Move instance selection logic out of router

The process of selecting a random (working) instance for a specified
service has been moved out of the router and into lib/farside.ex. Moving
forward, the router itself should have very simple and easy to follow
logic for all paths.

lib/farside.ex

@@ -1,5 +1,6 @@
 defmodule Farside do
   @service_prefix Application.fetch_env!(:farside, :service_prefix)
+  @fallback_str Application.fetch_env!(:farside, :fallback_str)
 
   def get_services_map do
     {:ok, service_list} = Redix.command(:redix, ["KEYS", "#{@service_prefix}*"])
@@ -24,6 +25,36 @@ defmodule Farside do
     end)
   end
 
+  def pick_instance(service) do
+    {:ok, instances} =
+      Redix.command(
+        :redix,
+        [
+          "LRANGE",
+          "#{@service_prefix}#{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
+        {:ok, result} =
+          Redix.command(
+            :redix,
+            ["GET", "#{service}#{@fallback_str}"]
+          )
+
+        result
+      end
+
+    instance
+  end
+
   def get_last_updated do
     {:ok, last_updated} =
       Redix.command(

lib/farside/router.ex

@@ -1,7 +1,5 @@
 defmodule Farside.Router do
   @index Application.fetch_env!(:farside, :index)
-  @fallback_str Application.fetch_env!(:farside, :fallback_str)
-  @service_prefix Application.fetch_env!(:farside, :service_prefix)
 
   use Plug.Router
 
@@ -27,32 +25,7 @@ defmodule Farside.Router do
 
   get "/:service/*glob" do
     path = Enum.join(glob, "/")
-
-    {:ok, instances} =
-      Redix.command(
-        :redix,
-        [
-          "LRANGE",
-          "#{@service_prefix}#{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
-        {:ok, result} =
-          Redix.command(
-            :redix,
-            ["GET", "#{service}#{@fallback_str}"]
-          )
-
-        result
-      end
+    instance = Farside.pick_instance(service)
 
     # Redirect to the available instance
     conn