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"}]
endDatabase Configuration
Maestro is intended to be used alongside an existing database/ecto repo.
config :maestro,
storage_adapter: Maestro.Store.Postgres,
repo: MyApp.RepoTo generate the migrations for the snapshot and event logs, do:
mix maestro.create.event_store_migrationExample
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))
endiex(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)