Question

在我的Django,我常常需要做类似于<条码>、植被_或出口()。例如,

User submits a tag. Need to see if that tag already is in the database. If not, create a new record for it. If it is, just update the existing record.

But looking into the doc for get_or_create() it looks like it s not threadsafe. Thread A checks and finds Record X does not exist. Then Thread B checks and finds that Record X does not exist. Now both Thread A and Thread B will create a new Record X.

这必须是非常常见的情况。我如何以可怕的方式处理该问题?

Answer 1

This must be a very common situation. How do I handle it in a threadsafe way?

Yes.

“标准”解决办法只是试图创造记录。如果它发挥作用,那是好的。持续进行。

如果试图创建记录,则从RDBMS处获得“复制”例外,则采用电子数据并保持运行。

然而,Django有一个ORM层,有自己的藏身。因此,该逻辑被推倒,无法直接和迅速地处理共同案件,不常见的情况(重复)则引起罕见的例外。

Answer 2

自2013年起或者,获得原产地为原子,因此,它处理的是共同点:

This method is atomic assuming correct usage, correct database configuration, and correct behavior of the underlying database. However, if uniqueness is not enforced at the database level for the kwargs used in a get_or_create call (see unique or unique_together), this method is prone to a race-condition which can result in multiple rows with the same parameters being inserted simultaneously.

If you are using MySQL, be sure to use the READ COMMITTED isolation level rather than REPEATABLE READ (the default), otherwise you may see cases where get_or_create will raise an IntegrityError but the object won’t appear in a subsequent get() call.

http://docs.djangoproject.com/en/dev/ref/models/querysets/#get-or-create”rel=“noreferer”

Here s an example of how you could do it:

1. 界定具有独特性的模式真:

class MyModel(models.Model):
    slug = models.SlugField(max_length=255, unique=True)
    name = models.CharField(max_length=255)

MyModel.objects.get_or_create(slug=<user_slug_here>, defaults={"name": <user_name_here>})

......或通过使用独一无二的“togheter”:

class MyModel(models.Model):
    prefix = models.CharField(max_length=3)
    slug = models.SlugField(max_length=255)
    name = models.CharField(max_length=255)

    class Meta:
        unique_together = ("prefix", "slug")

MyModel.objects.get_or_create(prefix=<user_prefix_here>, slug=<user_slug_here>, defaults={"name": <user_name_here>})

说明非岛屿田地是如何处于缺省状态的,NOT是独一无二的田地。这将确保你的创造是原子的。

这里指的是它如何在Django执行:rel=“noreferer”https://github.com/django/django/django/blob/fd60e688c986a102f0125d9dcdcf46pf61fmodgf - 制造物体,捕获最终的正直,并归还该笔物品。换言之,在数据库中处理原子问题。

Answer 3

审判交易.commit_on_success decorator for callsable where 你正在试图获得_or_create(**kwargs)

“利用承诺——分析工具,在功能上完成的所有工作中使用单一交易。如果职能成功恢复,那么Django就将完成该职能范围内的所有工作。如果该职能引起例外情况,Django将撤销交易。

除此以外,在同时要求获得_或_create时,两条镜子都试图以向它提出的论点来获取物体(但“过失”是一种在发出呼吁时使用的口号除外),因为(......)无法收回任何物体。如果二者都试图制造导致多个重复物体的物体,除非在数据库一级使用某些独特/独特的物体,同时使用(或)电话的外地。

it is similar to this post How do I deal with this race condition in django?

Answer 4

多年来,没有人写过<条码>读物。洛克希德/编码。如果你没有机会为<条码>unique 和进行迁徙,那么你会利用锁或<条码>进行翻新。 Semaphore Object. The pseudocode:

from concurrent.futures import ThreadPoolExecutor
from threading import Lock

_lock = Lock()


def get_staff(data: dict):
    _lock.acquire()
    try:
        staff, created = MyModel.objects.get_or_create(**data)
        return staff
    finally:
        _lock.release()


with ThreadPoolExecutor(max_workers=50) as pool:
    pool.map(get_staff, get_list_of_some_data())

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