Hey Simon,
It would be possible to write a method as you've suggested above. But it
entails the following problems:
1. There would have to be a wrapper for each of the transaction APIs, not
just transaction.atomic since all of them take the using kwarg.
2. I'm trying to build a library that helps achieve multi-tenancy (the
isolated DB approach) by dynamically routing DB queries to the appropriate
databases at runtime. For more info, check this out -> PyCon India 2021
proposal
<https://in.pycon.org/cfp/2021/proposals/django-and-multi-tenancy-the-isolated-database-approach~dR6oO/>.
If that be the case, for any new code base, I could expose a bunch of
methods as suggested above and ask them to incorporate it. But for
older/existing code bases which already directly use various transaction
APIs, it would be hard to replace all lines with another method.
By inspecting code, I figured that there is this method named
get_connection in the transaction.py module which is being used by all the
other transaction APIs for fetching the DB connection. Currently I've
patched this method with my own to achieve the functionality mentioned
above. Might I suggest an alternate implementation for this method as
follows:
#settings.py
TRANSACTION_DB_SELECTOR = "path_to_some_callable"
#transaction.py
...
transaction_db_selector = import_string(settings.TRANSACTION_DB_SELECTOR)
def get_connection():
if transaction_db_selector:
using = transaction_db_selector()
if using is None:
using = DEFAULT_DB_ALIAS
return connections[using]
Let me know your thoughts on this.
Regards,
Aditya N
On Friday, May 28, 2021 at 10:05:26 AM UTC+5:30 charettes wrote:
> Ticket that directed to the mailing list for wider feedback
> https://code.djangoproject.com/ticket/32788
>
> ---
>
> Can you think of places where this db_for_transaction hook would differ
> in any way from what db_for_write returns? That's what Django uses
> internally in such instances
>
> 1.
>
> https://github.com/django/django/blob/0d67481a6664a1e66d875eef59b96ed489060601/django/db/models/base.py#L745-L760
>
>
> <https://github.com/django/django/blob/0d67481a6664a1e66d875eef59b96ed489060601/django/db/models/base.py#L745-L760>
>
> 2.
>
> https://github.com/django/django/blob/0d67481a6664a1e66d875eef59b96ed489060601/django/db/models/base.py#L950
>
>
> <https://github.com/django/django/blob/0d67481a6664a1e66d875eef59b96ed489060601/django/db/models/base.py#L950>
>
> 3.
>
> https://github.com/django/django/blob/0d67481a6664a1e66d875eef59b96ed489060601/django/db/models/deletion.py#L400
>
>
> <https://github.com/django/django/blob/0d67481a6664a1e66d875eef59b96ed489060601/django/db/models/deletion.py#L400>
>
>
> I get that your asking for a way to avoid explicitly passing atomic(using)
> all over the place but I'm having a hard time figuring out in which cases a
> db_for_transaction hook, which cannot be provided any contextual details
> like other router methods do, can take an an advised decision without
> relying on contextual/global state.
>
> Is there a reason you can't replace your wide spread internal uses of
> atomic by something along these lines
>
> def routed_atomic(savepoint=True, durable=False):
>
> using = get_global_using()
>
> return transaction.atomic(using, savepoint=savepoint, durable=durable)
>
> Cheers,
>
> Simon
> Le jeudi 27 mai 2021 à 12:18:50 UTC-4, gojeta...@gmail.com a écrit :
>
>> From the Django docs, for any ORM query made, the DB alias to use is
>> determined by the following rules:
>>
>> - Checks if the using keyword is used either as a parameter in the
>> function call or chained with QuerySet.
>> - Consults the DB routers in order until a match is found.
>> - Falls back to the default router which always returns default as
>> the alias.
>>
>> Using the router scheme works perfectly for ORM queries. However, when it
>> comes to using transaction APIs (like the transaction.atomic construct),
>> it becomes mandatory to specify the *using* kwarg explicitly in all of
>> its publicly exposed APIs if a DB other than the default alias has to be
>> chosen.
>>
>> To illustrate why this is a problem, consider the following scenario:
>>
>> - A DB router exists such that it directs queries to a specific
>> database based on a value set in *thread-local* storage by a
>> middleware.
>> - When ORM queries from within the view are fired, they automatically
>> get forwarded to the right DB *without explicitly citing* the using
>> keyword
>> owing to the router.
>> - But if the transaction.atomic construct is used, the using keyword
>> would have to be explicitly specified each time. While this might seem
>> fine, it creates the following problems:
>> 1. For large code bases, it becomes highly unwieldy to make sure
>> that the *using* keyword has been mentioned in every transaction
>> API call. Also, if one tries to implement the above scheme in an
>> already
>> existing code base, it would be impractical to add the using keyword
>> in all
>> lines calling the transaction APIs.
>> 2. It doesn't gel well with the the routers framework.
>>
>> A proposed workaround could to be to add a separate method named
>> *db_for_transaction* to the routers framework which would be consulted
>> by the transaction APIs first, before falling back to using the default DB
>> alias.
>>
>>
>> 1. Having a separate method for transaction is good because it need
>> not be mangled up with the other methods i.e db_for_read / db_for_write
>> as
>> they clearly indicate certain operational semantics which aren't tied to
>> a
>> transaction since it can have both reads and writes within it.
>> 2. Also, if in the future, there is some special info that can be
>> delivered to the transaction based on which a decision regd which DB to
>> use
>> could be made, then it would be cleanly isolated into its own method.
>>
>>
>> If we can finalise on this, I could submit a PR for the same.
>>
>
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