Django v1.0 documentation

Managing database transactions

Django gives you a few ways to control how database transactions are managed, if you’re using a database that supports transactions.

Django’s default transaction behavior

Django’s default behavior is to commit automatically when any built-in, data-altering model function is called. For example, if you call model.save() or model.delete(), the change will be committed immediately.

This is much like the auto-commit setting for most databases. As soon as you perform an action that needs to write to the database, Django produces the INSERT/UPDATE/DELETE statements and then does the COMMIT. There’s no implicit ROLLBACK.

Tying transactions to HTTP requests

The recommended way to handle transactions in Web requests is to tie them to the request and response phases via Django’s TransactionMiddleware.

It works like this: When a request starts, Django starts a transaction. If the response is produced without problems, Django commits any pending transactions. If the view function produces an exception, Django rolls back any pending transactions.

To activate this feature, just add the TransactionMiddleware middleware to your MIDDLEWARE_CLASSES setting:

MIDDLEWARE_CLASSES = (
    'django.contrib.sessions.middleware.SessionMiddleware',
    'django.middleware.common.CommonMiddleware',
    'django.middleware.cache.CacheMiddleware',
    'django.middleware.transaction.TransactionMiddleware',
)

The order is quite important. The transaction middleware applies not only to view functions, but also for all middleware modules that come after it. So if you use the session middleware after the transaction middleware, session creation will be part of the transaction.

An exception is CacheMiddleware, which is never affected. The cache middleware uses its own database cursor (which is mapped to its own database connection internally).

Controlling transaction management in views

For most people, implicit request-based transactions work wonderfully. However, if you need more fine-grained control over how transactions are managed, you can use Python decorators to change the way transactions are handled by a particular view function.

Note

Although the examples below use view functions as examples, these decorators can be applied to non-view functions as well.

django.db.transaction.autocommit

Use the autocommit decorator to switch a view function to Django's default commit behavior, regardless of the global transaction setting.

Example:

from django.db import transaction

@transaction.autocommit
def viewfunc(request):
    ....

Within viewfunc(), transactions will be committed as soon as you call model.save(), model.delete(), or any other function that writes to the database.

django.db.transaction.commit_on_success

Use the commit_on_success decorator to use a single transaction for all the work done in a function:

from django.db import transaction

@transaction.commit_on_success
def viewfunc(request):
    ....

If the function returns successfully, then Django will commit all work done within the function at that point. If the function raises an exception, though, Django will roll back the transaction.

django.db.transaction.commit_manually

Use the commit_manually decorator if you need full control over transactions. It tells Django you'll be managing the transaction on your own.

If your view changes data and doesn't commit() or rollback(), Django will raise a TransactionManagementError exception.

Manual transaction management looks like this:

from django.db import transaction

@transaction.commit_manually
def viewfunc(request):
    ...
    # You can commit/rollback however and whenever you want
    transaction.commit()
    ...

    # But you've got to remember to do it yourself!
    try:
        ...
    except:
        transaction.rollback()
    else:
        transaction.commit()

An important note to users of earlier Django releases:

The database connection.commit() and connection.rollback() methods (called db.commit() and db.rollback() in 0.91 and earlier) no longer exist. They've been replaced by transaction.commit() and transaction.rollback().

How to globally deactivate transaction management

Control freaks can totally disable all transaction management by setting DISABLE_TRANSACTION_MANAGEMENT to True in the Django settings file.

If you do this, Django won't provide any automatic transaction management whatsoever. Middleware will no longer implicitly commit transactions, and you'll need to roll management yourself. This even requires you to commit changes done by middleware somewhere else.

Thus, this is best used in situations where you want to run your own transaction-controlling middleware or do something really strange. In almost all situations, you'll be better off using the default behavior, or the transaction middleware, and only modify selected functions as needed.

Transactions in MySQL

If you're using MySQL, your tables may or may not support transactions; it depends on your MySQL version and the table types you're using. (By "table types," we mean something like "InnoDB" or "MyISAM".) MySQL transaction peculiarities are outside the scope of this article, but the MySQL site has information on MySQL transactions.

If your MySQL setup does not support transactions, then Django will function in auto-commit mode: Statements will be executed and committed as soon as they're called. If your MySQL setup does support transactions, Django will handle transactions as explained in this document.