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.
>
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