View Source Supervisor behaviour (Elixir v1.15.0-dev)

A behaviour module for implementing supervisors.

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 provide fault-tolerance and encapsulate how our applications start and shutdown.

A supervisor may be started directly with a list of child specifications via start_link/2 or you may define a module-based supervisor that implements the required callbacks. The sections below use start_link/2 to start supervisors in most examples, but it also includes a specific section on module-based ones.

examples

Examples

In order to start a supervisor, we need to first define a child process that will be supervised. As an example, we will define a GenServer, a generic server, that keeps a counter. Other processes can then send messages to this process to read the counter and bump its value.

Note: in practice you would not define a counter as a GenServer. Instead, if you need a counter, you would pass it around as inputs and outputs to the functions that need it. The reason we picked a counter in this example is due to its simplicity, as it allows us to focus on how supervisors work.

defmodule Counter do
  use GenServer

  def start_link(arg) when is_integer(arg) do
    GenServer.start_link(__MODULE__, arg, name: __MODULE__)
  end

  ## Callbacks

  @impl true
  def init(counter) do
    {:ok, counter}
  end

  @impl true
  def handle_call(:get, _from, counter) do
    {:reply, counter, counter}
  end

  def handle_call({:bump, value}, _from, counter) do
    {:reply, counter, counter + value}
  end
end

The Counter receives an argument on start_link. This argument is passed to the init/1 callback which becomes the initial value of the counter. Our counter handles two operations (known as calls): :get, to get the current counter value, and :bump, that bumps the counter by the given value and returns the old counter.

We can now start a supervisor that will start and supervise our counter process. The first step is to define a list of child specifications that control how each child behaves. Each child specification is a map, as shown below:

children = [
  # The Counter is a child started via Counter.start_link(0)
  %{
    id: Counter,
    start: {Counter, :start_link, [0]}
  }
]

# Now we start the supervisor with the children and a strategy
{:ok, pid} = Supervisor.start_link(children, strategy: :one_for_one)

# After started, we can query the supervisor for information
Supervisor.count_children(pid)
#=> %{active: 1, specs: 1, supervisors: 0, workers: 1}

Note that when starting the GenServer, we are registering it with name Counter via the name: __MODULE__ option. This allows us to call it directly and get its value:

GenServer.call(Counter, :get)
#=> 0

GenServer.cast(Counter, {:bump, 3})
#=> 0

GenServer.call(Counter, :get)
#=> 3

However, there is a bug in our counter server. If we call :bump with a non-numeric value, it is going to crash:

GenServer.call(Counter, {:bump, "oops"})
** (exit) exited in: GenServer.call(Counter, {:bump, "oops"}, 5000)

Luckily, since the server is being supervised by a supervisor, the supervisor will automatically start a new one, reset back to its initial value of 0:

GenServer.call(Counter, :get)
#=> 0

Supervisors support different strategies; in the example above, we have chosen :one_for_one. Furthermore, each supervisor can have many workers and/or supervisors as children, with each one having its own configuration (as outlined in the "Child specification" section).

The rest of this document will cover how child processes are specified, how they can be started and stopped, different supervision strategies and more.

child-specification

Child specification

The child specification describes how the supervisor starts, shuts down, and restarts child processes.

The child specification is a map containing up to 6 elements. The first two keys in the following list are required, and the remaining ones are optional:

  • :id - any term used to identify the child specification internally by the supervisor; defaults to the given module. This key is required. For supervisors, in the case of conflicting :id values, the supervisor will refuse to initialize and require explicit IDs. This is not the case for dynamic supervisors though.

  • :start - a tuple with the module-function-args to be invoked to start the child process. This key is required.

  • :restart - an atom that defines when a terminated child process should be restarted (see the "Restart values" section below). This key is optional and defaults to :permanent.

  • :shutdown - an integer or atom that defines how a child process should be terminated (see the "Shutdown values" section below). This key is optional and defaults to 5_000 if the type is :worker or :infinity if the type is :supervisor.

  • :type - specifies that the child process is a :worker or a :supervisor. This key is optional and defaults to :worker.

  • :modules - a list of modules used by hot code upgrade mechanisms to determine which processes are using certain modules. It is typically set to the callback module of behaviours like GenServer, Supervisor, and such. It is set automatically based on the :start value and it is rarely changed in practice.

Let's understand what the :shutdown and :restart options control.

shutdown-values-shutdown

Shutdown values (:shutdown)

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

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

  • any integer >= 0 - the amount of time in milliseconds that the supervisor will wait for its children to terminate after emitting a Process.exit(child, :shutdown) signal. If the child process is not trapping exits, the initial :shutdown signal will terminate the child process immediately. If the child process is trapping exits, it has the given amount of time to terminate. If it doesn't terminate within the specified time, the child process is unconditionally terminated by the supervisor via Process.exit(child, :kill).

  • :infinity - works as an integer except the supervisor will wait indefinitely for the child to terminate. If the child process is a supervisor, the recommended value is :infinity to give the supervisor and its children enough time to shut down. This option can be used with regular workers but doing so is discouraged and requires extreme care. If not used carefully, the child process will never terminate, preventing your application from terminating as well.

restart-values-restart

Restart values (:restart)

The :restart option controls what the supervisor should consider to be a successful termination or not. If the termination is successful, the supervisor won't restart the child. If the child process crashed, the supervisor will start a new one.

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

  • :permanent - the child process is always restarted.

  • :temporary - the child process is never restarted, regardless of the supervision strategy: any termination (even abnormal) is considered successful.

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

For a more complete understanding of the exit reasons and their impact, see the "Exit reasons and restarts" section.

child_spec-1-function

child_spec/1 function

When starting a supervisor, we may pass a list of child specifications. Those specifications are maps that tell how the supervisor should start, stop and restart each of its children:

%{
  id: Counter,
  start: {Counter, :start_link, [0]}
}

The map above defines a child with :id of Counter that is started by calling Counter.start_link(0).

However, defining the child specification for each child as a map can be quite error prone, as we may change the Counter implementation and forget to update its specification. That's why Elixir allows you to pass a tuple with the module name and the start_link argument instead of the specification:

children = [
  {Counter, 0}
]

The supervisor will then invoke Counter.child_spec(0) to retrieve a child specification. Now the Counter module is responsible for building its own specification, for example, we could write:

def child_spec(arg) do
  %{
    id: Counter,
    start: {Counter, :start_link, [arg]}
  }
end

Luckily for us, use GenServer already defines a Counter.child_spec/1 exactly like above, so you don't need to write the definition above yourself. If you want to customize the automatically generated child_spec/1 function, you can pass the options directly to use GenServer:

use GenServer, restart: :transient

Finally, note it is also possible to simply pass the Counter module as a child:

children = [
  Counter
]

When only the module name is given, it is equivalent to {Counter, []}, which in our case would be invalid, which is why we always pass the initial counter explicitly.

By replacing the child specification with {Counter, 0}, we keep it encapsulated in the Counter module. We could now share our Counter implementation with other developers and they can add it directly to their supervision tree without worrying about the low-level details of the counter.

Overall, a child specification can be one of the following:

  • a map representing the child specification itself - as outlined in the "Child specification" section

  • a tuple with a module as first element and the start argument as second - such as {Counter, 0}. In this case, Counter.child_spec(0) is called to retrieve the child specification

  • a module - such as Counter. In this case, Counter.child_spec([]) would be called, which is invalid for the counter, but it is useful in many other cases, especially when you want to pass a list of options to the child process

If you need to convert a {module, arg} tuple or a module child specification to a child specification or modify a child specification itself, you can use the Supervisor.child_spec/2 function. For example, to run the counter with a different :id and a :shutdown value of 10 seconds (10_000 milliseconds):

children = [
  Supervisor.child_spec({Counter, 0}, id: MyCounter, shutdown: 10_000)
]

supervisor-strategies-and-options

Supervisor strategies and options

So far we have started the supervisor passing a single child as a tuple as well as a strategy called :one_for_one:

children = [
  {Counter, 0}
]

Supervisor.start_link(children, strategy: :one_for_one)

The first argument given to start_link/2 is a list of child specifications as defined in the "child_spec/1" section above.

The second argument is a keyword list of options:

  • :strategy - the supervision strategy option. It can be either :one_for_one, :rest_for_one or :one_for_all. Required. See the "Strategies" section.

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

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

  • :name - a name to register the supervisor process. Supported values are explained in the "Name registration" section in the documentation for GenServer. Optional.

strategies

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 terminated child process and the rest of the children started after it, are terminated and restarted.

In the above, process termination refers to unsuccessful termination, which is determined by the :restart option.

To efficiently supervise children started dynamically, see DynamicSupervisor.

name-registration

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.

module-based-supervisors

Module-based supervisors

In the example so far, the 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 defines child_spec/1
  use Supervisor

  def start_link(init_arg) do
    Supervisor.start_link(__MODULE__, init_arg, name: __MODULE__)
  end

  @impl true
  def init(_init_arg) do
    children = [
      {Counter, 0}
    ]

    Supervisor.init(children, strategy: :one_for_one)
  end
end

The difference between the two approaches is that a module-based supervisor gives you more direct control over how the supervisor is initialized. Instead of calling Supervisor.start_link/2 with a list of child specifications that are automatically initialized, we manually initialize the children by calling Supervisor.init/2 inside its init/1 callback. Supervisor.init/2 accepts the same :strategy, :max_restarts, and :max_seconds options as start_link/2.

use Supervisor also defines a child_spec/1 function which allows us to run MyApp.Supervisor as a child of another supervisor or at the top of your supervision tree as:

children = [
  MyApp.Supervisor
]

Supervisor.start_link(children, strategy: :one_for_one)

A general guideline is to use the supervisor without a callback module only at the top of your supervision tree, generally in the Application.start/2 callback. We recommend using module-based supervisors for any other supervisor in your application, so they can run as a child of another supervisor in the tree. The child_spec/1 generated automatically by Supervisor can be customized with the following options:

  • :id - the child specification identifier, defaults to the current module
  • :restart - when the supervisor should be restarted, defaults to :permanent

The @doc annotation immediately preceding use Supervisor will be attached to the generated child_spec/1 function.

start-and-shutdown

Start and shutdown

When the supervisor starts, it traverses all child specifications and then starts each child in the order they are defined. This is done by calling the function defined under the :start key in the child specification and typically defaults to start_link/1.

The start_link/1 (or a custom) is then called for each child process. The start_link/1 function must return {:ok, pid} where pid is the process identifier of a new process that is linked to the supervisor. The child process usually starts its work by executing the init/1 callback. Generally speaking, the init callback is where we initialize and configure the child process.

The shutdown process happens in reverse order.

When a supervisor shuts down, it terminates all children in the opposite order they are listed. The termination happens by sending a shutdown exit signal, via Process.exit(child_pid, :shutdown), to the child process and then awaiting for a time interval for the child process to terminate. This interval defaults to 5000 milliseconds. If the child process does not terminate in this interval, the supervisor abruptly terminates the child with reason :kill. The shutdown time can be configured in the child specification which is fully detailed in the next section.

If the child process is not trapping exits, it will shutdown immediately when it receives the first exit signal. If the child process is trapping exits, then the terminate callback is invoked, and the child process must terminate in a reasonable time interval before being abruptly terminated by the supervisor.

In other words, if it is important that a process cleans after itself when your application or the supervision tree is shutting down, then this process must trap exits and its child specification should specify the proper :shutdown value, ensuring it terminates within a reasonable interval.

exit-reasons-and-restarts

Exit reasons and restarts

A supervisor restarts a child process depending on its :restart configuration. For example, when :restart is set to :transient, the supervisor 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? 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

Note that the supervisor that reaches maximum restart intensity will exit with :shutdown reason. In this case the supervisor will only be restarted if its child specification was defined with the :restart option set to :permanent (the default).

Link to this section Summary

Types

A child process.

The supervisor child specification.

Options given to start_link/2 and init/2

The supervisor name

Return values of start_link functions

Return values of start_child functions

Option values used by the start* functions

Supported restart options

Supported shutdown options

Supported strategies

The supervisor flags returned on init

The supervisor reference

Supervisor type.

Callbacks

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

Functions

Builds and overrides a child specification.

Returns a map containing count values for the given supervisor.

Deletes the child specification identified by child_id.

Receives a list of child specifications to initialize and a set of options.

Restarts a child process identified by child_id.

Adds a child specification to supervisor and starts that child.

Starts a supervisor with the given children.

Starts a module-based supervisor process with the given module and init_arg.

Synchronously stops the given supervisor with the given reason.

Terminates the given child identified by child_id.

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

Link to this section Types

@type child() :: pid() | :undefined

A child process.

It can be a PID when the child process was started, or :undefined when the child was created by a dynamic supervisor.

@type child_spec() :: %{
  :id => atom() | term(),
  :start => {module(), function_name :: atom(), args :: [term()]},
  optional(:restart) => restart(),
  optional(:shutdown) => shutdown(),
  optional(:type) => type(),
  optional(:modules) => [module()] | :dynamic
}

The supervisor child specification.

It defines how the supervisor should start, stop and restart each of its children.

@type init_option() ::
  {:strategy, strategy()}
  | {:max_restarts, non_neg_integer()}
  | {:max_seconds, pos_integer()}

Options given to start_link/2 and init/2

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

The supervisor name

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

Return values of start_link functions

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

Return values of start_child functions

@type option() :: {:name, name()}

Option values used by the start* functions

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

Supported restart options

@type shutdown() :: timeout() | :brutal_kill

Supported shutdown options

@type strategy() :: :one_for_one | :one_for_all | :rest_for_one

Supported strategies

@type sup_flags() :: %{
  strategy: strategy(),
  intensity: non_neg_integer(),
  period: pos_integer()
}

The supervisor flags returned on init

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

The supervisor reference

@type type() :: :worker | :supervisor

Supervisor type.

Whether the supervisor is a worker or a supervisor.

Link to this section Callbacks

@callback init(init_arg :: term()) ::
  {:ok,
   {sup_flags(),
    [child_spec() | (old_erlang_child_spec :: :supervisor.child_spec())]}}
  | :ignore

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

Developers typically invoke Supervisor.init/2 at the end of their init callback to return the proper supervision flags.

Link to this section Functions

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child_spec(module_or_map, overrides)

View Source
@spec child_spec(
  child_spec() | {module(), arg :: term()} | module(),
  keyword()
) :: child_spec()

Builds and overrides a child specification.

Similar to start_link/2 and init/2, it expects a module, {module, arg}, or a child specification.

If a two-element tuple in the shape of {module, arg} is given, the child specification is retrieved by calling module.child_spec(arg).

If a module is given, the child specification is retrieved by calling module.child_spec([]).

After the child specification is retrieved, the fields on overrides are directly applied on the child spec. If overrides has keys that do not map to any child specification field, an error is raised.

See the "Child specification" section in the module documentation for all of the available keys for overriding.

examples

Examples

This function is often used to set an :id option when the same module needs to be started multiple times in the supervision tree:

Supervisor.child_spec({Agent, fn -> :ok end}, id: {Agent, 1})
#=> %{id: {Agent, 1},
#=>   start: {Agent, :start_link, [fn -> :ok end]}}
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count_children(supervisor)

View Source
@spec 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

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delete_child(supervisor, child_id)

View Source
@spec delete_child(supervisor(), term()) :: :ok | {:error, error}
when error: :not_found | :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.

Link to this function

init(children, options)

View Source (since 1.5.0)
@spec init(
  [
    child_spec()
    | {module(), term()}
    | module()
    | (old_erlang_child_spec :: :supervisor.child_spec())
  ],
  [init_option()]
) ::
  {:ok,
   {sup_flags(),
    [child_spec() | (old_erlang_child_spec :: :supervisor.child_spec())]}}

Receives a list of child specifications to initialize and a set of options.

This is typically invoked at the end of the init/1 callback of module-based supervisors. See the sections "Supervisor strategies and options" and "Module-based supervisors" in the module documentation for more information.

This function returns a tuple containing the supervisor flags and child specifications.

examples

Examples

def init(_init_arg) do
  children = [
    {Counter, 0}
  ]

  Supervisor.init(children, strategy: :one_for_one)
end

options

Options

  • :strategy - the supervision strategy option. It can be either :one_for_one, :rest_for_one, or :one_for_all

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

  • :max_seconds - the time frame in seconds 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.

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restart_child(supervisor, child_id)

View Source
@spec restart_child(supervisor(), term()) ::
  {:ok, child()} | {:ok, child(), term()} | {:error, error}
when error: :not_found | :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}.

Link to this function

start_child(supervisor, child_spec)

View Source
@spec start_child(
  supervisor(),
  child_spec()
  | {module(), term()}
  | module()
  | (old_erlang_child_spec :: :supervisor.child_spec())
) :: on_start_child()

Adds a child specification to supervisor and starts that child.

child_spec should be a valid child specification. The child process will be started as defined 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.

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start_link(children, options)

View Source
@spec start_link(
  [
    child_spec()
    | {module(), term()}
    | module()
    | (old_erlang_child_spec :: :supervisor.child_spec())
  ],
  [option() | init_option()]
) ::
  {:ok, pid()}
  | {:error, {:already_started, pid()} | {:shutdown, term()} | term()}
@spec start_link(module(), term()) :: on_start()

Starts a supervisor with the given children.

children is a list of the following forms:

  • a child specification

  • a module, where module.child_spec([]) will be invoked to retrieve its child specification

  • a two-element tuple in the shape of {module, arg}, where module.child_spec(arg) will be invoked to retrieve its child specification

A strategy is required to be provided through the :strategy option. See "Supervisor strategies and options" for examples and other options.

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 all child processes are successfully spawned (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 the supervisor is given a name and 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.

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

View Source
@spec start_link(module(), term(), [option()]) :: on_start()

Starts a module-based supervisor process with the given module and init_arg.

To start the supervisor, the init/1 callback will be invoked in the given module, with init_arg as its argument. The init/1 callback must return a supervisor specification which can be created with the help of the init/2 function.

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.

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

View Source
@spec stop(supervisor(), reason :: term(), timeout()) :: :ok

Synchronously 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.

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terminate_child(supervisor, child_id)

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@spec terminate_child(supervisor(), term()) :: :ok | {:error, :not_found}

Terminates the given child identified by child_id.

The process is terminated, if there's one. The child specification is kept unless the child is temporary.

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, this function returns {:error, :not_found}.

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which_children(supervisor)

View Source
@spec which_children(supervisor()) :: [
  {term() | :undefined, child() | :restarting, :worker | :supervisor,
   [module()] | :dynamic}
]

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

  • 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