GenStage.BroadcastDispatcher (gen_stage v1.1.0) View Source

A dispatcher that accumulates demand from all consumers before broadcasting events to all of them.

This dispatcher guarantees that events are dispatched to all consumers without exceeding the demand of any given consumer.

If a producer uses GenStage.BroadcastDispatcher, its subscribers can specify an optional :selector function that receives the event and returns a boolean in the subscription options.

Assume producer and consumer are stages exchanging events of type %{:key => String.t, any => any}, then by calling

  to: producer,
  selector: fn %{key: key} -> String.starts_with?(key, "foo-") end)

consumer will receive only the events broadcast from producer for which the selector function returns a truthy value.

The :selector option can be specified in sync and async subscriptions, as well as in the :subscribe_to list in the return tuple of GenStage.init/1. For example:

def init(:ok) do
  {:consumer, :ok, subscribe_to:
    [{producer, selector: fn %{key: key} -> String.starts_with?(key, "foo-") end}]}