==============================

Managing database transactions

==============================

.. currentmodule:: django.db

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 :setting:`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:

:class:`~django.middleware.cache.CacheMiddleware`,

:class:`~django.middleware.cache.UpdateCacheMiddleware`, and

:class:`~django.middleware.cache.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 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.

.. _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):

        ....

    @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.

``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):

        ....

    @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.

``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()

    @transaction.commit_manually(using="my_other_database")

    def viewfunc2(request):

        ....

.. 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()``.

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:

.. method:: transaction.savepoint(using=None)

    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, using=None)

    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, using=None)

    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.

Database-level autocommit

-------------------------

.. versionadded:: 1.1

With PostgreSQL 8.2 or later, there is an advanced option to run PostgreSQL

with :doc:`database-level autocommit </ref/databases>`. 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

    <topics-db-transactions-autocommit>`. 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.