Archive for September 2016

Django's get_or_create() may raise IntegrityError but subsequent get() raises DoesNotExist

Django 的 SomeModel.objects.get_or_create() 實際上的步驟是:

get, if-problem: save+some_trickery, if-still-problem: get again, if-still-problem: surrender and raise.

get_or_create() 拋出 IntegrityError 但是接著 get() 一次卻會是 DoesNotExist
這個錯誤常常會發生在 MySQL 的 AUTOCOMMIT=OFF 且 isolation level 是 REPEATABLE READ 的情況下
MySQL 默認採用 REPEATABLE READ(而 PostgreSQL 預設是 READ COMMITTED)
而 REPEATABLE READ 的定義是在同一個 transaction 之內 執行相同的 SELECT 一定會拿到相同的結果

with transaction.commit_on_success():
    try:
        user, created = User.objects.get_or_create(id=self.id, username=self.username)
    except IntegrityError:
        # 這個 get() 可能會發生 DoesNotExist
        user = User.objects.get(id=self.id)

會發生那個 DoesNotExist 通常是因為:

  • thread a 執行 get(),但是 DoesNotExist
  • 所以 thread a 接著 create(),但是在資料建立之前
  • thread b 幾乎在同一個時間點也執行 get(),也是 DoesNotExist
  • thread a 成功地 create()
  • 因為在 3 拿不到資料,所以 thread b 也執行 create(),但是因為 UNIQUE CONSTRAINT,導致失敗了
  • 所以 thread b 又會 get() 一次,但是因為 REPEATABLE READ,thread b 不會拿到 thread a 建立的資料,所以 DoesNotExist
  • 所以那個 User.DoesNotExist 就是 thread b 拋出來的錯誤
  • thread a 是成功的,你事後去資料庫裡查詢,會看到那個 User 的資料好端端地在那裡

ref:
http://stackoverflow.com/questions/2235318/how-do-i-deal-with-this-race-condition-in-django
https://blog.ionelmc.ro/2014/12/28/terrible-choices-mysql/

解決的方法 除了就乾脆不要在同一個 transaction 裡之外
基本上就只能把 isolation level 改成 READ COMMITTED 了
似乎沒有其他辦法了
因為這就是 isolation level 的定義

As of MariaDB/MySQL 5.1, if you use READ COMMITTED or enable innodb_locks_unsafe_for_binlog, you must use row-based binary logging.

SHOW VARIABLES LIKE 'binlog_format';
SET SESSION binlog_format = 'ROW';
SET GLOBAL binlog_format = 'ROW';

ref:
http://dba.stackexchange.com/questions/2678/how-do-i-show-the-binlog-format-on-a-mysql-server

Django 和 Celery 都建議使用 READ COMMITTED
https://docs.djangoproject.com/en/dev/ref/models/querysets/#get-or-create
https://code.djangoproject.com/ticket/13906
https://code.djangoproject.com/ticket/6641
http://docs.celeryproject.org/en/latest/faq.html#mysql-is-throwing-deadlock-errors-what-can-i-do

方法一

SHOW VARIABLES LIKE 'tx_isolation';
SET SESSION tx_isolation='READ-COMMITTED';
SET GLOBAL tx_isolation='READ-COMMITTED';

# or

SET SESSION TRANSACTION ISOLATION LEVEL READ COMMITTED;
SET GLOBAL TRANSACTION ISOLATION LEVEL READ COMMITTED;

ref:
http://dev.mysql.com/doc/refman/5.5/en/set-transaction.html

方法二

/etc/mysql/my.cnf

[mysqld]
transaction-isolation = READ-COMMITTED

方法三

in settings.py

DATABASES = {
    'default': {
        'ENGINE': 'django.db.backends.mysql',
        ...
        'OPTIONS': {
            'init_command': 'SET SESSION tx_isolation="READ-COMMITTED"',
        },
    }
}

方法四

自己寫一個 get_or_create()

from django.db import transaction

@transaction.atomic
# or
@transaction.commit_on_success
def my_get_or_create(...):
    try:
        obj = MyObj.objects.create(...)
    except IntegrityError:
        transaction.commit()
        obj = MyObj.objects.get(...)
    return obj

ref:
http://stackoverflow.com/questions/2235318/how-do-i-deal-with-this-race-condition-in-django

--

MySQL 默認是 AUTOCOMMIT
http://dev.mysql.com/doc/refman/5.5/en/commit.html

Django 默認也是 AUTOCOMMIT
除非你使用了 TransactionMiddleware(Django 1.5 以前)或是設置 ATOMIC_REQUESTS = False(Django 1.6 以後)
TransactionMiddleware 的效果類似 @transaction.commit_on_success@transaction.atomic
都是把 AUTOCOMMIT 關掉
差別在於範圍不同
http://django.readthedocs.io/en/1.5.x/topics/db/transactions.html#django-s-default-transaction-behavior
http://django.readthedocs.io/en/1.6.x/topics/db/transactions.html#managing-autocommit
http://django.readthedocs.io/en/1.6.x/ref/settings.html#std:setting-DATABASE-ATOMIC_REQUESTS

Transaction Isolation Levels 隔離等級

Dirty reads 髒讀

Transaction 1 讀到了 Transaction 2 的未提交的值
READ COMMITED 可以避免這個問題
READ UNCOMMITTED 會有這個問題

A dirty read occurs when a transaction is allowed to read data from a row that has been modified by another running transaction and not yet committed.

Non-repeatable reads 不可重複讀

在同一個 Transaction 裡 不同的時間點 讀取了同一個 row 卻得到了不同的值(但是可能使用了不同的 SELECT 語句去讀到同一個 row)
REPEATABLE READ 可以避免這個問題
READ COMMITED 會有這個問題

A non-repeatable read occurs, when during the course of a transaction, a row is retrieved twice and the values within the row differ between reads.

When Transaction 2 commits successfully, which means that its changes to the row with id 1 should become visible. However, Transaction 1 has already seen a different value for age in that row. At the SERIALIZABLE and REPEATABLE READ isolation levels, the DBMS must return the old value for the second SELECT. At READ COMMITTED and READ UNCOMMITTED, the DBMS may return the updated value; this is a non-repeatable read.

ref:
http://www.oracle.com/technetwork/testcontent/o65asktom-082389.html

Phantom reads 幻讀

有點類似 Non-repeatable reads
在同一個 Transaction 裡 不同的時間點 兩個相同但是先後執行的 SQL 卻得到了不同的結果
差別在於 Phantom reads 專門指沒有用 range locks 時
先後兩個同樣的 SELECT 卻返回了不同數量的結果
SERIALIZABLE 可以避免以上的三種問題

也可以說 non-repeatable reads 的原因是 UPDATE
而 phantom reads 的原因則是 INSERT 和 DELETE

A phantom read occurs when, in the course of a transaction, two identical queries are executed, and the collection of rows returned by the second query is different from the first.

READ UNCOMMITTED

允許 dirty reads
可能會讀到其他 transaction 尚未提交的數據

READ COMMITED

不會有 dirty reads
只會讀到其他 transaction 已經提交的數據
但是允許 non-repeatable reads

大部份資料庫默認的 isolation level 都是 READ COMMITED

REPEATABLE READ

不會有 dirty reads 和 non-repeatable reads
但是允許 phantom reads

MySQL 默認的 isolation level 是 REPEATABLE READ
但是 InnoDB 透過 next-key lock 也防止了 phantom reads

SERIALIZABLE

不會有 dirty reads、non-repeatable reads 和 phantom reads
讀寫互斥
所以讀和寫都只能一個一個依序執行

ref:
https://www.percona.com/blog/2012/08/28/differences-between-read-committed-and-repeatable-read-transaction-isolation-levels/