Django 提供多种方式控制数据库事务。
Django 默认的事务行为是自动提交。除非事务正在执行，每个查询将会马上自动提交到数据库。 详见.
Django 自动使用事务或还原点，以确保需多次查询的 ORM 操作的一致性，特别是 delete() 和 update() 操作.
由于性能原因，Django 的 TestCase 类同样将每个测试用事务封装起来。
在 Web 里，处理事务比较常用的方式是将每个请求封装在一个事务中。 在你想启用该行为的数据库中，把配置中的参数 ATOMIC_REQUESTS 设置为 True。
它是这样工作的：在调用试图方法前，Django 先生成一个事务。如果响应能正常生成，Django 会提交该事务。而如果视图出现异常，Django 则会回滚该事务。
你可以在你的视图代码中使用还原点执行子事务，一般会使用 :func:`atomic 上下文管理器。但是，在视图结束时，要么所有的更改都被提交，要么所有的更改都不被提交。
警告
While the simplicity of this transaction model is appealing, it also makes it inefficient when traffic increases. Opening a transaction for every view has some overhead. The impact on performance depends on the query patterns of your application and on how well your database handles locking.
Per-request transactions and streaming responses
When a view returns a StreamingHttpResponse, reading
the contents of the response will often execute code to generate the
content. Since the view has already returned, such code runs outside of
the transaction.
Generally speaking, it isn't advisable to write to the database while generating a streaming response, since there's no sensible way to handle errors after starting to send the response.
In practice, this feature simply wraps every view function in the atomic()
decorator described below.
Note that only the execution of your view is enclosed in the transactions. Middleware runs outside of the transaction, and so does the rendering of template responses.
When ATOMIC_REQUESTS is enabled, it's
still possible to prevent views from running in a transaction.
non_atomic_requests(using=None)[源代码]¶This decorator will negate the effect of ATOMIC_REQUESTS for a given view:
from django.db import transaction
@transaction.non_atomic_requests
def my_view(request):
do_stuff()
@transaction.non_atomic_requests(using='other')
def my_other_view(request):
do_stuff_on_the_other_database()
It only works if it's applied to the view itself.
Django provides a single API to control database transactions.
atomic(using=None, savepoint=True)[源代码]¶Atomicity is the defining property of database transactions. atomic
allows us to create a block of code within which the atomicity on the
database is guaranteed. If the block of code is successfully completed, the
changes are committed to the database. If there is an exception, the
changes are rolled back.
atomic blocks can be nested. In this case, when an inner block
completes successfully, its effects can still be rolled back if an
exception is raised in the outer block at a later point.
atomic is usable both as a decorator:
from django.db import transaction
@transaction.atomic
def viewfunc(request):
# This code executes inside a transaction.
do_stuff()
and as a context manager:
from django.db import transaction
def viewfunc(request):
# This code executes in autocommit mode (Django's default).
do_stuff()
with transaction.atomic():
# This code executes inside a transaction.
do_more_stuff()
Wrapping atomic in a try/except block allows for natural handling of
integrity errors:
from django.db import IntegrityError, transaction
@transaction.atomic
def viewfunc(request):
create_parent()
try:
with transaction.atomic():
generate_relationships()
except IntegrityError:
handle_exception()
add_children()
In this example, even if generate_relationships() causes a database
error by breaking an integrity constraint, you can execute queries in
add_children(), and the changes from create_parent() are still
there. Note that any operations attempted in generate_relationships()
will already have been rolled back safely when handle_exception() is
called, so the exception handler can also operate on the database if
necessary.
Avoid catching exceptions inside atomic!
When exiting an atomic block, Django looks at whether it's exited
normally or with an exception to determine whether to commit or roll
back. If you catch and handle exceptions inside an atomic block,
you may hide from Django the fact that a problem has happened. This
can result in unexpected behavior.
This is mostly a concern for DatabaseError and its
subclasses such as IntegrityError. After such an
error, the transaction is broken and Django will perform a rollback at
the end of the atomic block. If you attempt to run database
queries before the rollback happens, Django will raise a
TransactionManagementError. You may
also encounter this behavior when an ORM-related signal handler raises
an exception.
The correct way to catch database errors is around an atomic block
as shown above. If necessary, add an extra atomic block for this
purpose. This pattern has another advantage: it delimits explicitly
which operations will be rolled back if an exception occurs.
If you catch exceptions raised by raw SQL queries, Django's behavior is unspecified and database-dependent.
You may need to manually revert model state when rolling back a transaction.
The values of a model's fields won't be reverted when a transaction rollback happens. This could lead to an inconsistent model state unless you manually restore the original field values.
For example, given MyModel with an active field, this snippet
ensures that the if obj.active check at the end uses the correct
value if updating active to True fails in the transaction:
from django.db import DatabaseError, transaction
obj = MyModel(active=False)
obj.active = True
try:
with transaction.atomic():
obj.save()
except DatabaseError:
obj.active = False
if obj.active:
...
In order to guarantee atomicity, atomic disables some APIs. Attempting
to commit, roll back, or change the autocommit state of the database
connection within an atomic block will raise an exception.
atomic takes a using argument which should be the name of a
database. If this argument isn't provided, Django uses the "default"
database.
Under the hood, Django's transaction management code:
atomic block;atomic block;You can disable the creation of savepoints for inner blocks by setting the
savepoint argument to False. If an exception occurs, Django will
perform the rollback when exiting the first parent block with a savepoint
if there is one, and the outermost block otherwise. Atomicity is still
guaranteed by the outer transaction. This option should only be used if
the overhead of savepoints is noticeable. It has the drawback of breaking
the error handling described above.
You may use atomic when autocommit is turned off. It will only use
savepoints, even for the outermost block.
Performance considerations
Open transactions have a performance cost for your database server. To
minimize this overhead, keep your transactions as short as possible. This
is especially important if you're using atomic() in long-running
processes, outside of Django's request / response cycle.
In the SQL standards, each SQL query starts a transaction, unless one is already active. Such transactions must then be explicitly committed or rolled back.
This isn't always convenient for application developers. To alleviate this problem, most databases provide an autocommit mode. When autocommit is turned on and no transaction is active, each SQL query gets wrapped in its own transaction. In other words, not only does each such query start a transaction, but the transaction also gets automatically committed or rolled back, depending on whether the query succeeded.
PEP 249, the Python Database API Specification v2.0, requires autocommit to be initially turned off. Django overrides this default and turns autocommit on.
为了避免这种情况，你可以参考 deactivate the transaction management<deactivate-transaction-management> ，但并不推荐这样做。
You can totally disable Django's transaction management for a given database
by setting AUTOCOMMIT to False in its
configuration. If you do this, Django won't enable autocommit, and won't
perform any commits. You'll get the regular behavior of the underlying
database library.
This requires you to commit explicitly every transaction, even those started by Django or by third-party libraries. Thus, this is best used in situations where you want to run your own transaction-controlling middleware or do something really strange.
Sometimes you need to perform an action related to the current database transaction, but only if the transaction successfully commits. Examples might include a Celery task, an email notification, or a cache invalidation.
Django provides the on_commit() function to register callback functions
that should be executed after a transaction is successfully committed:
将任意函数（无参数）传递给 on_commit():
from django.db import transaction
def do_something():
pass # send a mail, invalidate a cache, fire off a Celery task, etc.
transaction.on_commit(do_something)
你也可以使用 lambda:: 包装函数
transaction.on_commit(lambda: some_celery_task.delay('arg1'))
The function you pass in will be called immediately after a hypothetical
database write made where on_commit() is called would be successfully
committed.
无任何活动事务时调用 on_commit() ，则回调函数会立即执行。
If that hypothetical database write is instead rolled back (typically when an
unhandled exception is raised in an atomic() block), your function will
be discarded and never called.
Savepoints (i.e. nested atomic() blocks) are handled correctly. That is,
an on_commit() callable registered after a savepoint (in a nested
atomic() block) will be called after the outer transaction is committed,
but not if a rollback to that savepoint or any previous savepoint occurred
during the transaction:
with transaction.atomic(): # Outer atomic, start a new transaction
transaction.on_commit(foo)
with transaction.atomic(): # Inner atomic block, create a savepoint
transaction.on_commit(bar)
# foo() and then bar() will be called when leaving the outermost block
On the other hand, when a savepoint is rolled back (due to an exception being raised), the inner callable will not be called:
with transaction.atomic(): # Outer atomic, start a new transaction
transaction.on_commit(foo)
try:
with transaction.atomic(): # Inner atomic block, create a savepoint
transaction.on_commit(bar)
raise SomeError() # Raising an exception - abort the savepoint
except SomeError:
pass
# foo() will be called, but not bar()
事务提交后的的回调函数执行顺序与当初注册时的顺序一致。
If one on-commit function within a given transaction raises an uncaught
exception, no later registered functions in that same transaction will run.
This is, of course, the same behavior as if you'd executed the functions
sequentially yourself without on_commit().
Your callbacks are executed after a successful commit, so a failure in a
callback will not cause the transaction to roll back. They are executed
conditionally upon the success of the transaction, but they are not part of
the transaction. For the intended use cases (mail notifications, Celery tasks,
etc.), this should be fine. If it's not (if your follow-up action is so
critical that its failure should mean the failure of the transaction itself),
then you don't want to use the on_commit() hook. Instead, you may want
two-phase commit such as the psycopg Two-Phase Commit protocol support
and the optional Two-Phase Commit Extensions in the Python DB-API
specification.
Callbacks are not run until autocommit is restored on the connection following the commit (because otherwise any queries done in a callback would open an implicit transaction, preventing the connection from going back into autocommit mode).
When in autocommit mode and outside of an atomic() block, the function
will run immediately, not on commit.
On-commit functions only work with autocommit mode
and the atomic() (or ATOMIC_REQUESTS) transaction API. Calling on_commit() when
autocommit is disabled and you are not within an atomic block will result in an
error.
Django's TestCase class wraps each test in a transaction
and rolls back that transaction after each test, in order to provide test
isolation. This means that no transaction is ever actually committed, thus your
on_commit() callbacks will never be run. If you need to test the results
of an on_commit() callback, use a
TransactionTestCase instead.
事务回滚钩子相比事务提交钩子更难实现，因为各种各样的情况都可能造成隐式回滚。
For instance, if your database connection is dropped because your process was killed without a chance to shut down gracefully, your rollback hook will never run.
解决方法很简单，与其在执行事务时（原子操作）进行某项操作，当事务执行失败后再取消这项操作，不如使用 on_commit() 来延迟该项操作，直到事务成功后再进行操作。毕竟事务成功后你才能确保之后的操作是有意义的。
警告
Always prefer atomic() if possible at all. It accounts for the
idiosyncrasies of each database and prevents invalid operations.
The low level APIs are only useful if you're implementing your own transaction management.
Django provides a straightforward API in the django.db.transaction
module to manage the autocommit state of each database connection.
这些函数使接受一个 using 参数表示所要操作的数据库。如果未提供，则 Django 使用 "default" 数据库。
自动提交默认为开启，如果你将它关闭，自己承担后果。
一旦你关闭了自动提交， Django 将无法帮助你，数据库将会按照你使用的数据库适配器的默认行为进行操作。虽然适配器的标准经过了 PEP 249 详细规定，但不同适配器的实现方式并不总是一致的。你需要谨慎地查看你所使用的适配器的文档。
在关闭自动提交之前，你必须确保当前没有活动的事务，通常你可以执行 commit() 或者 rollback() 函数以达到该条件。
当一个原子 atomic() 事务处于活动状态时， Django 将会拒绝关闭自动提交的请求，因为这样会破坏原子性。
事务是指具有原子性的一系列数据库操作。即使你的程序崩溃，数据库也会确保这些操作要么全部完成要么全部都未执行。
Django doesn't provide an API to start a transaction. The expected way to
start a transaction is to disable autocommit with set_autocommit().
Once you're in a transaction, you can choose either to apply the changes
you've performed until this point with commit(), or to cancel them with
rollback(). These functions are defined in django.db.transaction.
这些函数使接受一个 using 参数表示所要操作的数据库。如果未提供，则 Django 使用 "default" 数据库。
当一个原子 atomic() 事务处于活动状态时， Django 将会拒绝进行事务提交或者事务回滚，因为这样会破坏原子性。
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 with the SQLite, PostgreSQL, Oracle, and MySQL (when using the InnoDB storage engine) backends. Other backends provide the savepoint functions, but they're empty operations -- they don't actually do anything.
Savepoints aren't especially useful if you are using autocommit, the default
behavior of Django. However, once you open a transaction with atomic(),
you 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().
When the atomic() decorator is nested, it creates a savepoint to allow
partial commit or rollback. You're strongly encouraged to use atomic()
rather than the functions described below, but they're still part of the
public API, and there's no plan to deprecate them.
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 functions in django.db.transaction:
savepoint(using=None)[源代码]¶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).
savepoint_commit(sid, using=None)[源代码]¶Releases savepoint sid. The changes performed since the savepoint was
created become part of the transaction.
These functions do nothing if savepoints aren't supported or if the database is in autocommit mode.
In addition, there's a utility function:
The following example demonstrates the use of savepoints:
from django.db import transaction
# open a transaction
@transaction.atomic
def viewfunc(request):
a.save()
# transaction now contains a.save()
sid = transaction.savepoint()
b.save()
# 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()
Savepoints may be used to recover from a database error by performing a partial
rollback. If you're doing this inside an atomic() block, the entire block
will still be rolled back, because it doesn't know you've handled the situation
at a lower level! To prevent this, you can control the rollback behavior with
the following functions.
Setting the rollback flag to True forces a rollback when exiting the
innermost atomic block. This may be useful to trigger a rollback without
raising an exception.
Setting it to False prevents such a rollback. Before doing that, make sure
you've rolled back the transaction to a known-good savepoint within the current
atomic block! Otherwise you're breaking atomicity and data corruption may
occur.
While SQLite supports savepoints, a flaw in the design of the sqlite3
module makes them hardly usable.
When autocommit is enabled, savepoints don't make sense. When it's disabled,
sqlite3 commits implicitly before savepoint statements. (In fact, it
commits before any statement other than SELECT, INSERT, UPDATE,
DELETE and REPLACE.) This bug has two consequences:
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 always function in autocommit 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.
注解
This section is relevant only if you're implementing your own transaction
management. This problem cannot occur in Django's default mode and
atomic() handles it automatically.
Inside a transaction, 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". While 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.
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
sid = transaction.savepoint()
try:
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.
1月 11, 2019