Hello,

Currently you cannot do this:

from django.db import connection
connection.xid()  # or any other TPC method

Adding implementations of TPC methods in BaseDatabaseWrapper() that simply 
forward to the underlying connection object is the first step for a database 
agnostic implementation in Django.


For the high level API, here’s one possibility:

# entering the block calls .xid(format_id , global_transaction_id , 
branch_qualifier) and .tpc_begin(xid)
with transaction.atomic2(format_id , global_transaction_id , branch_qualifier) 
as prepare: 

    # run statements

    prepare()

    # check if others are ready to commit
    # raise an exception to abort

# exiting the block calls .tpc_commit() or .tpc_rollback() depending on whether 
there’s an exception

I’m proposing a separate context manager because I’m worried about increasing 
again the complexity of transaction.atomic. There will be a significant amount 
of duplication between the two implementations, though.


The proposal above doesn’t account for recovery: .tpc_recover(), 
.tpc_commit(xid), .tpc_rollback(xid). I’m not sure what recovery of a two phase 
transaction is and I can’t say if it needs to be supported in the API.


Also I didn’t talk about savepoints. I assume they can be supported like in 
regular transactions.


I hope this helps,

-- 
Aymeric.

> On 1 Dec 2016, at 12:27, Mateusz Mikołajczyk <mikolajczyk.mate...@gmail.com> 
> wrote:
> 
> Hello, fellow devs.
> 
> I have been googling intensively in order to see whether somebody already 
> raised such issue, but so far I have been unsuccesful. Therefore, trembling 
> on my legs, I decided to write to the devlist as suggested in the docs.
> 
> I am trying to extend the atomic decorator / context statement in order to do 
> 'prepare transaction \'foo\'' rather than usual 'commit' on succesful 
> transaction. It is, however, not the usual scenario where django would talk 
> to multiple databases. What I have in mind is a bunch of microservices, one 
> of them which would be django application. Therefore django app would be 
> talking to the external transaction manager which would then take care of 
> executing the appropriate transactions inside of each microservice.
> I suppose that after a bunch of hacking I could implement this with some 
> monkey patching of the original atomic() code but it clearly is not the way 
> to go.
> 
> I then started to think how this could be done database-agnostic way. I know 
> that PostgreSQL supports this with 'prepare transaction' statement, but I 
> suppose that other databases have different syntax for this kind of behaviour 
> and some don't support this feature at all. Therefore I thought that in e.g. 
> SQLite3 (or other database which doesn't support this natively), this 
> behavior could be 'emulated'. Therefore I thought of the following pseudocode:
> 
> ```
> with transaction.atomic(commit_hook=lambda connection: 
> connection.prepare_transaction('foo'))
> 
> OR
> 
> with transaction.atomic(prepare_transaction='foo')
> ```
> 
> When you would do CRUD operations they would be instead serialized inside a 
> special table, and then, after issuing another command, say ..
> 
> from django.db import transaction
> transaction.commit_prepared('foo')
> 
> they would be applied using regular 'atomic' call. (or.. I don't know, Django 
> could raise an IntegrityError if the database doesn't support distributed 
> transactions and the code would try to execute them)
> 
> Do you think that this is realistic or is it a wrong approach to the subject?
> 
> kind regards,
> 
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