View Source DynamicSupervisor behaviour (Elixir v1.18.0)

A supervisor optimized to only start children dynamically.

The Supervisor module was designed to handle mostly static children that are started in the given order when the supervisor starts. A DynamicSupervisor starts with no children. Instead, children are started on demand via start_child/2 and there is no ordering between children. This allows the DynamicSupervisor to hold millions of children by using efficient data structures and to execute certain operations, such as shutting down, concurrently.

Examples

A dynamic supervisor is started with no children and often a name:

children = [
  {DynamicSupervisor, name: MyApp.DynamicSupervisor, strategy: :one_for_one}
]

Supervisor.start_link(children, strategy: :one_for_one)

The options given in the child specification are documented in start_link/1.

Once the dynamic supervisor is running, we can use it to start children on demand. Given this sample GenServer:

defmodule Counter do
  use GenServer

  def start_link(initial) do
    GenServer.start_link(__MODULE__, initial)
  end

  def inc(pid) do
    GenServer.call(pid, :inc)
  end

  def init(initial) do
    {:ok, initial}
  end

  def handle_call(:inc, _, count) do
    {:reply, count, count + 1}
  end
end

We can use start_child/2 with a child specification to start a Counter server:

{:ok, counter1} = DynamicSupervisor.start_child(MyApp.DynamicSupervisor, {Counter, 0})
Counter.inc(counter1)
#=> 0

{:ok, counter2} = DynamicSupervisor.start_child(MyApp.DynamicSupervisor, {Counter, 10})
Counter.inc(counter2)
#=> 10

DynamicSupervisor.count_children(MyApp.DynamicSupervisor)
#=> %{active: 2, specs: 2, supervisors: 0, workers: 2}

Scalability and partitioning

The DynamicSupervisor is a single process responsible for starting other processes. In some applications, the DynamicSupervisor may become a bottleneck. To address this, you can start multiple instances of the DynamicSupervisor and then pick a "random" instance to start the child on.

Instead of:

children = [
  {DynamicSupervisor, name: MyApp.DynamicSupervisor}
]

and:

DynamicSupervisor.start_child(MyApp.DynamicSupervisor, {Counter, 0})

You can do this:

children = [
  {PartitionSupervisor,
   child_spec: DynamicSupervisor,
   name: MyApp.DynamicSupervisors}
]

and then:

DynamicSupervisor.start_child(
  {:via, PartitionSupervisor, {MyApp.DynamicSupervisors, self()}},
  {Counter, 0}
)

In the code above, we start a partition supervisor that will by default start a dynamic supervisor for each core in your machine. Then, instead of calling the DynamicSupervisor by name, you call it through the partition supervisor, using self() as the routing key. This means each process will be assigned one of the existing dynamic supervisors. Read the PartitionSupervisor docs for more information.

Module-based supervisors

Similar to Supervisor, dynamic supervisors also support module-based supervisors.

defmodule MyApp.DynamicSupervisor do
  # Automatically defines child_spec/1
  use DynamicSupervisor

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

  @impl true
  def init(_init_arg) do
    DynamicSupervisor.init(strategy: :one_for_one)
  end
end

See the Supervisor docs for a discussion of when you may want to use module-based supervisors. A @doc annotation immediately preceding use DynamicSupervisor will be attached to the generated child_spec/1 function.

use DynamicSupervisor

When you use DynamicSupervisor, the DynamicSupervisor module will set @behaviour DynamicSupervisor and define a child_spec/1 function, so your module can be used as a child in a supervision tree.

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.

Migrating from Supervisor's :simple_one_for_one

In case you were using the deprecated :simple_one_for_one strategy from the Supervisor module, you can migrate to the DynamicSupervisor in few steps.

Imagine the given "old" code:

defmodule MySupervisor do
  use Supervisor

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

  def start_child(foo, bar, baz) do
    # This will start child by calling MyWorker.start_link(init_arg, foo, bar, baz)
    Supervisor.start_child(__MODULE__, [foo, bar, baz])
  end

  @impl true
  def init(init_arg) do
    children = [
      # Or the deprecated: worker(MyWorker, [init_arg])
      %{id: MyWorker, start: {MyWorker, :start_link, [init_arg]}}
    ]

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

It can be upgraded to the DynamicSupervisor like this:

defmodule MySupervisor do
  use DynamicSupervisor

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

  def start_child(foo, bar, baz) do
    # If MyWorker is not using the new child specs, we need to pass a map:
    # spec = %{id: MyWorker, start: {MyWorker, :start_link, [foo, bar, baz]}}
    spec = {MyWorker, foo: foo, bar: bar, baz: baz}
    DynamicSupervisor.start_child(__MODULE__, spec)
  end

  @impl true
  def init(init_arg) do
    DynamicSupervisor.init(
      strategy: :one_for_one,
      extra_arguments: [init_arg]
    )
  end
end

The difference is that the DynamicSupervisor expects the child specification at the moment start_child/2 is called, and no longer on the init callback. If there are any initial arguments given on initialization, such as [initial_arg], it can be given in the :extra_arguments flag on DynamicSupervisor.init/1.

Summary

Types

init_option()

Options given to start_link/1 and init/1 functions

on_start_child()

Return values of start_child functions

strategy()

Supported strategies

sup_flags()

The supervisor flags returned on init

Callbacks

init(init_arg)

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

Functions

child_spec(options)

Returns a specification to start a dynamic supervisor under a supervisor.

count_children(supervisor)

Returns a map containing count values for the supervisor.

init(options)

Receives a set of options that initializes a dynamic supervisor.

start_child(supervisor, child_spec)

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

start_link(options)

Starts a supervisor with the given options.

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

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

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

Synchronously stops the given supervisor with the given reason.

terminate_child(supervisor, pid)

Terminates the given child identified by pid.

which_children(supervisor)

Returns a list with information about all children.

Types

init_option()

@type init_option() ::
  {:strategy, strategy()}
  | {:max_restarts, non_neg_integer()}
  | {:max_seconds, pos_integer()}
  | {:max_children, non_neg_integer() | :infinity}
  | {:extra_arguments, [term()]}

Options given to start_link/1 and init/1 functions

on_start_child()

@type on_start_child() ::
  {:ok, pid()}
  | {:ok, pid(), info :: term()}
  | :ignore
  | {:error, {:already_started, pid()} | :max_children | term()}

Return values of start_child functions

strategy()

@type strategy() :: :one_for_one

Supported strategies

sup_flags()

@type sup_flags() :: %{
  strategy: strategy(),
  intensity: non_neg_integer(),
  period: pos_integer(),
  max_children: non_neg_integer() | :infinity,
  extra_arguments: [term()]
}

The supervisor flags returned on init

Callbacks

init(init_arg)

@callback init(init_arg :: term()) :: {:ok, sup_flags()} | :ignore

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

Developers typically invoke DynamicSupervisor.init/1 at the end of their init callback to return the proper supervision flags.

Functions

child_spec(options)

(since 1.6.1)

Returns a specification to start a dynamic supervisor under a supervisor.

It accepts the same options as start_link/1.

See Supervisor for more information about child specifications.

count_children(supervisor)

(since 1.6.0) 
@spec count_children(Supervisor.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 supervisor.

The map contains the following keys:

  • :specs - the number of children processes

  • :active - the count of all actively running child processes managed by this supervisor

  • :supervisors - the count of all supervisors whether or not the child process is still alive

  • :workers - the count of all workers, whether or not the child process is still alive

init(options)

(since 1.6.0) 
@spec init([init_option()]) :: {:ok, sup_flags()}

Receives a set of options that initializes a dynamic supervisor.

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

It accepts the same options as start_link/1 (except for :name) and it returns a tuple containing the supervisor options.

Examples 

def init(_arg) do
  DynamicSupervisor.init(max_children: 1000)
end

start_child(supervisor, child_spec)

(since 1.6.0) 
@spec start_child(
  Supervisor.supervisor(),
  Supervisor.child_spec()
  | {module(), term()}
  | module()
  | (old_erlang_child_spec :: :supervisor.child_spec())
) :: on_start_child()

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

child_spec should be a valid child specification as detailed in the "Child specification" section of the documentation for Supervisor. The child process will be started as defined in the child specification. Note that while the :id field is still required in the spec, the value is ignored and therefore does not need to be unique.

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, then no child is added to the supervision tree and this function returns :ignore too.

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 the error or erroneous value returned from child process start function, or failure reason if it fails.

If the supervisor already has N children in a way that N exceeds the amount of :max_children set on the supervisor initialization (see init/1), then this function returns {:error, :max_children}.

start_link(options)

(since 1.6.0) 
@spec start_link([init_option() | GenServer.option()]) :: Supervisor.on_start()

Starts a supervisor with the given options.

This function is typically not invoked directly, instead it is invoked when using a DynamicSupervisor as a child of another supervisor:

children = [
  {DynamicSupervisor, name: MySupervisor}
]

If the supervisor is successfully spawned, 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.

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.

Options

  • :name - registers the supervisor under the given name. The supported values are described under the "Name registration" section in the GenServer module docs.

  • :strategy - the restart strategy option. The only supported value is :one_for_one which means that no other child is terminated if a child process terminates. You can learn more about strategies in the Supervisor module docs.

  • :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.

  • :max_children - the maximum amount of children to be running under this supervisor at the same time. When :max_children is exceeded, start_child/2 returns {:error, :max_children}. Defaults to :infinity.

  • :extra_arguments - arguments that are prepended to the arguments specified in the child spec given to start_child/2. Defaults to an empty list.

  • Any of the standard GenServer options

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

(since 1.6.0) 
@spec start_link(module(), term(), [GenServer.option()]) :: Supervisor.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/1 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.

If the supervisor is successfully spawned, 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.

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.

Options

This function accepts any regular GenServer options. Options specific to DynamicSupervisor must be returned from the init/1 callback.

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

(since 1.7.0) 
@spec stop(Supervisor.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.

terminate_child(supervisor, pid)

(since 1.6.0) 
@spec terminate_child(Supervisor.supervisor(), pid()) :: :ok | {:error, :not_found}

Terminates the given child identified by pid.

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

which_children(supervisor)

(since 1.6.0) 
@spec which_children(Supervisor.supervisor()) :: [
  {:undefined, pid() | :restarting, :worker | :supervisor,
   [module()] | :dynamic}
]

Returns a list with information about all children.

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 tuples containing:

  • id - it is always :undefined for dynamic supervisors

  • child - the PID of the corresponding child process or the atom :restarting if the process is about to be restarted

  • type - :worker or :supervisor as defined in the child specification

  • modules - as defined in the child specification