将操作推迟到提交之后

Question

我想找到一个好的模式来推迟执行操作，直到当前事务成功提交之后。例如，我可能有：

with pool(...)  as connection:
    execute_steps(connection) 

其中子函数通过将异步任务提交到外部消息队列(rabbit/AMQP，...(来启动多个异步任务之一......仅当事务成功提交时。那是

 def substep(connection): 
       submit_to_queue()

。如果事务随后失败(在提交到队列之后(，或者如果放置在队列中的作业在事务提交之前执行，则可能会失败。所以 -- submit_to_queue必须在 connection.commit 之后执行。

执行此操作的一种自然方法是收集回调步骤：

def substep(connection):
  def submit_to_queue(): ...
  ...
  connection.on_after_commit(submit_to_queue)
  ...

并具有：

@contextmanager
def db_pool(db_name="lb2", current_connection=None):
    connection = connection_pool.getconn()
    connection.on_after_commit = []
    try:
        # Do the work
        yield connection
        connection.commit()
        # Reset the after_success list and execute
        for job in connection.after_success():
            job()
        connection.after_success = [], 
    except Exception:
        connection.rollback()
        connection.after_success = []
        raise

但是，psycopg2的连接是不可变的对象。 那么... 有没有另一种方法可以使用psycopg2注册执行后回调？

谢谢。

Answer 1

子类化psycopg2.extensions.connection将允许您向对象添加属性(请参阅此处的解释以了解原因(：

In [44]: import psycopg2
In [45]: import psycopg2.extensions
In [46]: conn = psycopg2.connect('dbname=postgres')
In [47]: conn.foo = 'bar'
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-47-3ab9f2c6e77e> in <module>()
----> 1 conn.foo = 'bar'
AttributeError: 'psycopg2.extensions.connection' object has no attribute 'foo'
In [48]: class MyConnection(psycopg2.extensions.connection):
   ...       pass
   ...
In [49]: conn2 = MyConnection('dbname=postgres')
In [50]: conn2.foo = 'bar'
In [51]: conn2.foo
Out[51]: 'bar'