gen_stage v1.0.0 ConsumerSupervisor behaviour View Source

A supervisor that starts children as events flow in.

A ConsumerSupervisor can be used as the consumer in a GenStage pipeline. A new child process will be started per event, where the event is appended to the arguments in the child specification.

A ConsumerSupervisor can be attached to a producer by returning :subscribe_to from init/1 or explicitly with GenStage.sync_subscribe/3 and GenStage.async_subscribe/2.

Once subscribed, the supervisor will ask the producer for :max_demand events and start child processes as events arrive. As child processes terminate, the supervisor will accumulate demand and request more events once :min_demand is reached. This allows the ConsumerSupervisor to work similar to a pool, except a child process is started per event. The minimum amount of concurrent children per producer is specified by :min_demand and the maximum is given by :max_demand.

Example

Let's define a GenStage consumer as a ConsumerSupervisor that subscribes to a producer named Producer and starts a new process for each event received from the producer. Each new process will be started by calling Printer.start_link/1, which simply starts a task that will print the incoming event to the terminal.

defmodule Consumer do
  use ConsumerSupervisor

  def start_link(arg) do
    ConsumerSupervisor.start_link(__MODULE__, arg)
  end

  def init(_arg) do
    # Note: By default the restart for a child is set to :permanent
    # which is not supported in ConsumerSupervisor. You need to explicitly
    # set the :restart option either to :temporary or :transient.
    children = [%{id: Printer, start: {Printer, :start_link, []}, restart: :transient}]
    opts = [strategy: :one_for_one, subscribe_to: [{Producer, max_demand: 50}]]
    ConsumerSupervisor.init(children, opts)
  end
end

Then in the Printer module:

defmodule Printer do
  def start_link(event) do
    # Note: this function must return the format of `{:ok, pid}` and like
    # all children started by a Supervisor, the process must be linked
    # back to the supervisor (if you use [`Task.start_link/1`](https://hexdocs.pm/elixir/Task.html#start_link/1) then both
    # these requirements are met automatically)
    Task.start_link(fn ->
      IO.inspect({self(), event})
    end)
  end
end

Similar to Supervisor, ConsumerSupervisor also provides start_link/3, which allows developers to start a supervisor with the help of a callback module.

Name Registration

A supervisor is bound to the same name registration rules as a GenServer. Read more about it in the GenServer docs.

Link to this section Summary

Types

option()

Options used by the start* functions

Functions

count_children(supervisor)

Returns a map containing count values for the supervisor.

init(list, opts)

Receives a template to initialize and a set of options.

start_child(supervisor, args)

Starts a child in the consumer supervisor.

start_link(children, options)

Starts a supervisor with the given children.

start_link(mod, args, opts \\ [])

Starts a consumer supervisor module with the given args.

terminate_child(supervisor, pid)

Terminates the given child pid.

which_children(supervisor)

Returns a list with information about all children.

Callbacks

init(args)

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

Link to this section Types

option()

option() ::
  {:registry, atom()}
  | {:name, Supervisor.name()}
  | {:strategy, Supervisor.Spec.strategy()}
  | {:max_restarts, non_neg_integer()}
  | {:max_seconds, non_neg_integer()}
  | {:subscribe_to, [GenStage.stage() | {GenStage.stage(), keyword()}]}

Options used by the start* functions

Link to this section Functions

count_children(supervisor)

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 - always 1 as consumer supervisors have a single specification
: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(list, opts)

Receives a template to initialize and a set of options.

This is typically invoked at the end of the init/1 callback of module-based supervisors.

This function returns a the child specification and the supervisor flags.

Examples

Using the child specification changes introduced in Elixir 1.5:

defmodule MyConsumerSupervisor do
  use ConsumerSupervisor

  def start_link(arg) do
    ConsumerSupervisor.start_link(__MODULE__, arg)
  end

  def init(_arg) do
    ConsumerSupervisor.init([MyConsumer], strategy: :one_for_one, subscribe_to: MyProducer)
  end
end

start_child(supervisor, args)

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

Starts a child in the consumer supervisor.

The child process will be started by appending the given list of args to the existing function arguments in the child specification.

This child is started separately from any producer and does not count towards the demand of any of them.

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

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

start_link(children, options)

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

start_link(module(), any()) :: Supervisor.on_start()

Starts a supervisor with the given children.

A strategy is required to be given as an option. Furthermore, the :max_restarts, :max_seconds, and :subscribe_to values can be configured as described in the documentation for the init/1 callback.

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.

The child processes specified in children will be started by appending the event to process to the existing function arguments in the child specification.

Note that the consumer supervisor is linked to the parent process and will exit not only on crashes but also if the parent process exits with :normal reason.

start_link(mod, args, opts \\ [])

start_link(module(), any(), [option()]) :: Supervisor.on_start()

Starts a consumer supervisor module with the given args.

To start the supervisor, the init/1 callback will be invoked in the given module, with args passed to it. The init/1 callback must return a supervision specification which can be created with the help of the Supervisor module.

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 under the "Name Registration" section in the GenServer module docs.

terminate_child(supervisor, pid)

terminate_child(Supervisor.supervisor(), pid()) :: :ok | {:error, :not_found}

Terminates the given child pid.

If successful, the function returns :ok. If there is no such pid, the function returns {:error, :not_found}.

which_children(supervisor)

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

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 - as defined in the child specification but is always set to :undefined for consumer 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

Link to this section Callbacks

init(args)

init(args :: term()) ::
  {:ok, [:supervisor.child_spec()], options :: keyword()} | :ignore

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

Options

:strategy - the restart strategy option. Only :one_for_one is supported by consumer supervisors.
:max_restarts - the maximum amount of restarts allowed in a time frame. Defaults to 3 times.
:max_seconds - the time frame in which :max_restarts applies in seconds. Defaults to 5 seconds.
:subscribe_to - a list of producers to subscribe to. Each element represents the producer or a tuple with the producer and the subscription options, for example, [Producer] or [{Producer, max_demand: 20, min_demand: 10}].