#23718: TEST_MIRROR setting doesn't work as expected (and has no tests)
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Reporter: Ilya Baryshev | Owner: Simon
| Charette
Type: Bug | Status: assigned
Component: Testing framework | Version: 1.7
Severity: Normal | Resolution:
Keywords: replica testing | Triage Stage: Accepted
Has patch: 1 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 1
Easy pickings: 0 | UI/UX: 0
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Comment (by Simon Charette):
@Rag Sagar.V, sure feel free to assign the ticket to yourself.
I'd start by having a look at [https://github.com/django/django/pull/9603
the previous PR] and make sure to understand the rationale behind why it
got rejected.
For what it's worth I gave this ticket a 2-3 hours try by trying to point
`connections['mirror']` at `connections['mirrored']` Python objects for
the duration of `TestCase` and I got some failures related to transaction
handling. I think
[https://github.com/django/django/pull/9603#issuecomment-388347770 Tim did
a good job at nailing the issues] with this approach. In short transaction
handing is alias based so if something goes wrong with
`connections['mirrored']` then it's also surfaced in
`connections['mirror']` which differs from what would happen in a real
life scenario.
The further I got from getting things working were by setting the
isolation level on the miror/replica to `READ UNCOMMITED` for the duration
of the test to allow it to see changes within the testcase transaction.
This isolation level is not supported on all backends though (e.g.
PostgreSQL) and was quite unreliable on MySQL from my minimal testing.
That one might be hard one if you're not familiar with how `TestCase`
transactions wrapping takes place but you'll definitely learn a lot by
giving this ticket a shot and seeing how the suite fails. Happy to review
your changes if you get stuck on Github.
--
Ticket URL: <https://code.djangoproject.com/ticket/23718#comment:25>
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