This being MySQL I wouldn't be surprised if changing the isolation level would introduce new problems. Also, user code might rely on Django using repeatable read. If we introduce the change in to stable releases, we risk breaking sites that work perfectly well currently. To me this is against our backwards compatibility rules which state that we avoid doing such changes except when completely unavoidable.
- Anssi On Mon, Mar 21, 2016 at 10:40 AM, Shai Berger <s...@platonix.com> wrote: > My recommendation to backport is based on the observation that the peculiar > REPEATABLE READ behavior is highly conductive to data loss in the presence > of concurrency, combined with a sense that it is not very well known; I find > it much more likely that the change will fix broken code than break really > working code. > > On 21 במרץ 2016 09:59:25 GMT+02:00, "Anssi Kääriäinen" <akaar...@gmail.com> > wrote: >> >> I'm strongly -1 on changing the default isolation level in a minor >> release. We can recommend users switch the level and complain loudly >> if they don't. But just changing the isolation level has potential for >> breaking working code. >> >> - Anssi >> >> On Mon, Mar 21, 2016 at 9:27 AM, Shai Berger <s...@platonix.com> wrote: >>> >>> First of all, I would like to say that I strongly support the move to >>> READ >>> COMITTED, including backporting it to 1.8.x. >>> >>> But we also need to explain: REPEATABLE READ is a higher transaction >>> isolation >>> level than READ COMMITTED. If you have problematic code, it should lead >>> to >>> more deadlocks and/or transactions failing at commit time (compared to >>> READ >>> COMMITTED), not to data loss. The reason we get data losses is MySql's >>> unique >>> interpretation of REPEATABLE READ. If you're interested in the details >>> (and if >>> you use MySql, you should be), read on. >>> >>> With MySql's REPEATABLE READ, the "read" operations -- SELECT statements >>> -- >>> indeed act like they act in the usual REPEATABLE READ: Once you've read >>> some >>> table, changes made to that table by other transactions will not be >>> visible >>> within your transaction. But "write" operations -- UPDATE, DELETE, >>> INSERT and >>> the like -- act as if they're under READ COMMITTED, affecting (and >>> affected by) >>> changes committed by other transactions. The result is, essentially, >>> that >>> within a transaction, the reads are not guaranteed to be consistent with >>> the >>> writes [1]. >>> >>> In particular, in the bug[2] that caused this discussion, we get the >>> following >>> behavior in one transaction: >>> >>> (1) BEGIN TRANSACTION >>> >>> (2) SELECT ... FROM some_table WHERE some_field=some_value >>> (1 row returned) >>> >>> (3) (some other transactions commit) >>> >>> (4) SELECT ... FROM some_table WHERE some_field=some_value >>> (1 row returned, same as above) >>> >>> (5) DELETE some_table WHERE some_field=some_value >>> (answer: 1 row deleted) >>> >>> (6) SELECT ... FROM some_table WHERE some_field=some_value >>> (1 row returned, same as above) >>> >>> (7) COMMIT >>> (the row that was returned earlier is no longer in the >>> database) >>> >>> Take a minute to read this. Up to step (5), everything is as you would >>> expect; >>> you should find steps (6) and (7) quite surprising. >>> >>> This happens because the other transactions in (3) deleted the row that >>> is >>> returned in (2), (4) & (6), and inserted another one where >>> some_field=some_value; that other row is the row that was deleted in >>> (5). The >>> row that this transaction selects was not seen by the DELETE, and hence >>> not >>> changed by it, and hence continues to be visible by the SELECTs in our >>> transaction. But when we commit, the row (which has been deleted) no >>> longer >>> exists. >>> >>> I have expressed elsewhere my opinion of this behavior as a general >>> database >>> feature, and feel no need to repeat it here; but I think that, if >>> possible, it >>> is Django's job as a framework to protect its users from it, at least as >>> a >>> default. >>> >>> On Monday 21 March 2016 02:25:37 Cristiano Coelho wrote: >>>> >>>> What performance changes can you expect doing this change? It is >>>> probably >>>> that default on MySQL for a good reason. >>> >>> >>> The Django project is usually willing to give up quite a lot of >>> performance in >>> order to prevent data losses. I agree that this default on MySql is >>> probably >>> for a reason, but I don't think it can be a good reason for Django. >>> >>> Have fun, >>> Shai. >>> >>> [1] https://dev.mysql.com/doc/refman/5.7/en/innodb-consistent-read.html >>> [2] https://code.djangoproject.com/ticket/26347 > > > -- > Sent from my Android device with K-9 Mail. 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