@macrocallback __before_compile__(env :: Macro.Env.t()) :: Macro.t()

The callback invoked in case the adapter needs to inject code.

@callback checked_out?(adapter_meta()) :: boolean()

Returns true if a connection has been checked out.

@callback checkout(adapter_meta(), config :: Keyword.t(), (-> result)) :: result
when result: var

Checks out a connection for the duration of the given function.

In case the adapter provides a pool, this guarantees all of the code inside the given fun runs against the same connection, which might improve performance by for instance allowing multiple related calls to the datastore to share cache information:

Repo.checkout(fn ->
  for _ <- 100 do
    Repo.insert!(%Post{})
  end
end)

If the adapter does not provide a pool, just calling the passed function and returning its result are enough.

If the adapter provides a pool, it is supposed to "check out" one of the pool connections for the duration of the function call. Which connection is checked out is not passed to the calling function, so it should be done using a stateful method like using the current process' dictionary, process tracking, or some kind of other lookup method. Make sure that this stored connection is then used in the other callbacks implementations, such as Ecto.Adapter.Queryable and Ecto.Adapter.Schema.

@callback dumpers(primitive_type :: Ecto.Type.primitive(), ecto_type :: Ecto.Type.t()) ::
  [
    (term() -> {:ok, term()} | :error) | Ecto.Type.t()
  ]

Returns the dumpers for a given type.

It receives the primitive type and the Ecto type (which may be primitive as well). It returns a list of dumpers with the given type usually at the beginning.

This allows developers to properly translate values coming from the Ecto into adapter ones. For example, if the database does not support booleans but instead returns 0 and 1 for them, you could add:

def dumpers(:boolean, type), do: [type, &bool_encode/1]
def dumpers(_primitive, type), do: [type]

defp bool_encode(false), do: {:ok, 0}
defp bool_encode(true), do: {:ok, 1}

All adapters are required to implement a clause for :binary_id types, since they are adapter specific. If your adapter does not provide binary ids, you may simply use Ecto.UUID:

def dumpers(:binary_id, type), do: [type, Ecto.UUID]
def dumpers(_primitive, type), do: [type]

@callback ensure_all_started(
  config :: Keyword.t(),
  type :: :permanent | :transient | :temporary
) ::
  {:ok, [atom()]} | {:error, atom()}

Ensure all applications necessary to run the adapter are started.

@callback init(config :: Keyword.t()) :: {:ok, :supervisor.child_spec(), adapter_meta()}

Initializes the adapter supervision tree by returning the children and adapter metadata.

@callback loaders(primitive_type :: Ecto.Type.primitive(), ecto_type :: Ecto.Type.t()) ::
  [
    (term() -> {:ok, term()} | :error) | Ecto.Type.t()
  ]

Returns the loaders for a given type.

It receives the primitive type and the Ecto type (which may be primitive as well). It returns a list of loaders with the given type usually at the end.

This allows developers to properly translate values coming from the adapters into Ecto ones. For example, if the database does not support booleans but instead returns 0 and 1 for them, you could add:

def loaders(:boolean, type), do: [&bool_decode/1, type]
def loaders(_primitive, type), do: [type]

defp bool_decode(0), do: {:ok, false}
defp bool_decode(1), do: {:ok, true}

All adapters are required to implement a clause for :binary_id types, since they are adapter specific. If your adapter does not provide binary ids, you may simply use Ecto.UUID:

def loaders(:binary_id, type), do: [Ecto.UUID, type]
def loaders(_primitive, type), do: [type]