[openstack-dev] [Nova] question about DB migration difficulty

Michael Still mikal at stillhq.com
Wed Nov 13 21:27:17 UTC 2013

On Thu, Nov 14, 2013 at 5:18 AM, Mike Spreitzer <mspreitz at us.ibm.com> wrote:
> This is a follow-up to the design summit discussion about DB migrations.
> There was concern about the undo-ability of some migrations.  The specific
> example cited was removal of a column.  Could that be done with the
> following three migrations, each undo-able?  First, change the code to keep
> writing the column but no longer read the column.  Second migration changes
> the code to neither read nor write the column.  Third migration physically
> removes the column.

This was actually discussed in the session as an example of how other
projects handle these problems. Our concerns (IIRC) were that it would
take even more patches to land, and each of those patches is quite
hard to land in nova these days. Additionally, it increases the
complexity of our code a lot, because we have to handle databases in
all possible states because of how our continuous deployment model

Objects bring us closer to being able to do this, but we need objects
finished first.


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