# `TdsCdc.Persistence`

Behaviour for LSN position persistence.

By default, `TdsCdc.Persistence.File` writes LSN positions to disk so they
survive application restarts. You can implement this behaviour to store
positions in a database, Redis, or any other backend.

## Example: custom persistence module

    defmodule MyApp.DbPersistence do
      @behaviour TdsCdc.Persistence

      @impl true
      def save_positions(_name, positions) do
        # Write positions to your database
        :ok
      end

      @impl true
      def load_positions(_name) do
        # Read positions from your database
        {:ok, %{}}  # or {:error, reason}
      end
    end

Then pass it to the client:

    TdsCdc.start_link(
      conn: [...],
      capture_instances: ["dbo_users"],
      persistence: {MyApp.DbPersistence, []}
    )

# `opts`

```elixir
@type opts() :: keyword()
```

# `positions`

```elixir
@type positions() :: %{required(String.t()) => binary()}
```

# `load_positions`

```elixir
@callback load_positions(name :: atom()) :: {:ok, positions()} | {:error, term()}
```

Loads the saved LSN positions.

- `name` is the client's registered name (atom).

Returns `{:ok, positions}` where positions is a map of capture instance
name to LSN binary, or `{:error, reason}` if no positions are found.

# `save_positions`

```elixir
@callback save_positions(name :: atom(), positions :: positions()) ::
  :ok | {:error, term()}
```

Saves the current LSN positions for all capture instances.

- `name` is the client's registered name (atom).
- `positions` is a map of capture instance name to LSN binary.

---

*Consult [api-reference.md](api-reference.md) for complete listing*