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(