author | Lennard de Rijk <ljvderijk@gmail.com> |
Mon, 13 Jul 2009 00:53:42 +0200 | |
changeset 2616 | 9321e100a7fb |
parent 109 | 620f9b141567 |
permissions | -rw-r--r-- |
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Managing database transactions |
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Django gives you a few ways to control how database transactions are managed, |
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if you're using a database that supports transactions. |
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Django's default transaction behavior |
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===================================== |
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Django's default behavior is to commit automatically when any built-in, |
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data-altering model function is called. For example, if you call |
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``model.save()`` or ``model.delete()``, the change will be committed |
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immediately. |
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This is much like the auto-commit setting for most databases. As soon as you |
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perform an action that needs to write to the database, Django produces the |
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``INSERT``/``UPDATE``/``DELETE`` statements and then does the ``COMMIT``. |
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There's no implicit ``ROLLBACK``. |
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Tying transactions to HTTP requests |
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=================================== |
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The recommended way to handle transactions in Web requests is to tie them to |
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the request and response phases via Django's ``TransactionMiddleware``. |
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It works like this: When a request starts, Django starts a transaction. If the |
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response is produced without problems, Django commits any pending transactions. |
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If the view function produces an exception, Django rolls back any pending |
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transactions. |
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To activate this feature, just add the ``TransactionMiddleware`` middleware to |
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your ``MIDDLEWARE_CLASSES`` setting:: |
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MIDDLEWARE_CLASSES = ( |
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'django.contrib.sessions.middleware.SessionMiddleware', |
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'django.middleware.common.CommonMiddleware', |
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'django.middleware.cache.CacheMiddleware', |
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'django.middleware.transaction.TransactionMiddleware', |
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) |
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The order is quite important. The transaction middleware applies not only to |
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view functions, but also for all middleware modules that come after it. So if |
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you use the session middleware after the transaction middleware, session |
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creation will be part of the transaction. |
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An exception is ``CacheMiddleware``, which is never affected. The cache |
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middleware uses its own database cursor (which is mapped to its own database |
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connection internally). |
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Controlling transaction management in views |
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=========================================== |
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For most people, implicit request-based transactions work wonderfully. However, |
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if you need more fine-grained control over how transactions are managed, you |
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can use Python decorators to change the way transactions are handled by a |
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particular view function. |
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.. note:: |
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Although the examples below use view functions as examples, these |
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decorators can be applied to non-view functions as well. |
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``django.db.transaction.autocommit`` |
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------------------------------------ |
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Use the ``autocommit`` decorator to switch a view function to Django's default |
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commit behavior, regardless of the global transaction setting. |
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Example:: |
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from django.db import transaction |
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@transaction.autocommit |
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def viewfunc(request): |
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.... |
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Within ``viewfunc()``, transactions will be committed as soon as you call |
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``model.save()``, ``model.delete()``, or any other function that writes to the |
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database. |
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``django.db.transaction.commit_on_success`` |
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------------------------------------------- |
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Use the ``commit_on_success`` decorator to use a single transaction for |
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all the work done in a function:: |
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from django.db import transaction |
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@transaction.commit_on_success |
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def viewfunc(request): |
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.... |
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If the function returns successfully, then Django will commit all work done |
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within the function at that point. If the function raises an exception, though, |
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Django will roll back the transaction. |
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``django.db.transaction.commit_manually`` |
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----------------------------------------- |
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Use the ``commit_manually`` decorator if you need full control over |
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transactions. It tells Django you'll be managing the transaction on your own. |
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If your view changes data and doesn't ``commit()`` or ``rollback()``, Django |
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will raise a ``TransactionManagementError`` exception. |
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Manual transaction management looks like this:: |
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from django.db import transaction |
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@transaction.commit_manually |
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def viewfunc(request): |
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... |
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# You can commit/rollback however and whenever you want |
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transaction.commit() |
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... |
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|
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# But you've got to remember to do it yourself! |
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try: |
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... |
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except: |
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transaction.rollback() |
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else: |
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transaction.commit() |
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|
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.. admonition:: An important note to users of earlier Django releases: |
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|
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The database ``connection.commit()`` and ``connection.rollback()`` methods |
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(called ``db.commit()`` and ``db.rollback()`` in 0.91 and earlier) no longer |
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exist. They've been replaced by ``transaction.commit()`` and |
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``transaction.rollback()``. |
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How to globally deactivate transaction management |
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================================================= |
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Control freaks can totally disable all transaction management by setting |
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``DISABLE_TRANSACTION_MANAGEMENT`` to ``True`` in the Django settings file. |
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|
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If you do this, Django won't provide any automatic transaction management |
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whatsoever. Middleware will no longer implicitly commit transactions, and |
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you'll need to roll management yourself. This even requires you to commit |
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changes done by middleware somewhere else. |
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|
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Thus, this is best used in situations where you want to run your own |
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transaction-controlling middleware or do something really strange. In almost |
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all situations, you'll be better off using the default behavior, or the |
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transaction middleware, and only modify selected functions as needed. |
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Transactions in MySQL |
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===================== |
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If you're using MySQL, your tables may or may not support transactions; it |
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depends on your MySQL version and the table types you're using. (By |
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"table types," we mean something like "InnoDB" or "MyISAM".) MySQL transaction |
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peculiarities are outside the scope of this article, but the MySQL site has |
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`information on MySQL transactions`_. |
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If your MySQL setup does *not* support transactions, then Django will function |
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in auto-commit mode: Statements will be executed and committed as soon as |
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they're called. If your MySQL setup *does* support transactions, Django will |
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handle transactions as explained in this document. |
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.. _information on MySQL transactions: http://dev.mysql.com/books/mysqlpress/mysql-tutorial/ch10.html |