# `Tesla.Adapter`
[🔗](https://github.com/elixir-tesla/tesla/blob/v1.17.0/lib/tesla/adapter.ex#L1)

The adapter specification.

Adapter is a module that denormalize request data stored in `Tesla.Env` in order to make
request with lower level http client (e.g. `:httpc` or `:hackney`) and normalize response data
in order to store it back to `Tesla.Env`. It has to implement `c:Tesla.Adapter.call/2`.

## Writing custom adapter

Create a module implementing `c:Tesla.Adapter.call/2`.

See `c:Tesla.Adapter.call/2` for details.

### Examples

    defmodule MyProject.CustomAdapter do
      alias Tesla.Multipart

      @behaviour Tesla.Adapter

      @override_defaults [follow_redirect: false]

      @impl Tesla.Adapter
      def call(env, opts) do
        opts = Tesla.Adapter.opts(@override_defaults, env, opts)

        with {:ok, {status, headers, body}} <- request(env.method, env.body, env.headers, opts) do
          {:ok, normalize_response(env, status, headers, body)}
        end
      end

      defp request(_method, %Stream{}, _headers, _opts) do
        {:error, "stream not supported by adapter"}
      end

      defp request(_method, %Multipart{}, _headers, _opts) do
        {:error, "multipart not supported by adapter"}
      end

      defp request(method, body, headers, opts) do
        :lower_level_http.request(method, body, denormalize_headers(headers), opts)
      end

      defp denormalize_headers(headers), do: ...
      defp normalize_response(env, status, headers, body), do: %Tesla.Env{env | ...}
    end

# `options`

```elixir
@type options() :: any()
```

Unstructured data passed to the adapter using `opts[:adapter]`.

# `call`

```elixir
@callback call(env :: Tesla.Env.t(), options :: options()) :: Tesla.Env.result()
```

Invoked when a request runs.

## Arguments

- `env` - `t:Tesla.Env.t/0` struct that stores request/response data.
- `options` - middleware options provided by user.

# `opts`

```elixir
@spec opts(Keyword.t(), Tesla.Env.t(), Keyword.t()) :: Keyword.t()
```

Helper function that merges all adapter options.

## Arguments

- `defaults` (optional) - useful to override lower level http client default
  configuration.
- `env` - `t:Tesla.Env.t()`
- `opts` - options provided to the adapter from `Tesla.client/2`.

## Precedence rules

The options are merged in the following order of precedence (highest to lowest):

1. Options from `env.opts[:adapter]` (highest precedence).
2. Options provided to the adapter from `Tesla.client/2`.
3. Default options (lowest precedence).

This means that options specified in `env.opts[:adapter]` will override any
conflicting options from the other sources, allowing for fine-grained control
on a per-request basis.

---

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