Merge pull request openshift#297 from markmc/upgrade-downgrade-expect…

…ations

Template: add specific guidance in upgrade/downgrade section

guidelines/enhancement_template.md

@@ -216,6 +216,34 @@ enhancement:
 - What changes (in invocations, configurations, API use, etc.) is an existing
   cluster required to make on upgrade in order to make use of the enhancement?
 
+Upgrade expectations:
+- Each component should remain available for user requests and
+  workloads during upgrades. Any exception to this should be
+  identified and discussed here.
+- Micro version upgrades - users should be able to skip forward versions within a
+  minor release stream without being required to pass through intermediate
+  versions - i.e. `x.y.N->x.y.N+2` should work without requiring `x.y.N->x.y.N+1`
+  as an intermediate step.
+- Minor version upgrades - you only need to support `x.N->x.N+1` upgrade
+  steps. So, for example, it is acceptable to require a user running 4.3 to
+  upgrade to 4.5 with a `4.3->4.4` step followed by a `4.4->4.5` step.
+- While an upgrade is in progress, new component versions should
+  continue to operate correctly in concert with older component
+  versions (aka "version skew"). For example, if a node is down, and
+  an operator is rolling out a daemonset, the old and new daemonset
+  pods must continue to work correctly even while the cluster remains
+  in this partially upgraded state for some time.
+
+Downgrade expectations:
+- If an `N->N+1` upgrade fails mid-way through, or if the `N+1` cluster is
+  misbehaving, it should be possible for the user to rollback to `N`. It is
+  acceptable to require some documented manual steps in order to fully restore
+  the downgraded cluster to its previous state. Examples of acceptable steps
+  include:
+  - Deleting any CVO-managed resources added by the new version. The
+    CVO does not currently delete resources that no longer exist in
+    the target version.
+
 ### Version Skew Strategy
 
 How will the component handle version skew with other components?