non_neg_integer()]

Options used by the start* functions

supervisor()

supervisor() :: pid() | name() | {atom(), node()}

The supervisor reference

Functions

count_children(supervisor)

count_children(supervisor()) :: %{specs: non_neg_integer(), active: non_neg_integer(), supervisors: non_neg_integer(), workers: non_neg_integer()}

Returns a map containing count values for the given supervisor.

The map contains the following keys:

:specs - the total count of children, dead or alive
:active - the count of all actively running child processes managed by this supervisor
:supervisors - the count of all supervisors whether or not these child supervisors are still alive
:workers - the count of all workers, whether or not these child workers are still alive

delete_child(supervisor, child_id)

delete_child(supervisor(), Supervisor.Spec.child_id()) ::
  :ok |
  {:error, error} when error: :not_found | :simple_one_for_one | :running | :restarting

Deletes the child specification identified by child_id.

The corresponding child process must not be running; use terminate_child/2 to terminate it if it’s running.

If successful, this function returns :ok. This function may return an error with an appropriate error tuple if the child_id is not found, or if the current process is running or being restarted.

This operation is not supported by :simple_one_for_one supervisors.

restart_child(supervisor, child_id)

restart_child(supervisor(), Supervisor.Spec.child_id()) ::
  {:ok, child()} |
  {:ok, child(), term()} |
  {:error, error} when error: :not_found | :simple_one_for_one | :running | :restarting | term()

Restarts a child process identified by child_id.

The child specification must exist and the corresponding child process must not be running.

Note that for temporary children, the child specification is automatically deleted when the child terminates, and thus it is not possible to restart such children.

If the child process start function returns {:ok, child} or {:ok, child, info}, the PID is added to the supervisor and this function returns the same value.

If the child process start function returns :ignore, the PID remains set to :undefined and this function returns {:ok, :undefined}.

This function may return an error with an appropriate error tuple if the child_id is not found, or if the current process is running or being restarted.

If the child process start function returns an error tuple or an erroneous value, or if it fails, this function returns {:error, error}.

This operation is not supported by :simple_one_for_one supervisors.

start_child(supervisor, child_spec_or_args)

start_child(supervisor(), Supervisor.Spec.spec() | [term()]) :: on_start_child()

Dynamically adds a child specification to supervisor and starts that child.

child_spec should be a valid child specification (unless the supervisor is a :simple_one_for_one supervisor, see below). The child process will be started as defined in the child specification.

In the case of :simple_one_for_one, the child specification defined in the supervisor is used and instead of a child_spec, an arbitrary list of terms is expected. The child process will then be started by appending the given list to the existing function arguments in the child specification.

If a child specification with the specified id already exists, child_spec is discarded and this function returns an error with :already_started or :already_present if the corresponding child process is running or not, respectively.

If the child process start function returns {:ok, child} or {:ok, child, info}, then child specification and PID are added to the supervisor and this function returns the same value.

If the child process start function returns :ignore, the child specification is added to the supervisor, the PID is set to :undefined and this function returns {:ok, :undefined}.

If the child process start function returns an error tuple or an erroneous value, or if it fails, the child specification is discarded and this function returns {:error, error} where error is a term containing information about the error and child specification.

start_link(children, options)

start_link([Supervisor.Spec.spec()], options()) :: on_start()

start_link(module(), term()) :: on_start()

Starts a supervisor with the given children.

A strategy is required to be provided through the :strategy option. Furthermore, the :max_restarts and :max_seconds options can be configured as described in the documentation for Supervisor.Spec.supervise/2.

The options can also be used to register a supervisor name. The supported values are described under the “Name registration” section in the GenServer module docs.

If the supervisor and its child processes are successfully created (i.e., if the start function of each child process returns {:ok, child}, {:ok, child, info}, or :ignore) this function returns {:ok, pid}, where pid is the PID of the supervisor. If a process with the specified name already exists, the function returns {:error, {:already_started, pid}}, where pid is the PID of that process.

If the start function of any of the child processes fails or returns an error tuple or an erroneous value, the supervisor first terminates with reason :shutdown all the child processes that have already been started, and then terminates itself and returns {:error, {:shutdown, reason}}.

Note that a supervisor started with this function is linked to the parent process and exits not only on crashes but also if the parent process exits with :normal reason.

start_link(module, arg, options \\ [])

start_link(module(), term(), options()) :: on_start()

Starts a supervisor process with the given module and arg.

To start the supervisor, the init/1 callback will be invoked in the given module, with arg as its argument. The init/1 callback must return a supervisor specification which can be created with the help of the functions in the Supervisor.Spec module (especially Supervisor.Spec.supervise/2).

If the init/1 callback returns :ignore, this function returns :ignore as well and the supervisor terminates with reason :normal. If it fails or returns an incorrect value, this function returns {:error, term} where term is a term with information about the error, and the supervisor terminates with reason term.

The :name option can also be given in order to register a supervisor name, the supported values are described in the “Name registration” section in the GenServer module docs.

stop(supervisor, reason \\ :normal, timeout \\ :infinity)

stop(supervisor(), reason :: term(), timeout()) :: :ok

Stops the given supervisor with the given reason.

It returns :ok if the supervisor terminates with the given reason. If it terminates with another reason, the call exits.

This function keeps OTP semantics regarding error reporting. If the reason is any other than :normal, :shutdown or {:shutdown, _}, an error report is logged.

terminate_child(supervisor, pid_or_child_id)

terminate_child(supervisor(), pid() | Supervisor.Spec.child_id()) ::
  :ok |
  {:error, error} when error: :not_found | :simple_one_for_one

Terminates the given children, identified by PID or child id.

If the supervisor is not a :simple_one_for_one, the child id is expected and the process, if there’s one, is terminated; the child specification is kept unless the child is temporary.

In case of a :simple_one_for_one supervisor, a PID is expected. If the child specification identifier is given instead of a pid, this function returns {:error, :simple_one_for_one}.

A non-temporary child process may later be restarted by the supervisor. The child process can also be restarted explicitly by calling restart_child/2. Use delete_child/2 to remove the child specification.

If successful, this function returns :ok. If there is no child specification for the given child id or there is no process with the given PID, this function returns {:error, :not_found}.

which_children(supervisor)

which_children(supervisor()) :: [{Supervisor.Spec.child_id() | :undefined, child() | :restarting, Supervisor.Spec.worker(), Supervisor.Spec.modules()}]

Returns a list with information about all children of the given supervisor.

Note that calling this function when supervising a large number of children under low memory conditions can cause an out of memory exception.

This function returns a list of {id, child, type, modules} tuples, where:

id - as defined in the child specification or :undefined in the case of a simple_one_for_one supervisor
child - the PID of the corresponding child process, :restarting if the process is about to be restarted, or :undefined if there is no such process
type - :worker or :supervisor, as specified by the child specification
modules - as specified by the child specification

Callbacks

init(args)

init(args :: term()) ::
  {:ok, {:supervisor.sup_flags(), [Supervisor.Spec.spec()]}} |
  :ignore

Callback invoked to start the supervisor and during hot code upgrades.