# Data Layers

Data layers determine how resources are stored and retrieved. Examples of data layers:

- **Postgres**: For storing resources in PostgreSQL (via `AshPostgres`)
- **ETS**: For in-memory storage (`Ash.DataLayer.Ets`)
- **Mnesia**: For distributed storage (`Ash.DataLayer.Mnesia`)
- **Embedded**: For resources embedded in other resources (`data_layer: :embedded`) (typically JSON under the hood)
- **Ash.DataLayer.Simple**: For resources that aren't persisted at all. Leave off the data layer, as this is the default.

Specify a data layer when defining a resource:

```elixir
defmodule MyApp.Post do
  use Ash.Resource,
    domain: MyApp.Blog,
    data_layer: AshPostgres.DataLayer

  postgres do
    table "posts"
    repo MyApp.Repo
  end

  # ... attributes, relationships, etc.
end
```

For embedded resources:

```elixir
defmodule MyApp.Address do
  use Ash.Resource,
    data_layer: :embedded

  attributes do
    attribute :street, :string
    attribute :city, :string
    attribute :state, :string
    attribute :zip, :string
  end
end
```

Each data layer has its own configuration options and capabilities. Refer to the rules & documentation of the specific data layer package for more details.