[Koha-devel] Rolling DB upgrades

[Joonas Kylmälä]

Hi,

On 03/03/2021 11:39, Julian Maurice wrote:
> I'm not too much concerned about the "push-test-release" process, but
> more about how we consistently make the code aware of the multiple
> possible database states. Surely we'd have to enforce a new set of rules
> for writing database updates. And we'd have to test these changes very
> carefully, since a tiny mistake could mean a loss of data. Of course you
> have backups, but DB rollback means downtime, right ?

Depends on the rollback I guess, some things you could probably revert
without downtime. The possibility of loss of data due to programming
error happens now as well and is no different.

> So, that sounds like a lot of extra work for avoiding a few seconds of
> downtime (most of the time).

Not really sure it is more work than writing the current db upgrade
procedures, maybe tiny bit, and a bit more if you want to be able to
revert the DB upgrade as well (which is optional and I don't think we
should even consider that for MVP implementation of this).

> Also, what about template translations ? It takes some time to generate
> translated templates, and it can do weird things if the new code is used
> while using the old templates (or the old code with the new templates).
> Wouldn't that prevent zero-downtime upgrade ?

You redirect the traffic during upgrade to other nodes that are in
working state and after the node being upgraded is ready to receive
traffic you then can re-enable the traffic redirection there.

Joonas

-- 
Joonas Kylmälä
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