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