[Openstack-operators] [all] SQL Schema Downgrades: A Good Idea?

Morgan Fainberg morgan.fainberg at gmail.com
Thu Jan 29 19:26:35 UTC 2015

From an operator perspective I wanted to get input on the SQL Schema Downgrades.

Today most projects (all?) provide a way to downgrade the SQL Schemas after you’ve upgraded. Example would be moving from Juno to Kilo and then back to Juno. There are some odd concepts when handling a SQL migration downgrade specifically around the state of the data. A downgrade, in many cases, causes permanent and irrevocable data loss. When phrased like that (and dusting off my deployer/operator hat) I would be hesitant to run a downgrade in any production, stagings, or even QA environment.

In light of what a downgrade actually means I would like to get the views of the operators on SQL Migration Downgrades:

1) Would you actually perform a programatic downgrade via the cli tools or would you just do a restore-to-last-known-good-before-upgrade (e.g. from a DB dump)?
2) Would you trust the data after a programatic downgrade or would the data only really be trustworthy if from a restore? Specifically the new code *could* be relying on new data structures and a downgrade could result in weird states of services.

I’m looking at the expectation that a downgrade is possible. Each time I look at the downgrades I feel that it doesn’t make sense to ever really perform a downgrade outside of a development environment. The potential for permanent loss of data / inconsistent data leads me to believe the downgrade is a flawed design. Input from the operators on real-world cases would be great to have.

This is an operator specific set of questions related to a post I made to the OpenStack development mailing list: http://lists.openstack.org/pipermail/openstack-dev/2015-January/055586.html <http://lists.openstack.org/pipermail/openstack-dev/2015-January/055586.html>

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