diff --git a/user-stories/proposed/ha_vm.rst b/user-stories/proposed/ha_vm.rst
index 949e9ca..b48fb64 100644
--- a/user-stories/proposed/ha_vm.rst
+++ b/user-stories/proposed/ha_vm.rst
@@ -211,3 +211,11 @@ Glossary
   operational performance for a higher than normal period.  Not to be
   confused with 100% availability, which is sometimes described as
   `fault tolerance <https://en.wikipedia.org/wiki/Fault_tolerance>`_.
+
+* `Pets and cattle
+  <http://www.theregister.co.uk/2013/03/18/servers_pets_or_cattle_cern/>`_ -
+  a metaphor commonly used in the OpenStack community to describe the
+  difference between two service architecture models: cloud-native,
+  stateless, disposable instances with built-in resilience in the
+  application layer (cattle), vs. legacy, stateful instances with no
+  built-in resilience (pets).