Brook

Brook provides an event stream client interface for distributed applications to communicate indirectly and asynchronously. Brook sends and receives messages with the event stream (typically a message queue service) via a driver module and persists an application-specific view of the event stream via a storage module (defaulting to ETS).

Installation

The package can be installed from Hex by adding brook to your list of dependencies in mix.exs:

def deps do
  [
    {:brook, "~> 0.6"}
  ]
end

Documentation can be found at https://hexdocs.pm/brook.

Testing

Elsa uses the standard ExUnit testing library for unit testing. For integration testing interactions with Kafka, it uses the divo library. Run tests with the command mix test.integration.

Sample Configuration

Brook is configured within the application environment by defining a keyword list with three primary keys: driver, handler, and storage.

config :my_app, :brook,
  driver: [
    module: Brook.Driver.Json,
    init_arg: []
  ],
  handler: [MyApp.Event.Handler],
  storage: [
    modules: Brook.Storage.Ets,
    init_arg: []
  ]

Driver

The Brook driver implements a behaviour that sends messages to the event stream. Events are Brook structs that contain an event type, an author (or source), a creation timestamp, the event data, and an ack reference and ack data (following the lead of the broadway library.)

The default driver sends the event message to the Brook server via Genserver.cast

Additional drivers provided at this time are a json-encoding version of the default driver and a Kafka driver using the elsa library.

Handler

The Brook handler implements a behaviour that provides a handle_event/1 function. Handlers receive a Brook event and take appropriate action according to the implementing application's business logic.

Applications implement as many function heads for the event handler as necessary and return one of four tuples depending on how the storage module should treat the event with respect to persistence. Events can:

• create a record in the view state via the {:create, collection, key, value} return
• update an existing record via the {:merge, collection, key, value} return
• delete a record via the {:delete, collection, key} return
• discard the record and do not effect the persistent view via the :discard return

Storage

The Brook storage module implements yet another behaviour that persists event data to an application view state specific to the application importing the Brook library, allowing the application to only store information received from the event stream that is relevant to its own domain and retrieve it when necessary.

Storage modules implement basic CRUD operations that correlate to the return values of the event handler module.

The default module uses ETS for fast, local, in-memory storage and retrieval (great for testing purposes!) with an additional Redis-based module as well.