Django’s default transaction behavior¶

Django’s default behavior is to run with an open transaction which it commits 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.

from django.db import transaction

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

from django.db import transaction

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

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()

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.

Savepoints¶

A savepoint is a marker within a transaction that enables you to roll back part of a transaction, rather than the full transaction. Savepoints are available to the PostgreSQL 8 and Oracle backends. Other backends will provide the savepoint functions, but they are empty operations - they won't actually do anything.

Savepoints aren't especially useful if you are using the default autocommit behaviour of Django. However, if you are using commit_on_success or commit_manually, each open transaction will build up a series of database operations, awaiting a commit or rollback. If you issue a rollback, the entire transaction is rolled back. Savepoints provide the ability to perform a fine-grained rollback, rather than the full rollback that would be performed by transaction.rollback().

Savepoints are controlled by three methods on the transaction object:

transaction.savepoint()¶

Creates a new savepoint. This marks a point in the transaction that is known to be in a "good" state.

Returns the savepoint ID (sid).

transaction.savepoint_commit(sid)¶

Updates the savepoint to include any operations that have been performed since the savepoint was created, or since the last commit.

transaction.savepoint_rollback(sid)¶

Rolls the transaction back to the last point at which the savepoint was committed.

The following example demonstrates the use of savepoints:

from django.db import transaction

@transaction.commit_manually
def viewfunc(request):

  a.save()
  # open transaction now contains a.save()
  sid = transaction.savepoint()

  b.save()
  # open transaction now contains a.save() and b.save()

  if want_to_keep_b:
      transaction.savepoint_commit(sid)
      # open transaction still contains a.save() and b.save()
  else:
      transaction.savepoint_rollback(sid)
      # open transaction now contains only a.save()

  transaction.commit()

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.

Handling exceptions within PostgreSQL transactions¶

When a call to a PostgreSQL cursor raises an exception (typically IntegrityError), all subsequent SQL in the same transaction will fail with the error "current transaction is aborted, queries ignored until end of transaction block". Whilst simple use of save() is unlikely to raise an exception in PostgreSQL, there are more advanced usage patterns which might, such as saving objects with unique fields, saving using the force_insert/force_update flag, or invoking custom SQL.

There are several ways to recover from this sort of error.

a.save() # Succeeds, but may be undone by transaction rollback
try:
    b.save() # Could throw exception
except IntegrityError:
    transaction.rollback()
c.save() # Succeeds, but a.save() may have been undone

a.save() # Succeeds, and never undone by savepoint rollback
try:
    sid = transaction.savepoint()
    b.save() # Could throw exception
    transaction.savepoint_commit(sid)
except IntegrityError:
    transaction.savepoint_rollback(sid)
c.save() # Succeeds, and a.save() is never undone

a.save() # succeeds
try:
    b.save() # Could throw exception
except IntegrityError:
    pass
c.save() # succeeds