Re: [Django] #27683: Change default transaction isolation level to READ COMMITTED on MySQL

[Django]

#27683: Change default transaction isolation level to READ COMMITTED on MySQL
-+-
 Reporter:  Shai Berger  |Owner:  GitHub
 Type:   |  
  Cleanup/optimization   |   Status:  closed
Component:  Database layer   |  Version:  master
  (models, ORM)  |
 Severity:  Normal   |   Resolution:  fixed
 Keywords:   | Triage Stage:  Ready for
 |  checkin
Has patch:  1|  Needs documentation:  0
  Needs tests:  0|  Patch needs improvement:  0
Easy pickings:  0|UI/UX:  0
-+-

Comment (by GitHub ):

 In [changeset:"ab83d4d8fe76c2c6bc3b0a96aa2baed9c9f07f19" ab83d4d8]:
 {{{
 #!CommitTicketReference repository=""
 revision="ab83d4d8fe76c2c6bc3b0a96aa2baed9c9f07f19"
 Added multi_db=True to test cases that access the 'other' db connection.

 Fixed a failure in the context processors tests when running in
 reverse on MySQL due to an extra query after refs #27683.
 }}}

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Re: [Django] #27683: Change default transaction isolation level to READ COMMITTED on MySQL

[Django]

#27683: Change default transaction isolation level to READ COMMITTED on MySQL
-+-
 Reporter:  Shai Berger  |Owner:  GitHub
 Type:   |  
  Cleanup/optimization   |   Status:  closed
Component:  Database layer   |  Version:  master
  (models, ORM)  |
 Severity:  Normal   |   Resolution:  fixed
 Keywords:   | Triage Stage:  Ready for
 |  checkin
Has patch:  1|  Needs documentation:  0
  Needs tests:  0|  Patch needs improvement:  0
Easy pickings:  0|UI/UX:  0
-+-
Changes (by GitHub ):

 * owner:  (none) => GitHub 
 * status:  new => closed
 * resolution:   => fixed


Comment:

 In [changeset:"924af638e4d4fb8eb46a19ac0cafcb2e83480cf3" 924af638]:
 {{{
 #!CommitTicketReference repository=""
 revision="924af638e4d4fb8eb46a19ac0cafcb2e83480cf3"
 Fixed #27683 -- Made MySQL default to the read committed isolation level.

 Thanks Shai Berger for test help and Adam Johnson for review.
 }}}

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Re: [Django] #27683: Change default transaction isolation level to READ COMMITTED on MySQL

[Django]

#27683: Change default transaction isolation level to READ COMMITTED on MySQL
-+-
 Reporter:  Shai Berger  |Owner:  (none)
 Type:   |   Status:  new
  Cleanup/optimization   |
Component:  Database layer   |  Version:  master
  (models, ORM)  |
 Severity:  Normal   |   Resolution:
 Keywords:   | Triage Stage:  Ready for
 |  checkin
Has patch:  1|  Needs documentation:  0
  Needs tests:  0|  Patch needs improvement:  0
Easy pickings:  0|UI/UX:  0
-+-
Changes (by Shai Berger):

 * owner:  Shai Berger => (none)
 * status:  assigned => new
 * stage:  Accepted => Ready for checkin


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Re: [Django] #27683: Change default transaction isolation level to READ COMMITTED on MySQL

[Django]

#27683: Change default transaction isolation level to READ COMMITTED on MySQL
-+-
 Reporter:  Shai Berger  |Owner:  Shai
 Type:   |  Berger
  Cleanup/optimization   |   Status:  assigned
Component:  Database layer   |  Version:  master
  (models, ORM)  |
 Severity:  Normal   |   Resolution:
 Keywords:   | Triage Stage:  Accepted
Has patch:  1|  Needs documentation:  0
  Needs tests:  0|  Patch needs improvement:  0
Easy pickings:  0|UI/UX:  0
-+-

Comment (by GitHub ):

 In [changeset:"c4e18bb1cebc78e1e42c765608248ce3ead9deb7" c4e18bb]:
 {{{
 #!CommitTicketReference repository=""
 revision="c4e18bb1cebc78e1e42c765608248ce3ead9deb7"
 Refs #27683 -- Split up MySQL isolation level tests.
 }}}

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Re: [Django] #27683: Change default transaction isolation level to READ COMMITTED on MySQL

[Django]

#27683: Change default transaction isolation level to READ COMMITTED on MySQL
-+-
 Reporter:  Shai Berger  |Owner:  Shai
 Type:   |  Berger
  Cleanup/optimization   |   Status:  assigned
Component:  Database layer   |  Version:  master
  (models, ORM)  |
 Severity:  Normal   |   Resolution:
 Keywords:   | Triage Stage:  Accepted
Has patch:  1|  Needs documentation:  0
  Needs tests:  0|  Patch needs improvement:  0
Easy pickings:  0|UI/UX:  0
-+-
Changes (by Tim Graham):

 * has_patch:  0 => 1


Comment:

 [https://github.com/django/django/pull/7978 PR] to change the default
 isolation to read committed.

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Re: [Django] #27683: Change default transaction isolation level to READ COMMITTED on MySQL

[Django]

#27683: Change default transaction isolation level to READ COMMITTED on MySQL
-+-
 Reporter:  Shai Berger  |Owner:  Shai
 Type:   |  Berger
  Cleanup/optimization   |   Status:  assigned
Component:  Database layer   |  Version:  master
  (models, ORM)  |
 Severity:  Normal   |   Resolution:
 Keywords:   | Triage Stage:  Accepted
Has patch:  0|  Needs documentation:  0
  Needs tests:  0|  Patch needs improvement:  0
Easy pickings:  0|UI/UX:  0
-+-
Changes (by Tim Graham):

 * has_patch:  1 => 0


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Re: [Django] #27683: Change default transaction isolation level to READ COMMITTED on MySQL

[Django]

#27683: Change default transaction isolation level to READ COMMITTED on MySQL
-+-
 Reporter:  Shai Berger  |Owner:  Shai
 Type:   |  Berger
  Cleanup/optimization   |   Status:  assigned
Component:  Database layer   |  Version:  master
  (models, ORM)  |
 Severity:  Normal   |   Resolution:
 Keywords:   | Triage Stage:  Accepted
Has patch:  1|  Needs documentation:  0
  Needs tests:  0|  Patch needs improvement:  0
Easy pickings:  0|UI/UX:  0
-+-

Comment (by GitHub ):

 In [changeset:"f01ad1cb6a27669fd44b2079089e14f9dc4d3ca3" f01ad1c]:
 {{{
 #!CommitTicketReference repository=""
 revision="f01ad1cb6a27669fd44b2079089e14f9dc4d3ca3"
 Refs #27683 -- Allowed setting isolation level in DATABASES ['OPTIONS'] on
 MySQL.
 }}}

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Re: [Django] #27683: Change default transaction isolation level to READ COMMITTED on MySQL

[Django]

#27683: Change default transaction isolation level to READ COMMITTED on MySQL
-+-
 Reporter:  Shai Berger  |Owner:  Shai
 Type:   |  Berger
  Cleanup/optimization   |   Status:  assigned
Component:  Database layer   |  Version:  master
  (models, ORM)  |
 Severity:  Normal   |   Resolution:
 Keywords:   | Triage Stage:  Accepted
Has patch:  1|  Needs documentation:  0
  Needs tests:  0|  Patch needs improvement:  0
Easy pickings:  0|UI/UX:  0
-+-

Comment (by Tim Graham):

 Thanks. Here's the [https://groups.google.com/d/topic/django-
 developers/JiXalqmohUc/discussion django-developers thread].

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Re: [Django] #27683: Change default transaction isolation level to READ COMMITTED on MySQL

[Django]

#27683: Change default transaction isolation level to READ COMMITTED on MySQL
-+-
 Reporter:  Shai Berger  |Owner:  Shai
 Type:   |  Berger
  Cleanup/optimization   |   Status:  assigned
Component:  Database layer   |  Version:  master
  (models, ORM)  |
 Severity:  Normal   |   Resolution:
 Keywords:   | Triage Stage:  Accepted
Has patch:  1|  Needs documentation:  0
  Needs tests:  0|  Patch needs improvement:  0
Easy pickings:  0|UI/UX:  0
-+-

Comment (by Shai Berger):

 The system check was suggested by Adam in comment:1, and I thought the
 analogy with strict-mode holds. I felt odd about there being both the
 check and the deprecation warning (and commented about it in the original
 PR) but yes, I felt that the fact they were both mostly silent (the check
 is only vocal in `migrate` by default) sort-of justifies keeping both.

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Re: [Django] #27683: Change default transaction isolation level to READ COMMITTED on MySQL

[Django]

#27683: Change default transaction isolation level to READ COMMITTED on MySQL
-+-
 Reporter:  Shai Berger  |Owner:  Shai
 Type:   |  Berger
  Cleanup/optimization   |   Status:  assigned
Component:  Database layer   |  Version:  master
  (models, ORM)  |
 Severity:  Normal   |   Resolution:
 Keywords:   | Triage Stage:  Accepted
Has patch:  1|  Needs documentation:  0
  Needs tests:  0|  Patch needs improvement:  0
Easy pickings:  0|UI/UX:  0
-+-

Comment (by Tim Graham):

 Was your thinking that the system check is needed because the deprecation
 warning is silent by default or did you have another idea in mind?

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Re: [Django] #27683: Change default transaction isolation level to READ COMMITTED on MySQL

[Django]

#27683: Change default transaction isolation level to READ COMMITTED on MySQL
-+-
 Reporter:  Shai Berger  |Owner:  Shai
 Type:   |  Berger
  Cleanup/optimization   |   Status:  assigned
Component:  Database layer   |  Version:  master
  (models, ORM)  |
 Severity:  Normal   |   Resolution:
 Keywords:   | Triage Stage:  Accepted
Has patch:  1|  Needs documentation:  0
  Needs tests:  0|  Patch needs improvement:  0
Easy pickings:  0|UI/UX:  0
-+-

Comment (by Shai Berger):

 Replying to [comment:13 Tim Graham]:
 > With the current patch, would the system check ever trigger in Django
 2.1 when the default transaction level changes?

 No, it never would -- there will be no case where the user hasn't
 specified a preference and the isolation level would still be REPEATABLE
 READ.

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Re: [Django] #27683: Change default transaction isolation level to READ COMMITTED on MySQL

[Django]

#27683: Change default transaction isolation level to READ COMMITTED on MySQL
-+-
 Reporter:  Shai Berger  |Owner:  Shai
 Type:   |  Berger
  Cleanup/optimization   |   Status:  assigned
Component:  Database layer   |  Version:  master
  (models, ORM)  |
 Severity:  Normal   |   Resolution:
 Keywords:   | Triage Stage:  Accepted
Has patch:  1|  Needs documentation:  0
  Needs tests:  0|  Patch needs improvement:  0
Easy pickings:  0|UI/UX:  0
-+-

Comment (by Tim Graham):

 Ok, I'll write to the mailing list shortly. With the current patch, would
 the system check ever trigger in Django 2.1 when the default transaction
 level changes?

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Re: [Django] #27683: Change default transaction isolation level to READ COMMITTED on MySQL

[Django]

#27683: Change default transaction isolation level to READ COMMITTED on MySQL
-+-
 Reporter:  Shai Berger  |Owner:  Shai
 Type:   |  Berger
  Cleanup/optimization   |   Status:  assigned
Component:  Database layer   |  Version:  master
  (models, ORM)  |
 Severity:  Normal   |   Resolution:
 Keywords:   | Triage Stage:  Accepted
Has patch:  1|  Needs documentation:  0
  Needs tests:  0|  Patch needs improvement:  0
Easy pickings:  0|UI/UX:  0
-+-

Comment (by Shai Berger):

 At a first approximation, I suspect it is better to be cautious about this
 sort of change. However, this may be a good question to ask the community.

 The deprecation period may be required because although the warnings and
 settings are at the project level, there may be code changes required at
 the app level, and it may be better to keep the new projects aware of the
 issue while 3rd parties adapt.

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Re: [Django] #27683: Change default transaction isolation level to READ COMMITTED on MySQL

[Django]

#27683: Change default transaction isolation level to READ COMMITTED on MySQL
-+-
 Reporter:  Shai Berger  |Owner:  Shai
 Type:   |  Berger
  Cleanup/optimization   |   Status:  assigned
Component:  Database layer   |  Version:  master
  (models, ORM)  |
 Severity:  Normal   |   Resolution:
 Keywords:   | Triage Stage:  Accepted
Has patch:  1|  Needs documentation:  0
  Needs tests:  0|  Patch needs improvement:  0
Easy pickings:  0|UI/UX:  0
-+-

Comment (by Tim Graham):

 As I thought about this again, I wanted to ask if a deprecation cycle
 brings any value compared to this alternate strategy. It doesn't seem to
 me that projects need two release cycles to choose their isolation level
 -- in the easiest case they just specify what's being used now. If we
 changed the default isolation level now, we could have a "compatibility"
 system check raise a warning if `'isolation_level'` isn't specified in the
 database settings and end up in the same place (where all MySQL projects
 have to specify an isolation level to silence a warning). We could remove
 that check in Django 2.0 and avoid another 8 months of new MySQL projects
 having a check or deprecation warning out of the box.

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Re: [Django] #27683: Change default transaction isolation level to READ COMMITTED on MySQL

[Django]

#27683: Change default transaction isolation level to READ COMMITTED on MySQL
-+-
 Reporter:  Shai Berger  |Owner:  Shai
 Type:   |  Berger
  Cleanup/optimization   |   Status:  assigned
Component:  Database layer   |  Version:  master
  (models, ORM)  |
 Severity:  Normal   |   Resolution:
 Keywords:   | Triage Stage:  Accepted
Has patch:  1|  Needs documentation:  0
  Needs tests:  0|  Patch needs improvement:  0
Easy pickings:  0|UI/UX:  0
-+-
Changes (by Shai Berger):

 * needs_better_patch:  1 => 0


Comment:

 Review comments addressed.

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Re: [Django] #27683: Change default transaction isolation level to READ COMMITTED on MySQL

[Django]

#27683: Change default transaction isolation level to READ COMMITTED on MySQL
-+-
 Reporter:  Shai Berger  |Owner:  Shai
 Type:   |  Berger
  Cleanup/optimization   |   Status:  assigned
Component:  Database layer   |  Version:  master
  (models, ORM)  |
 Severity:  Normal   |   Resolution:
 Keywords:   | Triage Stage:  Accepted
Has patch:  1|  Needs documentation:  0
  Needs tests:  0|  Patch needs improvement:  1
Easy pickings:  0|UI/UX:  0
-+-
Changes (by Tim Graham):

 * needs_better_patch:  0 => 1


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Re: [Django] #27683: Change default transaction isolation level to READ COMMITTED on MySQL

[Django]

#27683: Change default transaction isolation level to READ COMMITTED on MySQL
-+-
 Reporter:  Shai Berger  |Owner:  Shai
 Type:   |  Berger
  Cleanup/optimization   |   Status:  assigned
Component:  Database layer   |  Version:  master
  (models, ORM)  |
 Severity:  Normal   |   Resolution:
 Keywords:   | Triage Stage:  Accepted
Has patch:  1|  Needs documentation:  0
  Needs tests:  0|  Patch needs improvement:  0
Easy pickings:  0|UI/UX:  0
-+-
Changes (by Shai Berger):

 * owner:  nobody => Shai Berger
 * status:  new => assigned
 * has_patch:  0 => 1


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Re: [Django] #27683: Change default transaction isolation level to READ COMMITTED on MySQL

[Django]

#27683: Change default transaction isolation level to READ COMMITTED on MySQL
-+-
 Reporter:  Shai Berger  |Owner:  nobody
 Type:   |   Status:  new
  Cleanup/optimization   |
Component:  Database layer   |  Version:  master
  (models, ORM)  |
 Severity:  Normal   |   Resolution:
 Keywords:   | Triage Stage:  Accepted
Has patch:  0|  Needs documentation:  0
  Needs tests:  0|  Patch needs improvement:  0
Easy pickings:  0|UI/UX:  0
-+-

Comment (by Shai Berger):

 [https://github.com/django/django/pull/7826 PR]

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Re: [Django] #27683: Change default transaction isolation level to READ COMMITTED on MySQL

[Django]

#27683: Change default transaction isolation level to READ COMMITTED on MySQL
-+-
 Reporter:  Shai Berger  |Owner:  nobody
 Type:   |   Status:  new
  Cleanup/optimization   |
Component:  Database layer   |  Version:  master
  (models, ORM)  |
 Severity:  Normal   |   Resolution:
 Keywords:   | Triage Stage:  Accepted
Has patch:  0|  Needs documentation:  0
  Needs tests:  0|  Patch needs improvement:  0
Easy pickings:  0|UI/UX:  0
-+-

Comment (by Shai Berger):

 So, with respect to the option to use checks and deprecation warnings:

 1) We can deprecate whatever we like whenever we like, the LTS limitation
 is only about removing deprecation shims in the same major version (so,
 shim introduced in 1.11 **can** be removed in 2.1).
 2) Annoyance stands, so I think the correct path is at least to minimize
 it by explicitly supporting isolation-level in `OPTIONS`. That's also the
 easy route (both for users and implementation) to deprecation.

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Re: [Django] #27683: Change default transaction isolation level to READ COMMITTED on MySQL

[Django]

#27683: Change default transaction isolation level to READ COMMITTED on MySQL
-+-
 Reporter:  Shai Berger  |Owner:  nobody
 Type:   |   Status:  new
  Cleanup/optimization   |
Component:  Database layer   |  Version:  master
  (models, ORM)  |
 Severity:  Normal   |   Resolution:
 Keywords:   | Triage Stage:  Accepted
Has patch:  0|  Needs documentation:  0
  Needs tests:  0|  Patch needs improvement:  0
Easy pickings:  0|UI/UX:  0
-+-
Changes (by Tim Graham):

 * needs_docs:  1 => 0
 * type:  New feature => Cleanup/optimization
 * severity:  Release blocker => Normal
 * stage:  Unreviewed => Accepted


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Re: [Django] #27683: Change default transaction isolation level to READ COMMITTED on MySQL

[Django]

#27683: Change default transaction isolation level to READ COMMITTED on MySQL
-+-
 Reporter:  Shai Berger  |Owner:  nobody
 Type:  New feature  |   Status:  new
Component:  Database layer   |  Version:  master
  (models, ORM)  |
 Severity:  Release blocker  |   Resolution:
 Keywords:   | Triage Stage:
 |  Unreviewed
Has patch:  0|  Needs documentation:  1
  Needs tests:  0|  Patch needs improvement:  0
Easy pickings:  0|UI/UX:  0
-+-

Comment (by Aymeric Augustin):

 Like I said in earlier discussions, I'm in favor of doing this change.

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Re: [Django] #27683: Change default transaction isolation level to READ COMMITTED on MySQL

[Django]

#27683: Change default transaction isolation level to READ COMMITTED on MySQL
-+-
 Reporter:  Shai Berger  |Owner:  nobody
 Type:  New feature  |   Status:  new
Component:  Database layer   |  Version:  master
  (models, ORM)  |
 Severity:  Release blocker  |   Resolution:
 Keywords:   | Triage Stage:
 |  Unreviewed
Has patch:  0|  Needs documentation:  1
  Needs tests:  0|  Patch needs improvement:  0
Easy pickings:  0|UI/UX:  0
-+-

Old description:

> There have been plenty of bug reports and discussions about the problems
> caused for users by MySQL's implementation of the REPEATABLE READ
> transaction isolation level, and the fact that it is the default. Django
> is mostly written for READ COMMITTED; of all supported backends, MySQL is
> the only one to use a different level by default. Things will probably
> run more in line with users expectations under READ COMMITTED, and
> reusable apps' behavior will be more similar across database backends.
>
> Some history: This has been raised already in the [#13906 1.2 era], and
> possibly even before that; the question was raised in different forms
> [#14026 again] and [#21670 again] until one more [#26347 instance] caused
> it to be brought up on the mailing list. To be sure, it wasn't the first
> time the issue was brought to the list either, but
> [https://groups.google.com/forum/#!msg/django-
> developers/6pWbpV1_6Us/a3CNmojACwAJ this discussion] seemed to conclude
> with a rough consensus that the default transaction isolation level for
> MySQL should change. As far as I'm aware, the issue has not been
> discussed since then.
>
> There are two kinds of backwards-compatibility problems I already see
> with this change:
>
> 1) The obvious one -- Apps that are written to be correct under MySQL's
> REPEATABLE READ isolation level may become subtly incorrect by default.
> This is a very real problem, and hard to tackle because writing tests to
> capture the problems is very hard -- one needs to make and use at least
> two separate connections to the same database, and the Django ORM (and
> testing framework) does not make that easy. The counter-argument to that
> is that we likely have plenty of apps -- reusable or otherwise -- that
> are currently subtly wrong because MySQL's REPEATABLE READ semantics is
> surprising and unintuitive.
>
> 2) The less obvious one -- the way to set transaction isolation levels is
> with SQL executed when the connection is opened, and we need to make sure
> we don't interrupt with the user's own `init_command` if there is one.
>
> Some very basic intro to transaction isolation levels in general and
> MySQL's REPEATABLE READ in particular is in my DC.EU.2016 lightning talk
> about this:
> https://opbeat.com/community/posts/lightning-talks-day-1/ (starting
> around 4:00).

New description:

 There have been plenty of bug reports and discussions about the problems
 caused for users by MySQL's implementation of the REPEATABLE READ
 transaction isolation level, and the fact that it is the default. Django
 is mostly written for READ COMMITTED; of all supported backends, MySQL is
 the only one to use a different level by default. Things will probably run
 more in line with users expectations under READ COMMITTED, and reusable
 apps' behavior will be more similar across database backends.

 Some history: This has been raised already in the [ticket:13906 1.2 era],
 and possibly even before that; the question was raised in different forms
 [ticket:14026 again] and [ticket:21670 again] until one more [ticket:26347
 instance] caused it to be brought up on the mailing list. To be sure, it
 wasn't the first time the issue was brought to the list either, but
 [https://groups.google.com/forum/#!msg/django-
 developers/6pWbpV1_6Us/a3CNmojACwAJ this discussion] seemed to conclude
 with a rough consensus that the default transaction isolation level for
 MySQL should change. As far as I'm aware, the issue has not been discussed
 since then.

 There are two kinds of backwards-compatibility problems I already see with
 this change:

 1) The obvious one -- Apps that are written to be correct under MySQL's
 REPEATABLE READ isolation level may become subtly incorrect by default.
 This is a very real problem, and hard to tackle because writing tests to
 capture the problems is very hard -- one needs to make and use at least
 two separate connections to the same database, and the Django ORM (and
 testing framework) does not make that easy. The counter-argument to that
 is that we likely have plenty of apps -- reusable or 

Re: [Django] #27683: Change default transaction isolation level to READ COMMITTED on MySQL

[Django]

#27683: Change default transaction isolation level to READ COMMITTED on MySQL
-+-
 Reporter:  Shai Berger  |Owner:  nobody
 Type:  New feature  |   Status:  new
Component:  Database layer   |  Version:  master
  (models, ORM)  |
 Severity:  Release blocker  |   Resolution:
 Keywords:   | Triage Stage:
 |  Unreviewed
Has patch:  0|  Needs documentation:  1
  Needs tests:  0|  Patch needs improvement:  0
Easy pickings:  0|UI/UX:  0
-+-

Comment (by Shai Berger):

 Replying to [comment:1 Adam Chainz]:
 > An alternative to changing the default would be to have a system check
 highly recommending READ-COMMITTED [...] "Warning: In Django 2.1 the
 default tx_isolation will change..."

 That may indeed be the way to go. Two issues arise:

 1) We need to make sure which deprecations can happen when (WRT our
 deprecation policies around LTSs)
 2) It may be seen as annoying: "if you know that it's correct to use that
 other level, why don't you just go and do it? why bug me?"

 The big advantage is that it completely solves my 2nd backward-
 compatibility issue.

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Ticket URL: 
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Re: [Django] #27683: Change default transaction isolation level to READ COMMITTED on MySQL

[Django]

#27683: Change default transaction isolation level to READ COMMITTED on MySQL
-+-
 Reporter:  Shai Berger  |Owner:  nobody
 Type:  New feature  |   Status:  new
Component:  Database layer   |  Version:  master
  (models, ORM)  |
 Severity:  Release blocker  |   Resolution:
 Keywords:   | Triage Stage:
 |  Unreviewed
Has patch:  0|  Needs documentation:  1
  Needs tests:  0|  Patch needs improvement:  0
Easy pickings:  0|UI/UX:  0
-+-

Comment (by Adam Chainz):

 An alternative to changing the default would be to have a system check
 highly recommending READ-COMMITTED, similar to what we do for MySQL's
 `sql_mode` (see
 
https://github.com/django/django/blob/master/django/db/backends/mysql/validation.py#L6
 ). This would also help educate users a bit more about the change.
 Possibly this could be part of a path to changing the default... "Warning:
 In Django 2.1 the default tx_isolation will change..."

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