[rabbitmq-discuss] tx_commit() semantics with async connections (e.g Pika SelectConnection)
mpietrek at skytap.com
Fri Aug 17 20:13:07 BST 2012
I've been drilling into an issue for a few days where my Pika channel is
abruptly closed. I'm pretty sure I've found either:
- A large gap in my understanding
- A misuse of the Rabbit/Pika APIs
- A serious bug in Pika.
My original understanding about transactions is that calling tx_select() is
a synchronous operation, and that when it returns, whatever actions you've
sent to the broker are committed.
However, in stepping through the Pika tx_commit() code, it seems like it
all it does is add the Tx.Commit message to an outgoing buffer, and that
the tx_commit() call returns without anything actually sent to the broker.
(I've set breakpoints in the debugger at strategic points to verify this.)
In my message handler, I'm writing a reply to the incoming message, using
the same channel. What I'm seeing (verified via WireShark) is that the
Tx.Commit message is followed by some number of Basic.Publish,
Content-Header, and Content-Body records corresponding to my reply message
writes. This violates the AMQP standard, which says that nothing should be
sent to the broker after sending a Tx.Commit and before receiving a
Tx.Commit-Ok reply. (If it helps understanding, my incoming message queue
has a few messages in it already, so I see a series of incoming messages
immediately after starting.)
I see that the tx_commit() method takes a callback method, but if I were to
simply block in my message handler, waiting for the callback to be invoked,
I'd deadlock because Pika's buffer processing code doesn't get a chance to
So given the above, is it me? Is it Pika?
My code looks like this:
"""Called when we are fully connected to RabbitMQ"""
# Open a channel
# Step #3
def echo_handle_delivery(channel, method, header, body):
print "Received message %s" % (body)
# Write a reply message on the same channel
body="replying to %s" % (body),
# Ack the incoming message, then force the write to be flushed.
connection_parameters = ConnectionParameters(host=broker_host)
echo_connection = SelectConnection(connection_parameters,
-------------- next part --------------
An HTML attachment was scrubbed...
More information about the rabbitmq-discuss