Create a project tag for zero-downtime upgrades

In transitioning projects towards a rolling upgrades strategy, it's
become clear that there are additional levels of granularity, reflecting
the additional difficulty of implementing and maintaining acceptable
solutions.

The existing asserts:supports-rolling-upgrade tag allows for minimal
downtime of the control plane during the upgrade process. This tag
raises the bar further by eliminating that allowance.

As a concession, this tag still allows for performance degradation
during the upgrade process, which we could again eliminate in a more
restrictive tag as projects begin to assert this one.

With this patch, the upgrade-related tags would then include:

1. asserts:supports-upgrade
2. asserts:supports-rolling-upgrade
3. asserts:supports-zero-downtime-upgrade

Change-Id: I8b191f7d56702aa21273c61417af4bdf1de2abb0

reference/tags/assert_supports-rolling-upgrade.rst

@@ -46,7 +46,9 @@ Requirements
   there are no such caveats. This does not require complete
   elimination of downtime during upgrades, but rather reducing the
   scope from "all services" to "some services at a time." In other
-  words, "restarting all API services together" is a reasonable restriction.
+  words, "restarting all API services together" is a reasonable restriction. A
+  more restrictive tag, :ref:`tag-assert:supports-zero-downtime-upgrade`
+  removes this allowance.
 * Full stack integration testing with services arranged in a
   mid-upgrade manner is performed on every proposed commit to validate
   that mixed-version services work together properly. This testing

reference/tags/assert_supports-zero-downtime-upgrade.rst

@@ -0,0 +1,78 @@
+::
+
+  This work is licensed under a Creative Commons Attribution 3.0
+  Unported License.
+  http://creativecommons.org/licenses/by/3.0/legalcode
+
+.. _`tag-assert:supports-zero-downtime-upgrade`:
+
+=====================================
+assert:supports-zero-downtime-upgrade
+=====================================
+
+This tag is part of the assert category of tags, which are assertions made by
+the project team themselves about their maturity.
+
+The ``assert:supports-zero-downtime-upgrade`` tag asserts that in addition to a
+project supporting `minimal rolling upgrade capabilities
+<https://governance.openstack.org/reference/tags/assert_supports-rolling-upgrade.html>`_,
+it does so *without incurring any disruption to API availability*.
+
+Application to current projects
+===============================
+
+.. tagged-projects:: assert:supports-zero-downtime-upgrade
+
+
+Rationale
+=========
+
+OpenStack projects represent code that is installed and deployed on many
+distributed systems in order to provide services to users. Operators need to
+know what projects support an upgrade process that eliminates downtime of the
+control plane entirely.
+
+Requirements
+============
+
+* The project is already tagged as ``type:service``.
+
+* The project has already successfully asserted both the
+  :ref:`tag-assert:supports-upgrade` and
+  :ref:`tag-assert:supports-rolling-upgrade` tags. All the requirements for
+  those tags are requirements for this tag, and assertion of both those tags
+  are requirements to assert this tag.
+
+* In addition to the plan required by the
+  :ref:`tag-assert:supports-rolling-upgrade` tag which allows for minimal
+  downtime, this tag requires services to completely eliminate API downtime of
+  the control plane during the upgrade. In other words, requiring operators to
+  "restart all API services together" is not reasonable under this tag.
+  Additionally, it is not reasonable for services to respond to requests with
+  HTTP 5xx codes when they otherwise would have returned successful responses,
+  even intermittently.
+
+* While all requests to the control plane must be eventually processed,
+  performance degradation during the upgrade is acceptable. This may include
+  slow HTTP responses and delayed request handling (such as messages queuing up
+  on the message bus).
+
+* In addition to the full stack integration testing required by the
+  :ref:`tag-assert:supports-rolling-upgrade` tag, services should be capable of
+  receiving and handling requests throughout the rolling upgrade process with a
+  normal success rate. In order to assert this tag, services should prevent
+  regression by implementing a zero-downtime gate job wherein both a new
+  version of the service and an old version of the service are run
+  concurrently.
+
+Tag application process
+=======================
+
+Assertion tags are set by the project team PTL or suitable designee based on
+meeting the above criteria. The Technical Committee may exceptionally remove
+the tag if they find that the project doesn't actually follow the requirements
+for the assertion.
+
+The project asserting this tag should do so when the above requirements are met
+for the reference implementation(s) or configuration(s) officially adopted by
+that project.

reference/tags/index.rst

@@ -30,6 +30,7 @@ Project Assertions Tags
    assert_follows-standard-deprecation
    assert_supports-upgrade
    assert_supports-rolling-upgrade
+   assert_supports-zero-downtime-upgrade
 
 Release Management Tags
 =======================