The aggregate specification.

An aggregate in Disco is a struct which has the fields representing the state of itself.

This module defines a behaviour with a set of default callback implementations to execute a command or a query on the aggregate.

Define an aggregate

Here’s how to implement a simple aggregate:

defmodule MyApp.Aggregate do
  defstruct id: nil, foo: nil

  use Disco.Aggregate,
    routes: %{
      commands: %{
        do_something: MyApp.Commands.DoSomething
      },
      queries: %{
        find_something: MyApp.Commands.FindSomething
      }
    },
    event_store_client: MyApp.EventStoreClient

  def apply_event(%{type: "SomethingDone"} = event, state) do
    %{state | foo: event.foo, id: event.aggregate_id}
  end
end

NOTE

It is very important to always specify an id field in the aggregate struct. Every aggregate needs it to be uniquely identified.

Overriding default functions

The simplest implementation only requires to implement apply_event/2 callback, while the others are already implemented by default. Sometimes you might need a custom implementation of the callbacks, that’s why it’s possible to easily override them.

Usage example

NOTE: Disco.Factories.ExampleAggregate has been defined in test/support/examples/example_aggregate.ex.

iex> alias Disco.Factories.ExampleAggregate, as: Aggregate
iex> Aggregate.commands()
[:do_something]
iex> Aggregate.routes()
[:find_something]
iex> Aggregate.dispatch(:do_something, %{foo: "bar"})
{:ok, %Disco.Factories.ExampleAggregate{
  foo: "bar",
  id: "4fd98a9e-8d6f-4e35-a8fc-aca5544596cb"
}}
iex> Aggregate.query(:find_something, %{foo: "bar"})
%{foo: "bar"}

Umbrella apps

Check Disco.Orchestrator.