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Is “transaction.atomic” same as “transaction.commit_on_success”?

Posted by: admin November 30, 2017 Leave a comment

Questions:

Django 1.6 proposes @transaction.atomic as part of the rehaul in the transaction management from 1.5.

I have a function which is called by a Django management command which is in turn called by cron, i.e. no HTTP request triggering transactions in this case. Snippet:

from django.db import transaction

@transaction.commit_on_success
def my_function():
    # code here

In the above code block commit_on_success uses a single transaction for all the work done in my_function.

Does replacing @transaction.commit_on_success with @transaction.atomic result in the identical behaviour? @transaction.atomic docs state:

Atomicity is the defining property of database transactions. atomic
allows us to create a block of code within which the atomicity on the
database is guaranteed. If the block of code is successfully
completed, the changes are committed to the database. If there is an
exception, the changes are rolled back.

I take it that they result in the same behaviour; correct?

Answers:

Yes. You should use atomic in the places where you previously used commit_on_success.

Since the new transaction system is designed to be more robust and consistent, though, it’s possible that you could see different behavior. For example, if you catch database errors and try to continue on you will see a TransactionManagementError, whereas the previous behavior was undefined and probably case-dependent.

But, if you’re doing things properly, everything should continue to work the same way.

Questions:
Answers:

Based on the documentation I have read on the subject, there is a significant difference when these decorators are nested.

Nesting two atomic blocks does not work the same as nesting two commit_on_success blocks.

The problem is that there are two guarantees that you would like to have from these blocks.

  • You would like the content of the block to be atomic, either everything inside the block is committed, or nothing is committed.
  • You would like durability, once you have left the block without an exception you are guaranteed, that everything you wrote inside the block is persistent.

It is impossible to provide both guarantees when blocks are nested. If an exception is raised after leaving the innermost block but before leaving the outermost block, you will have to fail in one of two ways:

  • Fail to provide durability for the innermost block.
  • Fail to provide atomicity for the outermost block.

Here is where you find the difference. Using commit_on_success would give durability for the innermost block, but no atomicity for the outermost block. Using atomic would give atomicity for the outermost block, but no durability for the innermost block.

Simply raising an exception in case of nesting could prevent you from running into the problem. The innermost block would always raise an exception, thus it never promises any durability. But this loses some flexibility.

A better solution would be to have more granularity about what you are asking for. If you can separately ask for atomicity and durability, then you can perform nesting. You just have to ensure that every block requesting durability is outside those requesting atomicity. Requesting durability inside a block requesting atomicity would have to raise an exception.

atomic is supposed to provide the atomicity part. As far as I can tell django 1.6.1 does not have a decorator, which can ask for durability. I tried to write one, and posted it on codereview.