Maestro

Maestro is an event sourcing library. It is inspired by CQRS and re-uses terminology where appropriate. The divergence from being a CQRS framework is intentional as Maestro focuses on processing commands in a consistent manner and replaying events in a consistent order.

Currently, the only storage adapter suited to a multi-node environment is the Maestro.Store.Postgres adapter. The Maestro.Store.InMemory adapter exists for testing purposes only.

Status

Documentation is available here.

Installation

def deps do
  [{:maestro, "~> 0.2"}]
end

Database Configuration

Maestro is intended to be used alongside an existing database/ecto repo.

config :maestro,
  storage_adapter: Maestro.Store.Postgres,
  repo: MyApp.Repo

To generate the migrations for the snapshot and event logs, do:

mix maestro.create.event_store_migration

Example

There are three behaviours that make the command/event lifecycle flow: Maestro.Aggregate.CommandHandler, Maestro.Aggregate.EventHandler, and Maestro.Aggregate.ProjectionHandler. Modules implementing the command and event handler behaviours are looked up via a configurable :command_prefix and :event_prefix respectively. Projections are reserved for maintaining other models/representations within the event's transaction.

defmodule MyApp.Aggregate do
  use Maestro.Aggregate.Root,
    command_prefix: MyApp.Aggregate.Commands,
    event_prefix: MyApp.Aggregate.Events

  def initial_state, do: %{"value" => 0}

  def prepare_snapshot(state), do: state

  def use_snapshot(_curr, %Maestro.Types.Snapshot{body: state}), do: state
end

defmodule MyApp.Aggregate.Commands.IncrementCounter do

  @behaviour Maestro.Aggregate.CommandHandler

  alias Maestro.Types.Event

  def eval(aggregate, _command) do
    [
      %Event{
        aggregate_id: aggregate.id,
        type: "counter_incremented",
        body: %{}
      }
    ]
  end
end

defmodule MyApp.Aggregate.Events.CounterIncremented do

  @behaviour Maestro.Aggregate.EventHandler

  def apply(state, _event), do: Map.update!(state, "value", &(&1 + 1))
end

iex(1)> {:ok, id} = MyApp.Aggregate.new()
iex(2)> :ok = MyApp.Aggregate.evaluate(%Maestro.Types.Command{aggregate_id: id, type: "increment_counter", data: %{}})
iex(3)> {:ok, %{"value" => 1}} = MyApp.Aggregate.get(id)