gen_stage v0.12.2 GenStage.Dispatcher behaviour 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

Callbacks

ask(demand, from, state)

Called every time a consumer sends demand

cancel(from, state)

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

dispatch(events, length, state)

Called every time a producer wants to dispatch an event

info(msg, state)

Used to send an info message to the current process

init(opts)

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

subscribe(opts, from, state)

Called every time the producer gets a new subscriber

Link to this section Callbacks

ask(demand, from, state)

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.

cancel(from, state)

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.

dispatch(events, length, state)

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])

info(msg, state)

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.

init(opts)

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

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