Queens compute node is not compatible with Pike Controller node

[omkar_telee]

Description
===========

We Have OpenStack Pike running on Ubuntu 16.04.
As per OpenStack documentation, compute node should support N+1 version of controller.
But when we upgrade the controller node, we get below error on compute side for all actions performed.

ERROR oslo_messaging.rpc.server UnsupportedVersion: Endpoint does not support RPC version 5.0. Attempted method: build_and_run_instance

The RPC version of Pike compute is not supported with RPC version of Queens controller.

Thus we are unable to upgrade this setup.

Steps to reproduce
==================

1. Setup openstack Pike on Ubuntu 16.04
2. Upgrade controller node to Queens by adding new keys
3. Synch db, restart (standard upgrade process)
4. After successful upgrade of controller node, check functions of nova (create instance, start/stop instance) Here controller is on Queens and Compute is on Pike
5. You should get error that RPC versions are not supported

Expected result
===============

Compute Should be compatible with N+1 version of OpenStack.
In This case, this scenario must be supported and compute functions must work.

Actual result
=============

All compute related functions fails. Start/stop/reboot instance fails.

Logs
============

ERROR oslo_messaging.rpc.server UnsupportedVersion: Endpoint does not support RPC version 5.0. Attempted method: build_and_run_instance

Environment
===========

Ubuntu 16.04 controller and compute with Pike installation.

Do let me know if you need more details.

[Matt Riedemann (mriedem)]

This isn't really supported. You should be configuring [upgrade_levels]/compute to pike:

https://docs.openstack.org/nova/latest/configuration/config.html#upgrade_levels.compute

Until you get everything upgraded to Queens at which point you can remove the RPC version pin.

[Matt Riedemann (mriedem)]

See https://docs.openstack.org/nova/latest/user/upgrade.html#rolling-upgrade-process for details on rolling upgrades.