[rabbitmq-discuss] py-amqplib basic_consume

[Matthias Radestock]

Tsuraan,

tsuraan wrote:
> Ok, so now I'm trying to get basic_consume to work, and it's behaving
> very strangely.  As before, I first run a producer that puts 1000
> messages into a queue called toshred.  Once that has run, I run my new
> consumer code, which just prints that it got something to consume.  It
> doesn't acknowledge the message, it doesn't do anything the slightest
> bit useful.  However, it does appear to devour messages like mad.
> Every time I run this "consumer", it calls the consume method once,
> but over 200 messages will be gone from the toshred queue (as
> determined by a passive declaration of that queue).

The passive declaration of a queue tells you the number of messages 
present in the queue (and committed if the channel on which they were 
published is transacted) *that are not awaiting acknowledgement*.

When a client has subscribed to a queue with basic.consume, the queue 
will send it messages as fast as it can, until network back pressure 
kicks in.

So it is perfectly normal for there to be a discrepancy between the 
number of messages published vs the number of messages received + the 
message count returned by queue.declare-ok - the difference is accounted 
for by messages "in flight".

> Two questions: why are messages being removed from the queue?

They are not actually being removed until an ack is received, but they 
are unavailable for consumption by another consumer.

> It doesn't seem like any messages are being acknowledged, so I wouldn't
> expect any messages to be dequeued.

Actually, in your example you are using ...

>   chan.basic_consume('toshred', no_ack=True, callback=consume)

with the no_ack flag set to True. That causes messages to be 
automatically "self-acknowledged" by the server. In other words, as soon 
as the queue has sent them on their way they are removed from the queue 
and gone for good.


Matthias.