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 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.middleware.cache.UpdateCacheMiddleware', 'django.contrib.sessions.middleware.SessionMiddleware', 'django.middleware.common.CommonMiddleware', 'django.middleware.transaction.TransactionMiddleware', 'django.middleware.cache.FetchFromCacheMiddleware', )
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.
The various cache middlewares are an exception: CacheMiddleware, UpdateCacheMiddleware, and FetchFromCacheMiddleware are never affected. Even when using database caching, Django's cache backend 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 a set of functions in django.db.transaction to control transactions on a per-function or per-code-block basis.
These functions, described in detail below, can be used in two different ways:
As a decorator on a particular function. For example:
from django.db import transaction @transaction.commit_on_success def viewfunc(request): # ... # this code executes inside a transaction # ...
This technique works with all supported version of Python (that is, with Python 2.4 and greater).
As a context manager around a particular block of code:
from django.db import transaction def viewfunc(request): # ... # this code executes using default transaction management # ... with transaction.commit_on_success(): # ... # this code executes inside a transaction # ...
The with statement is new in Python 2.5, and so this syntax can only be used with Python 2.5 and above.
For maximum compatibility, all of the examples below show transactions using the decorator syntax, but all of the follow functions may be used as context managers, too.
Although the examples below use view functions as examples, these decorators and context managers can be used anywhere in your code that you need to deal with transactions.
Use the autocommit decorator to switch a view function to Django's default commit behavior, regardless of the global transaction setting.
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): ....
Requirements for transaction handling¶
Django requires that every transaction that is opened is closed before the completion of a request. If you are using autocommit() (the default commit mode) or commit_on_success(), this will be done for you automatically. However, if you are manually managing transactions (using the commit_manually() decorator), you must ensure that the transaction is either committed or rolled back before a request is completed.
This applies to all database operations, not just write operations. Even if your transaction only reads from the database, the transaction must be committed or rolled back before you complete a request.
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.
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 behavior 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).
Updates the savepoint to include any operations that have been performed since the savepoint was created, or since the last commit.
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.
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
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.
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