Custom Producers

If you want to use Broadway but there is no existing Broadway producer for the technology of your choice, you can integrate any existing GenStage producer into the pipeline with relative ease.

Example

In general, producers must generate %Broadway.Message{} structs in order to be processed by Broadway. In case you need to use an existing GenStage producer and you don't want to change its original implementation, you'll have to set the producer's :transformer option to translate the generated events into Broadway messages.

In the following example the producer is a regular GenStage, i.e., it produces plain events that cannot be processed by Broadway directly:

defmodule Counter do
  use GenStage

  def start_link(number) do
    GenStage.start_link(Counter, number)
  end

  def init(counter) do
    {:producer, counter}
  end

  def handle_demand(demand, counter) when demand > 0 do
    events = Enum.to_list(counter..counter+demand-1)
    {:noreply, events, counter + demand}
  end
end

By using a transformer, you can tell Broadway to transform all events generated by the producer into proper Broadway messages:

defmodule MyBroadway do
  use Broadway

  alias Broadway.Message

  def start_link(_opts) do
    Broadway.start_link(__MODULE__,
      name: __MODULE__,
      producers: [
        default: [
          module: {Counter, 1},
          transformer: {__MODULE__, :transform, []}
        ],
      ],
      processors: [
        default: [stages: 10]
      ],
      batchers: [
        default: [stages: 2, batch_size: 5],
      ]
    )
  end

  ...callbacks...

  def transform(event, _opts) do
    %Message{
      data: event,
      acknowledger: {__MODULE__, :ack_id, :ack_data}
    }
  end

  def ack(:ack_id, successful, failed) do
    # Write ack code here
  end
end

Notice that you need to pass two options to the producer:

  • :module - a tuple representing the GenStage producer as {mod, arg}. Where mod is module that implements the GenStage behaviour and arg the argument that will be given to the init callback of the GenStage. It is very important to note that Broadway will not call the child_spec/1 or start_link/1 function of the producer. That's because Broadway wraps the producer to augment it with extra features.

  • :transformer - a module-function-args tuple that will be invoked inside the producer, for every producer message, that should create a Broadway.Message struct with the data and acknowledger fields.

See the Broadway.Acknowledger module for more information on defining and setting up acknowledgements.