Change the default MySQL backend transaction isolation level to the READ-COMMITED

[Bogdan Dobrelya]

A quote from http://lists.openstack.org/pipermail/openstack-dev/2015-February/056245.html :

MySQL uses repeatable read for transactions by default. Postgres uses read committed by default, and
SQLite uses serializable. We (note: Oslo.db) don't set the isolation level explicitly
anywhere, so our applications are running under different isolation
levels depending on backend.

Fuel should configure the MySQL transaction_isolation=READ-COMMITTED as it looks that this mode is implicit for backends by Oslo.db because it is a standard for DB transations.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/168842

[Jay Pipes (jaypipes)]

Bogdan, I don't believe this is a bug. I have seen no evidence (including the ML post referenced in the patch comments above) that the default MySQL transaction isolation level needs to be changed.

[OpenStack Infra (hudson-openstack)]

Change abandoned by Bogdan Dobrelya (<email address hidden>) on branch: master
Review: https://review.openstack.org/168842

[Bogdan Dobrelya (bogdando)]

This bug is valid I believe. Please read https://aphyr.com/posts/313-strong-consistency-models carefully. According to https://aphyr.com/data/posts/313/family-tree.jpg , MySQL's RR default isolation level is in "red zone" and cannot guarantee a fully AP system.

Although I'm not sure, could this change be made for CM side only. Perhaps Oslo.DB side should also be adjusted to rely on RC isolation layer. Does it assume default RR for transactions is good enough? Or does it rely only on RC guarantees and uses DLM for higher levels of consistency guarantees, for example? Anyway, the decision for CM can be made only after these questions answered.

[Bogdan Dobrelya (bogdando)]

According to http://lists.openstack.org/pipermail/openstack-dev/2015-February/056245.html there are few places there RR is assumed in the code. I believe this is a major Oslo.db architecture issue. RC should be the only assumption everythere. Very strange that that mail thread got no responces. No one cares?

[Eugene Nikanorov (enikanorov)]

RR is assumed everywhere, in fact.

Changing default Tx isolation level is a big deal and should not be any kind of downstream effort.

[Bogdan Dobrelya (bogdando)]

Agreed, this cannot be fixed downstream

[Roman Podoliaka (rpodolyaka)]

We no longer fix bugs with Importance lower than High in 8.0, closing as Won't Fix.