Add integration test against mocked HTTP client.

This is overkill for now, but will become necessary to prove we've got
everything wired up properly to forward trace context in HTTP headers.

plug_gateway/config/config.exs

@@ -8,3 +8,5 @@ config :plug_gateway,
   api_endpoint: System.get_env("BACKEND_API_URL"),
   auth_token: System.get_env("BACKEND_AUTH_TOKEN"),
   port: String.to_integer(System.get_env("PORT", "4000"))
+
+if Mix.env() in [:test], do: import_config("#{Mix.env()}.exs")

plug_gateway/config/test.exs

@@ -0,0 +1,6 @@
+use Mix.Config
+
+config :plug_gateway,
+  api_endpoint: "http://api.example.com/api",
+  auth_token: "BACKEND_AUTH_TOKEN",
+  http_module: PlugGateway.HTTPMock

plug_gateway/lib/plug_gateway/backend_client.ex

@@ -1,14 +1,12 @@
 defmodule PlugGateway.BackendClient do
+  @moduledoc """
+  PlugGateway back end client.
+  """
 
-  def get(url, opts \\ []) do
-    auth_token = PlugGateway.Config.get().auth_token
-    headers = [{"authorization", "Bearer #{auth_token}"}]
-
-    url
-    |> HTTPoison.get(headers)
-    |> case do
-      {:ok, %HTTPoison.Response{status_code: status_code, body: body}} -> {:ok, status_code, body}
-      {:error, reason} -> {:error, reason}
-    end
+  def get(path) do
+    config = PlugGateway.Config.get()
+    url = config.api_endpoint <> path
+    headers = [{"authorization", "Bearer #{config.auth_token}"}]
+    config.http_module.get(url, headers)
   end
 end

plug_gateway/lib/plug_gateway/config.ex

@@ -3,7 +3,7 @@ defmodule PlugGateway.Config do
   PlugGateway configuration.
   """
 
-  defstruct [:api_endpoint, :auth_token, :port]
+  defstruct [:api_endpoint, :auth_token, http_module: PlugGateway.HTTP.API.Poisonous, port: 4000]
 
   @doc "Get our configuration."
   def get do

plug_gateway/lib/plug_gateway/http_api.ex

@@ -0,0 +1,29 @@
+defmodule PlugGateway.HTTP.API do
+  @moduledoc """
+  PlugGateway API for speaking to HTTP back ends.
+  """
+
+  @doc "Get a URL."
+  @callback get(
+              url :: String.t(),
+              headers: [{String.t(), iodata()}]
+            ) :: {:ok, integer(), String.t()} | {:error, term()}
+end
+
+defmodule PlugGateway.HTTP.API.Poisonous do
+  @moduledoc """
+  PlugGateway implementation for speaking to HTTP back ends.
+
+  Adapts `c:PlugGateway.HTTP.API.get/1` to `HTTPoison.get/2`.
+  """
+
+  @behaviour PlugGateway.HTTP.API
+
+  @impl true
+  def get(url, headers) do
+    case HTTPoison.get(url, headers) do
+      {:ok, %HTTPoison.Response{status_code: status_code, body: body}} -> {:ok, status_code, body}
+      {:error, reason} -> {:error, reason}
+    end
+  end
+end

plug_gateway/lib/plug_gateway/router.ex

@@ -3,11 +3,15 @@ defmodule PlugGateway.Router do
 
   alias PlugGateway.BackendClient
 
-  plug :match
-  plug Plug.Parsers, parsers: [:urlencoded, :json],
+  plug(:match)
+
+  plug(Plug.Parsers,
+    parsers: [:urlencoded, :json],
     pass: ["*/*"],
     json_decoder: Jason
-  plug :dispatch
+  )
+
+  plug(:dispatch)
 
   get "/" do
     send_resp(conn, 200, "Hello World")
@@ -22,26 +26,20 @@ defmodule PlugGateway.Router do
   end
 
   get "/users" do
-    response =
-      BackendClient.get(backend_api_endpoint() <> "/users")
-      |> case do
-        {:ok, status_code, body} -> send_resp(conn, status_code, body)
-        {:error, reason} -> send_resp(conn, 502, ~s|{"errors":"#{inspect reason}"}|)
-      end
+    case BackendClient.get("/users") do
+      {:ok, status_code, body} -> send_resp(conn, status_code, body)
+      {:error, reason} -> send_resp(conn, 502, ~s|{"errors":"#{inspect(reason)}"}|)
+    end
   end
 
   get "/users_n_plus_1" do
-    response =
-      BackendClient.get(backend_api_endpoint() <> "/users_n_plus_1")
-      |> case do
-        {:ok, status_code, body} -> send_resp(conn, status_code, body)
-        {:error, reason} -> send_resp(conn, 502, ~s|{"errors":"#{inspect reason}"}|)
-      end
+    case BackendClient.get("/users_n_plus_1") do
+      {:ok, status_code, body} -> send_resp(conn, status_code, body)
+      {:error, reason} -> send_resp(conn, 502, ~s|{"errors":"#{inspect(reason)}"}|)
+    end
   end
 
   match _ do
     send_resp(conn, 404, "Not Found")
   end
-
-  defp backend_api_endpoint, do: PlugGateway.Config.get().api_endpoint
 end

plug_gateway/mix.exs

@@ -24,6 +24,7 @@ defmodule PlugGateway.MixProject do
     [
       {:httpoison, "~> 1.6"},
       {:jason, "~> 1.1"},
+      {:mox, "~> 0.5.2", only: :test},
       {:plug_cowboy, "~> 2.1"}
     ]
   end

plug_gateway/mix.lock

@@ -9,6 +9,7 @@
   "metrics": {:hex, :metrics, "1.0.1", "25f094dea2cda98213cecc3aeff09e940299d950904393b2a29d191c346a8486", [:rebar3], [], "hexpm", "69b09adddc4f74a40716ae54d140f93beb0fb8978d8636eaded0c31b6f099f16"},
   "mime": {:hex, :mime, "1.3.1", "30ce04ab3175b6ad0bdce0035cba77bba68b813d523d1aac73d9781b4d193cf8", [:mix], [], "hexpm", "6cbe761d6a0ca5a31a0931bf4c63204bceb64538e664a8ecf784a9a6f3b875f1"},
   "mimerl": {:hex, :mimerl, "1.2.0", "67e2d3f571088d5cfd3e550c383094b47159f3eee8ffa08e64106cdf5e981be3", [:rebar3], [], "hexpm", "f278585650aa581986264638ebf698f8bb19df297f66ad91b18910dfc6e19323"},
+  "mox": {:hex, :mox, "0.5.2", "55a0a5ba9ccc671518d068c8dddd20eeb436909ea79d1799e2209df7eaa98b6c", [:mix], [], "hexpm", "df4310628cd628ee181df93f50ddfd07be3e5ecc30232d3b6aadf30bdfe6092b"},
   "parse_trans": {:hex, :parse_trans, "3.3.0", "09765507a3c7590a784615cfd421d101aec25098d50b89d7aa1d66646bc571c1", [:rebar3], [], "hexpm", "17ef63abde837ad30680ea7f857dd9e7ced9476cdd7b0394432af4bfc241b960"},
   "plug": {:hex, :plug, "1.8.3", "12d5f9796dc72e8ac9614e94bda5e51c4c028d0d428e9297650d09e15a684478", [:mix], [{:mime, "~> 1.0", [hex: :mime, repo: "hexpm", optional: false]}, {:plug_crypto, "~> 1.0", [hex: :plug_crypto, repo: "hexpm", optional: false]}, {:telemetry, "~> 0.4", [hex: :telemetry, repo: "hexpm", optional: true]}], "hexpm", "164baaeb382d19beee0ec484492aa82a9c8685770aee33b24ec727a0971b34d0"},
   "plug_cowboy": {:hex, :plug_cowboy, "2.1.0", "b75768153c3a8a9e8039d4b25bb9b14efbc58e9c4a6e6a270abff1cd30cbe320", [:mix], [{:cowboy, "~> 2.5", [hex: :cowboy, repo: "hexpm", optional: false]}, {:plug, "~> 1.7", [hex: :plug, repo: "hexpm", optional: false]}], "hexpm", "6cd8ddd1bd1fbfa54d3fc61d4719c2057dae67615395d58d40437a919a46f132"},

plug_gateway/test/plug_gateway_test.exs

@@ -4,6 +4,8 @@ defmodule PlugGatewayTest do
 
   alias PlugGateway.Router
 
+  import Mox, only: [set_mox_from_context: 1, verify_on_exit!: 1]
+
   @router_opts Router.init([])
 
   test "root endpoint" do
@@ -29,6 +31,47 @@ defmodule PlugGatewayTest do
     assert conn.resp_body == "Not Found"
   end
 
+  test "users route to back end", ctx do
+    assert PlugGateway.Config.get().http_module == PlugGateway.HTTPMock
+    set_mox_from_context(ctx)
+    verify_on_exit!(ctx)
+
+    test_pid = self()
+
+    # Expect a call to PlugGateway.HTTPMock.get/2:
+    Mox.expect(PlugGateway.HTTPMock, :get, fn url, headers ->
+      try do
+        # Did PlugGateway.BackendClient compose the full URL?
+        assert url == "http://api.example.com/api/users"
+
+        # Did PlugGateway.BackendClient inject the authorization header?
+        assert {_, "Bearer BACKEND_AUTH_TOKEN"} = :lists.keyfind("authorization", 1, headers)
+
+        # Send an event matching the exception smuggling below:
+        send(test_pid, {:mock_result, :ok})
+
+        # Reply:
+        {:ok, 200, "yay"}
+      rescue
+        e ->
+          # Smuggle the exception back to the test so you get full colour diffs on assert failure:
+          send(test_pid, {:mock_result, :error, e, __STACKTRACE__})
+
+          # Reply as if the server didn't like what we sent:
+          {:ok, 400, "poot"}
+      end
+    end)
+
+    result = get("/users")
+
+    receive do
+      {:mock_result, :ok} -> :ok
+      {:mock_result, :error, e, stacktrace} -> reraise(e, stacktrace)
+    end
+
+    assert %{status: 200, resp_body: "yay"} = result
+  end
+
   # Helpers
 
   defp get(path) do

plug_gateway/test/test_helper.exs

@@ -1 +1,3 @@
 ExUnit.start()
+Mox.defmock(PlugGateway.HTTPMock, for: PlugGateway.HTTP.API)
+ExUnit.start(capture_log: true, timeout: 10_000, exclude: [:skip])