DynamicSupervisor — Elixir v1.16.3

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# View Source DynamicSupervisor behaviour (Elixir v1.16.3)

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` and `init/1` functions

on_start_child()

Return values of `start_child` functions

option()

Options given to `start_link` 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

Link to this type

# init_option()

View Source

    @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` and `init/1` functions

Link to this type

# on_start_child()

View Source

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

Return values of `start_child` functions

Link to this type

# option()

View Source

    @type option() :: GenServer.option()

Options given to `start_link` functions

Link to this type

# strategy()

View Source

    @type strategy() :: :one_for_one

Supported strategies

Link to this type

# sup_flags()

View Source

    @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

Link to this callback

# init(init_arg)

View Source

    @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

Link to this function

# child_spec(options)

View Source (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.

Link to this function

# count_children(supervisor)

View Source (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

Link to this function

# init(options)

View Source (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

Link to this function

# start_child(supervisor, child_spec)

View Source (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}`.

Link to this function

# start_link(options)

View Source (since 1.6.0)

    @spec start_link([option() | init_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.

Link to this function

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

View Source (since 1.6.0)

    @spec start_link(module(), term(), [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.

Link to this function

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

View Source (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.

Link to this function

# terminate_child(supervisor, pid)

View Source (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}`.

Link to this function

# which_children(supervisor)

View Source (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