using-multiple-repos.md

Using Multiple Repos

When scaling PostgreSQL you may want to setup read replicas to improve performance and availability. This can be achieved by configuring multiple repositories in your application.

Setup Read Replicas

Following the ecto docs, change your Repo configuration:

defmodule MyApp.Repo do
  use Ecto.Repo,
    otp_app: :my_app,
    adapter: Ecto.Adapters.Postgres

  @replicas [
    MyApp.Repo.Replica1,
    MyApp.Repo.Replica2,
    MyApp.Repo.Replica3,
    MyApp.Repo.Replica4
  ]

  def replica do
    Enum.random(@replicas)
  end

  for repo <- @replicas do
    defmodule repo do
      use Ecto.Repo,
        otp_app: :my_app,
        adapter: Ecto.Adapters.Postgres,
        read_only: true
    end
  end
end

Configure AshPostgres

Now change the repo argument for your postgres block as such:

defmodule MyApp.MyDomain.MyResource do
  use Ash.Resource,
    date_layer: AshPostgres.DataLayer

  postgres do
    table "my_resources"
    repo fn
      _resource, :read -> MyApp.Repo.replica()
      _resource, :mutate -> MyApp.Repo
    end
  end
end