Read Model Projections.md

Read model projections

Your read model can be built using a Commanded event handler and whichever storage provider you prefer. You can choose to use a SQL or NoSQL database, document store, the filesystem, a full text search index, or any other storage mechanism. You may even use multiple storage providers, optimised for the querying they must support.

Ecto projections

You can use the Commanded Ecto projections library to build a read model using one of the databases supported by Ecto (PostgreSQL, MySQL, et al).

Example

defmodule MyApp.ExampleProjector do
  use Commanded.Projections.Ecto, name: "ExampleProjector"

  project %AnEvent{name: name}, _metadata do
    Ecto.Multi.insert(multi, :example_projection, %ExampleProjection{name: name})
  end

  project %AnotherEvent{name: name} do
    Ecto.Multi.insert(multi, :example_projection, %ExampleProjection{name: name})
  end
end

Consistency guarantee

You will often choose to use :strong consistency for read model projections to ensure that you can query data affected by a dispatched command. In a typical web request using the POST/Redirect/GET pattern you want to ensure the read model is up-to-date before redirecting the user to the modified resource.

By opting in to strong consistency you are guaranteed that an :ok reply from command dispatch indicates all strongly consistent read models will have been updated.

Configure the consistency option in your projector:

defmodule MyApp.ExampleProjector do
  use Commanded.Projections.Ecto,
    name: "ExampleProjector",
    consistency: :strong
end