Reuse previous instance within rate-limit interval

Rather than blocking <1s back-to-back queries from the same IP, Farside
will now re-use the previously selected instance.

Fixes #20

lib/farside.ex

@@ -26,6 +26,15 @@ defmodule Farside do
     end)
   end
 
+  def last_instance(service) do
+    {:ok, previous} =
+      Redix.command(
+        :redix,
+        ["GET", "#{service}#{@previous_suffix}"]
+      )
+    previous
+  end
+
   def pick_instance(service) do
     {:ok, instances} =
       Redix.command(
@@ -38,7 +47,7 @@ defmodule Farside do
         ]
       )
 
-    # Either pick a random available instance, 
+    # Either pick a random available instance,
     # or fall back to the default one
     instance =
       if Enum.count(instances) > 0 do
@@ -48,14 +57,8 @@ defmodule Farside do
         else
           # ...otherwise pick a random one from the list, ensuring
           # that the same instance is never picked twice in a row.
-          {:ok, previous} =
-            Redix.command(
-              :redix,
-              ["GET", "#{service}#{@previous_suffix}"]
-            )
-
           instance =
-            Enum.filter(instances, &(&1 != previous))
+            Enum.filter(instances, &(&1 != last_instance(service)))
             |> Enum.random()
 
           Redix.command(

lib/farside/router.ex

@@ -40,7 +40,14 @@ defmodule Farside.Router do
 
   get "/:service/*glob" do
     path = Enum.join(glob, "/")
-    instance = Farside.pick_instance(service)
+    instance = cond do
+      conn.assigns[:throttle] != nil ->
+        Farside.last_instance(service)
+      true ->
+        Farside.pick_instance(service)
+    end
+    IO.inspect(get_req_header(conn, "throttle"))
+    IO.inspect(instance)
 
     params =
       cond do

lib/farside/throttle.ex

@@ -14,8 +14,7 @@ defmodule Farside.Throttle do
   def allow_action(conn, _data, _opts), do: conn
 
   def block_action(conn, _data, _opts) do
+    conn = assign(conn, :throttle, 1)
     conn
-    |> send_resp(:forbidden, "Exceeded rate limit\n")
-    |> halt
   end
 end