Django memberikan anda sedikit cara mengendalikan bagaimana transaksi basisdata dikelola.
\nKebiasaan awal Django adalah menjalankan suasana penyeraahn otomatis. Setiap query segera diserahkan ke basisdata, meskipun transaksi adalah aktif. Lihat dibawah ini untuk rincian.
\nDjango menggunakan transaksi atau titiksimpan otomatis untuk menjamin kesatuan dari operasi ORM yang membutuhkan banyak query, khususnya query delete() dan update().
\nDjango's TestCase class also wraps each test in a\ntransaction for performance reasons.
Cara umum untuk menangani transaksi di jaringan adalah membungkus setiap permintaan di transaksi. Setel ATOMIC_REQUESTS ke True di konfigurasi dari setiap basisdata untuk anda ingin adakan kebiasaan ini.
It works like this. Before calling a view function, Django starts a\ntransaction. If the response is produced without problems, Django commits the\ntransaction. If the view produces an exception, Django rolls back the\ntransaction.
\nYou may perform subtransactions using savepoints in your view code, typically\nwith the atomic() context manager. However, at the end of the view,\neither all or none of the changes will be committed.
In practice, this feature simply wraps every view function in the atomic()\ndecorator described below.
Note that only the execution of your view is enclosed in the transactions.\nMiddleware runs outside of the transaction, and so does the rendering of\ntemplate responses.
\nWhen ATOMIC_REQUESTS is enabled, it's\nstill possible to prevent views from running in a transaction.
This decorator will negate the effect of ATOMIC_REQUESTS for a given view:
from django.db import transaction\n\n@transaction.non_atomic_requests\ndef my_view(request):\n do_stuff()\n\n@transaction.non_atomic_requests(using='other')\ndef my_other_view(request):\n do_stuff_on_the_other_database()\nIt only works if it's applied to the view itself.
\nDjango provides a single API to control database transactions.
\nAtomicity is the defining property of database transactions. atomic\nallows us to create a block of code within which the atomicity on the\ndatabase is guaranteed. If the block of code is successfully completed, the\nchanges are committed to the database. If there is an exception, the\nchanges are rolled back.
atomic blocks can be nested. In this case, when an inner block\ncompletes successfully, its effects can still be rolled back if an\nexception is raised in the outer block at a later point.
atomic is usable both as a decorator:
from django.db import transaction\n\n@transaction.atomic\ndef viewfunc(request):\n # This code executes inside a transaction.\n do_stuff()\ndan sebagai context manager:
\nfrom django.db import transaction\n\ndef viewfunc(request):\n # This code executes in autocommit mode (Django's default).\n do_stuff()\n\n with transaction.atomic():\n # This code executes inside a transaction.\n do_more_stuff()\nWrapping atomic in a try/except block allows for natural handling of\nintegrity errors:
from django.db import IntegrityError, transaction\n\n@transaction.atomic\ndef viewfunc(request):\n create_parent()\n\n try:\n with transaction.atomic():\n generate_relationships()\n except IntegrityError:\n handle_exception()\n\n add_children()\nIn this example, even if generate_relationships() causes a database\nerror by breaking an integrity constraint, you can execute queries in\nadd_children(), and the changes from create_parent() are still\nthere. Note that any operations attempted in generate_relationships()\nwill already have been rolled back safely when handle_exception() is\ncalled, so the exception handler can also operate on the database if\nnecessary.
In order to guarantee atomicity, atomic disables some APIs. Attempting\nto commit, roll back, or change the autocommit state of the database\nconnection within an atomic block will raise an exception.
atomic takes a using argument which should be the name of a\ndatabase. If this argument isn't provided, Django uses the "default"\ndatabase.
Dibawah tenda, kode pengelolaan transaksi Django:
\nmembuka sebuah transaksi ketika memasukkan blok atomic paling luar;
membuat titiksimpan ketika memasukkan blok atomic paling dalam;
melepaskan atau digulung kembali ke titik simpan ketika keluar blok paling dalam;
menyerahkan atau gulung kembali transaksi ketika keluar blok paling luar
Anda dapat meniadakan pembuatan dari titik simpan untuk blok paling dalam dengan mengatur argumen savepoint ke False. Jika sebuah pengecualian muncul, DJango akan melakukan gulunh kembali ketika keluar blok induk pertama dengan sebuah titik simpan jika ada, dan blok paling luar jika tidak. Atomicity masih dijamin oleh transaksi paling luar. Pilihan ini harus hanya digunakan jika kelebihan dari titik simpan dapat dilihat. Itu mempunyai kekurangan dari memecahkan penanganan kesalahan yang digambarkan diatas.
Anda dapat menggunakan atomic ketika penyerahan otomatis dimatikan. Itu akan hanya menggunakan titik simpan, bahkan untuk blok paling luar.
Dalam standar SQL, setiap permintaan SQL memulai sebuah transaksi, meskipun satu sudah aktif. Transaksi tersebut harus secara eksplisit diserahkan dan digulung kembali.
\nIni tidak selalu mudah untuk pengembang aplikasi. Untuk meredakan masalah ini, kebanyakan basisdata menyediakan sebuah suasana penyerahan otomatis. Ketika penyerahan otomatis dinayalan dan tidak ada transaksi aktif, setiap permintaan SQL dibungkus dalam transaksinya sendiri. Dengan kata lain, bukan hanya melakukan setiap permintaan tersebut mulai sebuah transaksi, tetapi transaksi juga mendapatkan otomatis diserahkan atau digulung kembali, tergandunt pada apakah permintaan berhasil.
\nPEP 249, the Python Database API Specification v2.0, requires autocommit to\nbe initially turned off. Django overrides this default and turns autocommit\non.
\nUntuk menghindari ini, anda dapat menonaktifkan pengelolaan transaksi, tetapi itu sangat tidak dianjurkan.
\nYou can totally disable Django's transaction management for a given database\nby setting AUTOCOMMIT to False in its\nconfiguration. If you do this, Django won't enable autocommit, and won't\nperform any commits. You'll get the regular behavior of the underlying\ndatabase library.
Ini membutuhkan anda utnuk menyerahkan secara eksplisit setiap transaksi, bahkan yang dimulai oleh Django atau oleh pustaka pihak ketiga. Jadi, ini adalah penggunaan terbaik dalam keadaan dimana anda ingin menjalankan middleware pengendalian transaksi milik anda atau melakukan sesuatu yang sangat aneh.
\nTerkadang anda butuh melakukan sebuah tindakan terkait pada transaksi basisdata saat ini, tetapi hanya jika transaksi berhasil diserahkan. Contoh mungkin termasuk tugas Celery, sebuah pemberitahuan surel, atau penghapusan tembolok
\nDjango menyediakan fungsi on_commit() untuk mendaftarkan fungsi panggil kembali yang harusnya dijalankan setelah transaksi berhasil diserahkan:
Lewati fungsi apapun (yang tidak mengambil argumen) ke on_commit():
from django.db import transaction\n\ndef do_something():\n pass # send a mail, invalidate a cache, fire off a Celery task, etc.\n\ntransaction.on_commit(do_something)\nAnda dapat juga membungkus fungsi anda dalam lambda:
\ntransaction.on_commit(lambda: some_celery_task.delay('arg1'))\nFungsi anda lewati akan dipanggil segera setelah hipotetis penulisan basisdata dibuat dimana on_commit() dipanggil akan berhasil diserahkan.
Jika anda memanggil on_commit() selagi tidak ada sebiah transaksi aktif, panggil kembali akan dijalankan segera.
Jika hipotetis penulisan basisdata itu bukannya digulung kembali (khususnya ketika sebuah pengecualian tidak tertangani dimunculkan dalam sebuah blok atomic()), fungsi anda akan disingkirkan dan tidak pernah dipanggil.
Titik simpan (yaitu blok atomic() bersarang) ditangani dengan benar. Yaitu, sebuah pemganggilan on_commit() terdaftar setelah sebuah titik simpan (dalam blok atomic() bersarang) akan dipanggil setelah transaksi paling luar diserahkan, tetapi tidak jika digulung kembali ke titik simpan tersebut atau titik simpan sebelumnya yang muncul selama transaksi:
with transaction.atomic(): # Outer atomic, start a new transaction\n transaction.on_commit(foo)\n\n with transaction.atomic(): # Inner atomic block, create a savepoint\n transaction.on_commit(bar)\n\n# foo() and then bar() will be called when leaving the outermost block\nPada sisi lain, ketika sebuah titik simpan digulung kembali (karena sebuah pengecualian telah dimunculkan), bagian dalam dapat dipanggil tidak akan dipanggil:
\nwith transaction.atomic(): # Outer atomic, start a new transaction\n transaction.on_commit(foo)\n\n try:\n with transaction.atomic(): # Inner atomic block, create a savepoint\n transaction.on_commit(bar)\n raise SomeError() # Raising an exception - abort the savepoint\n except SomeError:\n pass\n\n# foo() will be called, but not bar()\nOn-commit functions for a given transaction are executed in the order they were\nregistered.
\nIf one on-commit function within a given transaction raises an uncaught\nexception, no later registered functions in that same transaction will run.\nThis is, of course, the same behavior as if you'd executed the functions\nsequentially yourself without on_commit().
Gulungan kembali anda dijalankan setelah penyerahan berhasil, jadi sebuah kegagalan dalam gulung kembali tidak akan menyebabkan transaksi di gulung kembali. Mereka dijalankan kondisional atas transaksi berhasil, tetapi mereka bukan bagian dari transaksi. Untuk penggunaan kasus dimaksud (pemberitahuan surat, tugas Celery, dll.), ini harus baik-baik saja. Jika tidak (jika tindakan mengikuti anda juga gawat dimana kegagalan itu harus berarti kegagalan dari transaksi itu sendiri), lalu anda tidak ingin menggunakan hubungan on_commit(). Sebagai gantinya, anda ingin two-phase commit seperti psycopg Two-Phase Commit protocol support dan optional Two-Phase Commit Extensions in the Python DB-API specification.
Callbacks are not run until autocommit is restored on the connection following\nthe commit (because otherwise any queries done in a callback would open an\nimplicit transaction, preventing the connection from going back into autocommit\nmode).
\nWhen in autocommit mode and outside of an atomic() block, the function\nwill run immediately, not on commit.
On-commit functions only work with autocommit mode\nand the atomic() (or ATOMIC_REQUESTS) transaction API. Calling on_commit() when\nautocommit is disabled and you are not within an atomic block will result in an\nerror.
Django's TestCase class wraps each test in a transaction\nand rolls back that transaction after each test, in order to provide test\nisolation. This means that no transaction is ever actually committed, thus your\non_commit() callbacks will never be run. If you need to test the results\nof an on_commit() callback, use a\nTransactionTestCase instead.
A rollback hook is harder to implement robustly than a commit hook, since a\nvariety of things can cause an implicit rollback.
\nFor instance, if your database connection is dropped because your process was\nkilled without a chance to shut down gracefully, your rollback hook will never\nrun.
\nThe solution is simple: instead of doing something during the atomic block\n(transaction) and then undoing it if the transaction fails, use\non_commit() to delay doing it in the first place until after the\ntransaction succeeds. It’s a lot easier to undo something you never did in the\nfirst place!
Django provides a straightforward API in the django.db.transaction\nmodule to manage the autocommit state of each database connection.
These functions take a using argument which should be the name of a\ndatabase. If it isn't provided, Django uses the "default" database.
Autocommit is initially turned on. If you turn it off, it's your\nresponsibility to restore it.
\nOnce you turn autocommit off, you get the default behavior of your database\nadapter, and Django won't help you. Although that behavior is specified in\nPEP 249, implementations of adapters aren't always consistent with one\nanother. Review the documentation of the adapter you're using carefully.
\nYou must ensure that no transaction is active, usually by issuing a\ncommit() or a rollback(), before turning autocommit back on.
Django will refuse to turn autocommit off when an atomic() block is\nactive, because that would break atomicity.
A transaction is an atomic set of database queries. Even if your program\ncrashes, the database guarantees that either all the changes will be applied,\nor none of them.
\nDjango doesn't provide an API to start a transaction. The expected way to\nstart a transaction is to disable autocommit with set_autocommit().
Once you're in a transaction, you can choose either to apply the changes\nyou've performed until this point with commit(), or to cancel them with\nrollback(). These functions are defined in django.db.transaction.
These functions take a using argument which should be the name of a\ndatabase. If it isn't provided, Django uses the "default" database.
Django will refuse to commit or to rollback when an atomic() block is\nactive, because that would break atomicity.
A savepoint is a marker within a transaction that enables you to roll back\npart of a transaction, rather than the full transaction. Savepoints are\navailable with the SQLite (≥ 3.6.8), PostgreSQL, Oracle and MySQL (when using\nthe InnoDB storage engine) backends. Other backends provide the savepoint\nfunctions, but they're empty operations -- they don't actually do anything.
\nSavepoints aren't especially useful if you are using autocommit, the default\nbehavior of Django. However, once you open a transaction with atomic(),\nyou build up a series of database operations awaiting a commit or rollback. If\nyou issue a rollback, the entire transaction is rolled back. Savepoints\nprovide the ability to perform a fine-grained rollback, rather than the full\nrollback that would be performed by transaction.rollback().
When the atomic() decorator is nested, it creates a savepoint to allow\npartial commit or rollback. You're strongly encouraged to use atomic()\nrather than the functions described below, but they're still part of the\npublic API, and there's no plan to deprecate them.
Each of these functions takes a using argument which should be the name of\na database for which the behavior applies. If no using argument is\nprovided then the "default" database is used.
Savepoints are controlled by three functions in django.db.transaction:
Creates a new savepoint. This marks a point in the transaction that is\nknown to be in a "good" state. Returns the savepoint ID (sid).
Releases savepoint sid. The changes performed since the savepoint was\ncreated become part of the transaction.
Rolls back the transaction to savepoint sid.
These functions do nothing if savepoints aren't supported or if the database\nis in autocommit mode.
\nIn addition, there's a utility function:
\nResets the counter used to generate unique savepoint IDs.
\nThe following example demonstrates the use of savepoints:
\nfrom django.db import transaction\n\n# open a transaction\n@transaction.atomic\ndef viewfunc(request):\n\n a.save()\n # transaction now contains a.save()\n\n sid = transaction.savepoint()\n\n b.save()\n # transaction now contains a.save() and b.save()\n\n if want_to_keep_b:\n transaction.savepoint_commit(sid)\n # open transaction still contains a.save() and b.save()\n else:\n transaction.savepoint_rollback(sid)\n # open transaction now contains only a.save()\nSavepoints may be used to recover from a database error by performing a partial\nrollback. If you're doing this inside an atomic() block, the entire block\nwill still be rolled back, because it doesn't know you've handled the situation\nat a lower level! To prevent this, you can control the rollback behavior with\nthe following functions.
Setting the rollback flag to True forces a rollback when exiting the\ninnermost atomic block. This may be useful to trigger a rollback without\nraising an exception.
Setting it to False prevents such a rollback. Before doing that, make sure\nyou've rolled back the transaction to a known-good savepoint within the current\natomic block! Otherwise you're breaking atomicity and data corruption may\noccur.
While SQLite ≥ 3.6.8 supports savepoints, a flaw in the design of the\nsqlite3 module makes them hardly usable.
When autocommit is enabled, savepoints don't make sense. When it's disabled,\nsqlite3 commits implicitly before savepoint statements. (In fact, it\ncommits before any statement other than SELECT, INSERT, UPDATE,\nDELETE and REPLACE.) This bug has two consequences:
If you're using MySQL, your tables may or may not support transactions; it\ndepends on your MySQL version and the table types you're using. (By\n"table types," we mean something like "InnoDB" or "MyISAM".) MySQL transaction\npeculiarities are outside the scope of this article, but the MySQL site has\ninformation on MySQL transactions.
\nIf your MySQL setup does not support transactions, then Django will always\nfunction in autocommit mode: statements will be executed and committed as soon\nas they're called. If your MySQL setup does support transactions, Django\nwill handle transactions as explained in this document.
\nInside a transaction, when a call to a PostgreSQL cursor raises an exception\n(typically IntegrityError), all subsequent SQL in the same transaction\nwill fail with the error "current transaction is aborted, queries ignored\nuntil end of transaction block". While simple use of save() is unlikely\nto raise an exception in PostgreSQL, there are more advanced usage patterns\nwhich might, such as saving objects with unique fields, saving using the\nforce_insert/force_update flag, or invoking custom SQL.
There are several ways to recover from this sort of error.
\nThe first option is to roll back the entire transaction. For example:
\na.save() # Succeeds, but may be undone by transaction rollback\ntry:\n b.save() # Could throw exception\nexcept IntegrityError:\n transaction.rollback()\nc.save() # Succeeds, but a.save() may have been undone\nCalling transaction.rollback() rolls back the entire transaction. Any\nuncommitted database operations will be lost. In this example, the changes\nmade by a.save() would be lost, even though that operation raised no error\nitself.
You can use savepoints to control\nthe extent of a rollback. Before performing a database operation that could\nfail, you can set or update the savepoint; that way, if the operation fails,\nyou can roll back the single offending operation, rather than the entire\ntransaction. For example:
\na.save() # Succeeds, and never undone by savepoint rollback\nsid = transaction.savepoint()\ntry:\n b.save() # Could throw exception\n transaction.savepoint_commit(sid)\nexcept IntegrityError:\n transaction.savepoint_rollback(sid)\nc.save() # Succeeds, and a.save() is never undone\nIn this example, a.save() will not be undone in the case where\nb.save() raises an exception.