Any feedback about my 2 proposals, to know if it is worth to spend time to 
propose a patch for one of those?

Le mardi 5 juillet 2016 18:54:27 UTC+1, Romain Garrigues a écrit :
> Markus, I like the idea, which is definitely better than my idea of new 
> option to recreate it manually when we know we have to.
> I can try to investigate a bit if you think that could lead to something 
> that makes sense.
> Two ideas I have in mind after a quick look at migrate command line code:
> 1/ Extract the code related to the "plan" (created by 
> executor.migration_plan(targets) function) to be also used somewhere else 
> (in clone_test_db for example...)
> 2/ Make "migrate" command return a sort of report (number of migrations 
> applied, ...) of what happened during a "migrate" call, that could then be 
> used in db.backends.base.creation.BaseDatabaseCreation.create_test_db and 
> passed to the connection.creation.clone_test_db loop block, moving from 
> "django.test.runner.setup_databases" to 
> "db.backends.base.creation.BaseDatabaseCreation.create_test_db" (as cloned 
> databases have a link with the state of the default one, it can justify 
> this move).
> This parallel option is really great and coming with some environment 
> constraints, as you said Aymeric, but for big projects, the gain is so 
> impressive that I will do all I can to help on that!
> I have the benchmark in my todo list, do you think it makes sense to 
> update the current PR with one of these 2 propositions explained above?
> Romain.
> Le mardi 5 juillet 2016 12:48:38 UTC+1, Markus Holtermann a écrit :
>> Hi, 
>> it might be a shot in the dark, but can't we check if Django's 
>> testrunner applied new migrations in which case we drop the cloned 
>> databases and recreate them. If all migrations already existed we keep 
>> the clones the way they are? 
>> /Markus 
>> On Tue, Jul 05, 2016 at 09:00:25AM +0200, Aymeric Augustin wrote: 
>> >Hello, 
>> > 
>> >I’ll try to clarify what I said in the PR below.. 
>> > 
>> >The main reason for the `--parallel` option was to make Django’s own 
>> test suite faster. A full run went down from ~8m to ~1m30 when I committed 
>> that patch, which really helps the development cycle on invasive patches. 
>> > 
>> >Since Django’s own test runner bypasses migrations, whenever you make 
>> changes to a model in Django’s test suite, you need a run without 
>> `--keepdb`. So the problem you’re describing doesn’t exist for the primary 
>> use case. 
>> > 
>> >Now let’s talk about models and migrations in users’ projects, which is 
>> the logical next step and the use case you’re trying to improve. 
>> > 
>> >Note that the `--parallel` option is experimental and often 
>> non-functional in this context: as soon as two tests hit a resource other 
>> than the database — say, the cache — they can stomp upon one another. 
>> > 
>> >> On 05 Jul 2016, at 00:22, Romain Garrigues <> 
>> wrote: 
>> >> 
>> >> We could have just documented this limitation, but I don't think that 
>> my situation is a really rare edge case in terms of process, so I was 
>> suggesting to add a new option to be able to reset the cloned databases if 
>> needed (let's name it --parallel-clone-reset). 
>> > 
>> >When I make changes to models, usually I keep removing and recreating a 
>> single migration, which is incompatible with using the `--keepdb` option. 
>> Whenever I make changes, I run without `--keepdb` once. 
>> > 
>> >> I don't really like the idea of adding a new option, as it impacts the 
>> test runner, the clone_test_db function signature, ... but I have not found 
>> a better idea to at the same time keep the performances with --keepdb and 
>> --parallel, and handle these newly added migrations to a project. 
>> > 
>> >I’m not a fan of a new option either… 
>> > 
>> >> To summarize my proposal, this option (--parallel-clone-reset, or any 
>> other name) should be used only if you are using --keepdb and --parallel 
>> options at the same time, and when you have added a new migration between 2 
>> test run. 
>> > 
>> >IIRC this will more than double the run time of Django’s own test suite 
>> on MySQL: it will increase from ~2m to ~4m (give or take 30s) because 
>> cloning databases is slow on MySQL. 
>> > 
>> >I’m quoting all these figures from memory and I may mix them up. As I 
>> said on the ticket it would be useful to redo the benchmark on a first run 
>> and subsequent run of `./` on PostgreSQL and MySQL. 
>> > 
>> >You could argue that it’s best to degrade the experience of a few Django 
>> contributors (original use case, Django’s test suite) for the benefits of 
>> the wider community (new use case, projects’ test suites). However the 
>> original use case is known to work and I don’t believe that the new use 
>> case works well enough in general, at least not without some engineering to 
>> isolate tests from one another. For this reason I’m not convinced by this 
>> argument. 
>> > 
>> >I hope this clarifies the context of the trade-off we’re discussing. 
>> > 
>> >Best regards, 
>> > 
>> >-- 
>> >Aymeric. 
>> > 
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