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 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.
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).
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. All of the decorators take an option using parameter which should be the alias for a database connection for which the behavior applies to. If no alias is specified then the "default" database is used.
Note
Although the examples below use view functions as examples, these decorators can be applied to non-view functions as well.
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):
....
@transaction.autocommit(using="my_other_database")
def viewfunc2(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. viewfunc2() will have this same behavior, but for the "my_other_database" connection.
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):
....
@transaction.commit_on_success(using="my_other_database")
def viewfunc2(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.
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()
@transaction.commit_manually(using="my_other_database")
def viewfunc2(request):
....
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().
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.
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().
Each of these functions takes a using argument which should be the name of a database for which the behavior applies. If no using argument is provided then the "default" database is used.
Savepoints are controlled by three methods on the transaction object:
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).
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()
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.
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.
The first option is to roll back the entire transaction. For example:
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
Calling transaction.rollback() rolls back the entire transaction. Any uncommitted database operations will be lost. In this example, the changes made by a.save() would be lost, even though that operation raised no error itself.
If you are using PostgreSQL 8 or later, you can use savepoints to control the extent of a rollback. Before performing a database operation that could fail, you can set or update the savepoint; that way, if the operation fails, you can roll back the single offending operation, rather than the entire transaction. For example:
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
In this example, a.save() will not be undone in the case where b.save() raises an exception.
With PostgreSQL 8.2 or later, there is an advanced option to run PostgreSQL with database-level autocommit. If you use this option, there is no constantly open transaction, so it is always possible to continue after catching an exception. For example:
a.save() # succeeds
try:
b.save() # Could throw exception
except IntegrityError:
pass
c.save() # succeeds
Note
This is not the same as the autocommit decorator. When using database level autocommit there is no database transaction at all. The autocommit decorator still uses transactions, automatically committing each transaction when a database modifying operation occurs.
Jul 05, 2010