.. _topics-db-transactions: ============================== 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.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. .. _topics-db-transactions-autocommit: ``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() .. admonition:: 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. .. _topics-db-transactions-savepoints: 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: .. method:: 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). .. method:: 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. .. method:: 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. .. _information on MySQL transactions: http://dev.mysql.com/doc/refman/5.0/en/sql-syntax-transactions.html 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. Transaction rollback -------------------- 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. Savepoint rollback ------------------ If you are using PostgreSQL 8 or later, you can use :ref:`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. Database-level autocommit ------------------------- .. versionadded:: 1.1 With PostgreSQL 8.2 or later, there is an advanced option to run PostgreSQL with :ref:`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 :ref:`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.