Req

Docs

Req is an HTTP client with a focus on ease of use and composability, built on top of Finch.

This is a work in progress!

Features

• Extensibility via request, response, and error steps

• Automatic body decompression (via decompress/2 step)

• Automatic body encoding and decoding (via encode/1 and decode/2 steps)

• Encode params as query string (via params/2 step)

• Basic authentication (via auth/2 step)

• .netrc file support (via netrc/2 step)

• Range requests (via range/2 step)

• Follows redirects (via follow_redirects/2 step)

• Retries on errors (via retry/3 step)

• Basic HTTP caching (via if_modified_since/2 step)

Usage

The easiest way to use Req is with Mix.install/2 (requires Elixir v1.12+):

Mix.install([
  {:req, "~> 0.1.0"}
])

Req.get!("https://api.github.com/repos/elixir-lang/elixir").body["description"]
#=> "Elixir is a dynamic, functional language designed for building scalable and maintainable applications"

If you want to use Req in a Mix project, you can add the above dependency to your mix.exs.

Low-level API

Under the hood, Req works by passing a request through a series of steps.

The request struct, %Req.Request{}, initially contains data like HTTP method and request headers. You can also add request, response, and error steps to it.

Request steps are used to refine the data that will be sent to the server.

After making the actual HTTP request, we'll either get a HTTP response or an error. The request, along with the response or error, will go through response or error steps, respectively.

Nothing is actually executed until we run the pipeline with Req.run/1.

The high-level API shown before:

Req.get!("https://api.github.com/repos/elixir-lang/elixir")

is equivalent to this composition of lower-level API functions:

Req.build(:get, "https://api.github.com/repos/elixir-lang/elixir")
|> Req.prepend_default_steps()
|> Req.run!()

(See Req.build/3, Req.prepend_default_steps/2, and Req.run!/1 for more information.)

We can also build more complex flows like returning a response from a request step or an error from a response step. We will explore those next.

Request steps

A request step is a function that accepts a request and returns one of the following:

• A request

• A {request, response_or_error} tuple. In that case no further request steps are executed and the return value goes through response or error steps

Examples:

def default_headers(request) do
  update_in(request.headers, &[{"user-agent", "req/0.1.0-dev"} | &1])
end

def read_from_cache(request) do
  case ResponseCache.fetch(request) do
    {:ok, response} -> {request, response}
    :error -> request
  end
end

Response and error steps

A response step is a function that accepts a {request, response} tuple and returns one of the following:

• A {request, response} tuple

• A {request, exception} tuple. In that case, no further response steps are executed but the exception goes through error steps

Similarly, an error step is a function that accepts a {request, exception} tuple and returns one of the following:

• A {request, exception} tuple

• A {request, response} tuple. In that case, no further error steps are executed but the response goes through response steps

Examples:

def decode({request, response}) do
  case List.keyfind(response.headers, "content-type", 0) do
    {_, "application/json" <> _} ->
      {request, update_in(response.body, &Jason.decode!/1)}

    _ ->
      {request, response}
  end
end

def log_error({request, exception}) do
  Logger.error(["#{request.method} #{request.url}: ", Exception.message(exception)])
  {request, exception}
end

Halting

Any step can call Req.Request.halt/1 to halt the pipeline. This will prevent any further steps from being invoked.

Examples:

def circuit_breaker(request) do
  if CircuitBreaker.open?() do
    {Req.Request.halt(request), RuntimeError.exception("circuit breaker is open")}
  else
    request
  end
end

Acknowledgments

Req is built on top of Finch and is inspired by cURL, Requests, Tesla, and many other HTTP clients - thank you!

License

Copyright (c) 2021 Wojtek Mach

Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with the License. You may obtain a copy of the License at http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License.