a tool for building in-order GenStage topologies for EventStoreDB¹

def deps do
  [
    {:beeline, "~> 1.0"},
    # these are optional: add whichever adapter your project needs
    {:kelvin, "~> 1.0"},
    {:volley, "~> 1.0"}
  ]
end

Check out the docs here: https://hexdocs.pm/beeline

Beeline is a Broadway-like library for building in-order event-processors for EventStoreDB. Writing a Beeline topology is roughly the same as writing a GenStage consumer but with more configuration in the start_link/1 function.

defmodule MyApp.MyEventHandler do
  use Beeline

  def start_link(_opts) do
    Beeline.start_link(__MODULE__,
      name: __MODULE__,
      producers: [
        default: [
          adapter: :kelvin,
          stream_name: "$ce-BoundedContext.Aggregate",
          connection: MyApp.ExtremeClient
        ]
      ],
      get_stream_position: fn producer ->
        MyApp.Repo.get(MyApp.StreamPosition, inspect(producer))
      end
    )
  end

  @impl GenStage
  def handle_events(subscription_events, _from, state) do
    # do something with the events

    {:noreply, [], state}
  end
end

See the hex documentation for Beeline.start_link/2 for the configuration schema and explanation.

Beeline champions in-order procesing because it can vastly simplify the writing of event processing code. By forcing strictly in-order processing, Beeline can guarantee either that all events in the stream will be handled in-order and exactly once or that the processor will halt. This avoids any potential concurrency dragons and allows an author to make assumptions about ordering.

In-order subscriptions are also very inexpensive on the EventStoreDB: stream reads and basic subscriptions cost next to nothing in terms of resources.

If you want multiple consumers or a pipeline with producer-consumers, you can write the topology using straight-up GenStage, Kelvin/Volley, and a Supervisor. Beeline is meant to be a convenience library for gluing together the most common processes and configuration for a simple in-order subscription.

If you want to handle events out of order with concurrency and batching, Broadway is an excellent choice when paired with the EventStoreDB persistent subscriptions feature. Broadway's acknowledgement system fits perfectly with the persistent subscriptions API.