.. _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.

``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
<topics-db-transactions-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.
