Bypass

Bypass provides a quick way to create a custom plug that can be put in place instead of an actual HTTP server to return prebaked responses to client requests. This is most useful in tests, when you want to create a mock HTTP server and test how your HTTP client handles different types of responses from the server.

Installation

Add bypass to your list of dependencies in mix.exs:

def deps do
  [{:bypass, "~> 0.1", only: :test}]
end

We do not recommended adding :bypass to the list of applications in your mix.exs. See below for usage info.

Bypass supports Elixir 1.0 and up.

Usage

Start Bypass in your test/test_helper.exs file to make it available in tests:

ExUnit.start
Application.ensure_all_started(:bypass)

To use Bypass in a test case, open a connection and use its port to connect your client to it.

If you want to test what happens when the HTTP server goes down, use Bypass.down/1 to close the TCP socket and Bypass.up/1 to start listening on the same port again. Both functions block until the socket updates its state.

In the following example TwitterClient.start_link() takes the endpoint URL as its argument allowing us to make sure it will connect to the running instance of Bypass.

defmodule TwitterClientTest do
  use ExUnit.Case, async: true

  setup do
    bypass = Bypass.open
    {:ok, bypass: bypass}
  end

  test "client can handle an error response", %{bypass: bypass} do
    Bypass.expect bypass, fn conn ->
      assert "/1.1/statuses/update.json" == conn.request_path
      assert "POST" == conn.method
      Plug.Conn.resp(conn, 429, ~s<{"errors": [{"code": 88, "message": "Rate limit exceeded"}]}>)
    end
    {:ok, client} = TwitterClient.start_link(url: endpoint_url(bypass.port))
    assert {:error, :rate_limited} == TwitterClient.post_tweet(client, "Elixir is awesome!")
  end

  test "client can recover from server downtime", %{bypass: bypass} do
    Bypass.expect bypass, fn conn ->
      # We don't care about `request_path` or `method` for this test.
      Plug.Conn.resp(conn, 200, "")
    end
    {:ok, client} = TwitterClient.start_link(url: endpoint_url(bypass.port))

    assert :ok == TwitterClient.post_tweet(client, "Elixir is awesome!")

    # Blocks until the TCP socket is closed.
    Bypass.down(bypass)

    assert {:error, :noconnect} == TwitterClient.post_tweet(client, "Elixir is awesome!")

    Bypass.up(bypass)

    # When testing a real client that is using e.g. https://github.com/fishcakez/connection
    # with https://github.com/ferd/backoff to handle reconnecting, we'd have to loop for
    # a while until the client has reconnected.

    assert :ok == TwitterClient.post_tweet(client, "Elixir is awesome!")
  end

  defp endpoint_url(port), do: "http://localhost:#{port}/"
end

That's all you need to do. Bypass automatically sets up an on_exit hook to close its socket when the test finishes running.

Multiple concurrent Bypass instances are supported, all will have a different unique port.

In case you need to assign a specific port to a Bypass instance to listen on, you can pass the port option to Bypass.open():

bypass = Bypass.open(port: 1234)

Configuration options

Set :enable_debug_log to true in the application environment to make Bypass log what it's doing:

config :bypass, enable_debug_log: true

License

This software is licensed under the MIT license.