python - 并发进程中的Django select_for_update函数

Question

当我在多线程条件下测试update_or_create功能时，我发现结果不是我想要的，他们在MySQL中创建了不止一条记录。如代码所示，update_or_create使用select .. for update锁定MySQL中的行，那么MySQL中应该只有一条记录。我使用了 SQLAlchemy 并且 row sql 已经证明了这一点。

那么，Django 代码是错误的吗？

使用 Django 代码:

def get_or_create_ins():
    p, created = OxalicAcid.objects.update_or_create(defaults={"formula": "20", "degree": "80"}, name="smart")

def run():
    for i in range(10):
        t = threading.Thread(target=get_or_create_ins, args=())
        t.start()

if __name__ == "__main__":
    # more than one record will be created
    run()

使用 SQLAlchemy 代码:

@contextmanager
def transaction_atomic():
    session = Session()
    try:
        yield session
        session.commit()
    except Exception as e:
        session.rollback()
        raise e


def get_result_with_update(session, name):
    sql = text("""
        select * from acid_oxalicacid where name = :name for update 
    """)
    params = dict(name=name)
    cursor = session.execute(sql, params)
    result = cursor.fetchall()
    return result


def get_result(session, name):
    sql = text("""
        select * from acid_oxalicacid where name = :name
    """)
    params = dict(name=name)
    cursor = session.execute(sql, params)
    result = cursor.fetchall()
    return result


def create_data(session, name, degree, formula):
    sql = text("""
        insert into acid_oxalicacid (name, degree, formula) values (:name, :degree, :formula)
    """)
    params = dict(
        name=name,
        degree=degree,
        formula=formula
    )
    session.execute(sql, params)


def get_or_create():
    name = "smart"
    degree = "50"
    formula = "100"

    with transaction_atomic() as session:
        res = get_result_with_update(session, name)
        if not res:
            create_data(session, name, degree, formula)
            res = get_result(session, name)
        return res


if __name__ == "__main__":
    # Only one record be created, that's correct 
    for i in range(10):
        t = threading.Thread(target=get_or_create, args=())
        t.start()

Answer 1

由于Django使用“读已提交”事务隔离级别，因此会出现多条记录，如果将其更改为“可重复读”，则数据库中将只有一条记录。