Prevent same instance from being selected twice in a row

Introduces a new db key "<service>-previous" to track which instance was
last selected for a particular service. This allows for filtering the
list of available instances to exclude the instance that was last
picked, to ensure a (slightly) more even distribution of traffic.
There's still the possiblity of the following scenario, however:

:service instances > 2

/:service request #1 -> instance #1
/:service request #2 -> instance #2
/:service request #3 -> instance #1
/:service request #4 -> instance #2

where there are many ignored instances for a particular service. One
possible solution would be to implement the "<service>-previous" value
to be a list, rather than a single value, and push to that list until
only one element is left in the original "instance" array after
filtering, and then delete the "<service>-previous" key.

1 files changed, 3 insertions, 3 deletions

diff --git a/update.exs b/update.exs
index 7452e3d..e0c367a 100644
--- a/update.exs
+++ b/update.exs
@@ -1,5 +1,5 @@
 defmodule Instances do
-  @fallback_str Application.fetch_env!(:farside, :fallback_str)
+  @fallback_suffix Application.fetch_env!(:farside, :fallback_suffix)
   @update_file Application.fetch_env!(:farside, :update_file)
   @services_json Application.fetch_env!(:farside, :services_json)
   @service_prefix Application.fetch_env!(:farside, :service_prefix)
@@ -59,13 +59,13 @@ defmodule Instances do
     if Enum.count(instances) > 0 do
       Redix.command(:redix, [
         "SET",
-        "#{service.type}#{@fallback_str}",
+        "#{service.type}#{@fallback_suffix}",
         Enum.random(instances)
       ])
     else
       Redix.command(:redix, [
         "SET",
-        "#{service.type}#{@fallback_str}",
+        "#{service.type}#{@fallback_suffix}",
         service.fallback
       ])
     end