GenStage.Dispatcher behaviour (gen_stage v1.1.0) View Source

This module defines the behaviour used by :producer and :producer_consumer to dispatch events.

When using a :producer or :producer_consumer, the dispatcher may be configured on init as follows:

{:producer, state, dispatcher: GenStage.BroadcastDispatcher}

Some dispatchers may require options to be given on initialization, those can be done with a tuple:

{:producer, state, dispatcher: {GenStage.PartitionDispatcher, partitions: 0..3}}

Elixir ships with the following dispatcher implementations:

  • GenStage.DemandDispatcher - dispatches the given batch of events to the consumer with the biggest demand in a FIFO ordering. This is the default dispatcher.

  • GenStage.BroadcastDispatcher - dispatches all events to all consumers. The demand is only sent upstream once all consumers ask for data.

  • GenStage.PartitionDispatcher - dispatches all events to a fixed amount of consumers that works as partitions according to a hash function.

Link to this section Summary

Types

Options used by init/1

Callbacks

Called every time a consumer sends demand.

Called every time a subscription is cancelled or the consumer goes down.

Called every time a producer wants to dispatch an event.

Used to send an info message to the current process.

Called on initialization with the options given on GenStage.init/1.

Called every time the producer gets a new subscriber.

Link to this section Types

Specs

options() :: keyword()

Options used by init/1

Link to this section Callbacks

Link to this callback

ask(demand, from, state)

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Specs

ask(demand :: pos_integer(), from :: {pid(), reference()}, state :: term()) ::
  {:ok, actual_demand :: non_neg_integer(), new_state}
when new_state: term()

Called every time a consumer sends demand.

The demand will always be a positive integer (more than 0). This callback must return the actual_demand as part of its return tuple. The returned demand is then sent to producers.

It is guaranteed the reference given in from points to a reference previously given in subscribe.

Specs

cancel(from :: {pid(), reference()}, state :: term()) ::
  {:ok, demand :: non_neg_integer(), new_state}
when new_state: term()

Called every time a subscription is cancelled or the consumer goes down.

It is guaranteed the reference given in from points to a reference previously given in subscribe.

Link to this callback

dispatch(events, length, state)

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Specs

dispatch(events :: [term(), ...], length :: pos_integer(), state :: term()) ::
  {:ok, leftover_events :: [term()], new_state}
when new_state: term()

Called every time a producer wants to dispatch an event.

The events will always be a non empty list. This callback may receive more events than previously asked and therefore must return events it cannot not effectively deliver as part of its return tuple. Any leftover_events will be stored by producers in their buffer.

It is important to emphasize that leftover_events can happen in any dispatcher implementation. After all, a consumer can subscribe, ask for events and crash. Eventually the events the consumer asked will be delivered while the consumer no longer exists, meaning they must be returned as left_over events until another consumer subscribes.

It is guaranteed the reference given in from points to a reference previously given in subscribe. It is also recommended for events to be sent with Process.send/3 and the [:noconnect] option as the consumers are all monitored by the producer. For example:

Process.send(consumer, {:"$gen_consumer", {self(), consumer_ref}, events}, [:noconnect])

Specs

info(msg :: term(), state :: term()) :: {:ok, new_state} when new_state: term()

Used to send an info message to the current process.

In case the dispatcher is doing buffering, the message must only be sent after all currently buffered consumer messages are delivered.

Specs

init(opts :: options()) :: {:ok, state} when state: any()

Called on initialization with the options given on GenStage.init/1.

Link to this callback

subscribe(opts, from, state)

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Specs

subscribe(opts :: keyword(), from :: {pid(), reference()}, state :: term()) ::
  {:ok, demand :: non_neg_integer(), new_state} | {:error, term()}
when new_state: term()

Called every time the producer gets a new subscriber.