Route service requests to an available instance

Router has been updated with a basic /:service/*glob endpoint, which
retrieves a list of instances for the requested service and picks a
random one to forward the user to (or falls back to a default instance
if none are found).

Should probably add a check to make sure the requested service exists
first.

lib/privacy_revolver/router.ex

@@ -5,6 +5,25 @@ defmodule PrivacyRevolver.Router do
   plug :dispatch
 
   get "/ping" do
-    send_resp(conn, 200, "pong")
+    # Useful for app healthcheck
+    {:ok, resp} = Redix.command(:redix, ["PING"])
+    send_resp(conn, 200, resp)
+  end
+
+  get "/:service/*glob" do
+    full_path = "/" <> Enum.join(glob, "/")
+    {:ok, instances} = Redix.command(:redix, ["LRANGE", service, "0", "-1"])
+
+    # Either pick a random available instance, or fall back to the default one
+    instance = if Enum.count(instances) > 0 do
+      Enum.random(instances)
+    else
+      Redix.command(:redix, ["GET", service <> "-fallback"])
+    end
+
+    # Redirect to the available instance
+    conn |>
+    Plug.Conn.resp(:found, "") |>
+    Plug.Conn.put_resp_header("location", instance <> full_path)
   end
 end