If anybody is interested, I created a proof of concept code for PostgreSQL which extends existing Atomic context:
https://github.com/toudi/django-tpc-proof-of-concept the actual implementation can be found inside tpc/atomic_tpc.py and there are two commands available: ./manage.py prepare ./manage.py commit if atomic() would have been extendible (i.e. if one could replace connection.commit() with something else ) then this whole lot of monkeypatching wouldn't be necesary. What do you guys think? Also, has nobody ever written stuff like this? Or am searching the wrong way? cheers, toudi W dniu piątek, 2 grudnia 2016 22:02:44 UTC+1 użytkownik Aymeric Augustin napisał: > > Hello, > > To be honest I’m pessimistic about the feasibility of emulating > transactional behavior — pretty much the most complicated and low level > thing databases do — in the application. I don’t think that would be > considered suitable for Django. > > Usually Django handles such cases with a database feature flag and make > the methods no-ops on databases that don’t support the corresponding > features. For instance that’s how Django ignores transactions on MySQL + > MyISAM. > > Best regards, > > -- > Aymeric. > > On 2 Dec 2016, at 20:43, Mateusz Mikołajczyk <mikolajcz...@gmail.com > <javascript:>> wrote: > > Well, I suppose that it would either lead to very obfuscated > implementation code, or very weird syntax (client code). As for your first > argument ( promise that the transaction is unlikely to fail ): > > from django.db import distributed: > > with distributed('foo') as foo: > MyModel.get_or_create(field=123) > > then, before calling the emulated behavior, the db would have to: > > * do all the operations (like it would normally do with regular commit - > thus checking every constrainst and so on) > * then do a rollback (so that it doesn't store the actual values in the db) > * then serialize them in separate journal (the additional model I > mentioned - an analogy to an actual separate journal of PostgreSQL) > > Utterly ugly / hacky solution if you ask me, but please keep in mind that > this would be only emulation of the actual algorithm for the databases > which don't support this standart > > As for the relations, I have thought a lot about it and the only > pseudocode I could think of was utterly ugly as well: > > with distributed('foo') as foo: > foo.add(MyModel.objects.get_or_create, {'field': 123}, > namespace='mymodel') > foo.add(MyOtherModel.objects.create, {'my_model_id': > ('from-namespace', 'mymodel')}) > > Theoretically, both of these syntaxes could co-exist. If you wouldn't have > any relations, you could use the cleaner syntax. > > So I'd say it would technically be possible but would lead to very, very, > very ugly code (at least in the second scenario with relations). And I > realize that this is not an option in the Django world. > > I understand that because of all the above it is unlikely to create a nice > interface which would work in database-agnostic way, therefore Django would > have to throw IntegrityError if somebody would be trying to do distributed > transaction on non-supported database? But if that's the case then this > code doesn't really belong in the django core, does it? Which means that > I'm probably left with the monkey-patching thing :( Or .. ? I have to > prepare this functionality either way - because I need it ;) > > Thank you for all the answers ! > > W dniu piątek, 2 grudnia 2016 14:32:51 UTC+1 użytkownik Patryk Zawadzki > napisał: >> >> W dniu piątek, 2 grudnia 2016 12:05:11 UTC+1 użytkownik Mateusz >> Mikołajczyk napisał: >>> >>> What would you say about checking which CRUD operations were executed >>> within atomic() call (in order to serialize them and save into a special >>> model for databases which don't support this functionality) ? Is it >>> realistic? >>> >> >> It would likely break the promise that distributed two-step transactions >> give you: that once all statements are prepared the transaction is unlikely >> to fail during commit. In this case the commit would mean "start over and >> try to repeat my steps" at which point any of the recorded statements is >> likely to fail constraint checks. (Even more so if your code used >> get_or_create().) >> >> Also how would relations work? You begin a transaction, create a Foo >> instance and the returned PK is 5. You assign it to child models. At this >> point the transaction is saved and rolled back. During replay the insert >> returns PK = 7, at this point there's no way to detect that some of the >> stored fives should now be treated as sevens while some should remain fives. >> > > -- > You received this message because you are subscribed to the Google Groups > "Django developers (Contributions to Django itself)" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to django-developers+unsubscr...@googlegroups.com <javascript:>. > To post to this group, send email to django-d...@googlegroups.com > <javascript:>. > Visit this group at https://groups.google.com/group/django-developers. > To view this discussion on the web visit > https://groups.google.com/d/msgid/django-developers/8d4ca04f-cdcd-4d8b-a060-fc5598b3baf8%40googlegroups.com > > <https://groups.google.com/d/msgid/django-developers/8d4ca04f-cdcd-4d8b-a060-fc5598b3baf8%40googlegroups.com?utm_medium=email&utm_source=footer> > . > For more options, visit https://groups.google.com/d/optout. > > > -- You received this message because you are subscribed to the Google Groups "Django developers (Contributions to Django itself)" group. 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