AMQP? Maybe not yet. At least not with Python.

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In my last Python project I thought about using AMQP messaging. A quick look at Python AMQP libraries shows two possibilities:

• Apache Qpid python client
• Barry’s py-amqplib

Both libraries are developed tightly with their brokers: Apache uses Qpid, py-amqplib uses RabbitMQ.

Apache Qpid

I started my adventure by downloading the source. They don’t have binary packages so this is what’s needed:

$ wget http://www.apache.org/dist/incubator/qpid/M3-incubating/qpid-incubating-M3.tar.gz
$ tar xvzf qpid-incubating-M3.tar.gz
$ cd qpid-incubating-M3/

Hmm. What’s next... No README files... After some googling I found that there’s getting stated guide. Following it this should be the magic command:

$ cd java/broker
$ PATH=$PATH:bin QPID_HOME=$PWD ./bin/qpid-server -c etc/persistent_config.xml
./bin/qpid-server: line 37: qpid-run: No such file or directory

Okay, let’s look for the missing file somewhere:

$ find ../.. -name qpid-run
../../java/common/bin/qpid-run
$ cp ../../java/common/bin/qpid-run bin

Next try:

$ PATH=$PATH:bin QPID_HOME=$PWD ./bin/qpid-server -c etc/persistent_config.xml
Setting QPID_WORK to /home/majek as default
System Properties set to -Damqj.logging.level=info -DQPID_HOME=/home/majek/b/qpid-incubating-M3/java/broker -DQPID_WORK=/home/majek
Using QPID_CLASSPATH /home/majek/b/qpid-incubating-M3/java/broker/lib/qpid-incubating.jar:/home/majek/b/qpid-incubating-M3/java/broker/lib/bdbstore-launch.jar
Info: QPID_JAVA_GC not set. Defaulting to JAVA_GC -XX:+UseConcMarkSweepGC -XX:+HeapDumpOnOutOfMemoryError
Info: QPID_JAVA_MEM not set. Defaulting to JAVA_MEM -Xmx1024m
Exception in thread "main" java.lang.NoClassDefFoundError: org/apache/qpid/server/Main
Caused by: java.lang.ClassNotFoundException: org.apache.qpid.server.Main

That’s enough. I’m definitely too stupid to run this server and too busy to waste more time here (there is also other reason why I’m not really interested in qpid, read on).

py-amqplib + RabbitMQ

Here installation went smoothly.

$ python easy_install amqplib

The broker:

$ wget http://www.rabbitmq.com/releases/rabbitmq-server/v1.4.0/rabbitmq-server_1.4.0-1_all.deb
$ sudo dpkg -i rabbitmq-server_1.4.0-1_all.deb

The good

I also downloaded the py-amqlib sources and found out that the examples are working out of the box:

~/amqplib-0.5/demo$ ./demo_send.py  1
~/amqplib-0.5/demo$ ./demo_receive.py

Most common way of using Barry’s library is to write software that only consumes data from AMQP. Use one event loop that blocks forever and when something happens on AMQP the callback is executed. I have slightly different use case. In the project I’m writing I use asynchronous programming. This means that I have one event loop that is not the AMQP event loop. I want to run AMQP stack every time something happened on a socket. This is how it would look like in pseudocode:

while True:
nonblocking_consume_amqp_events()
<magically_wait_for_activity_on_amqp_socket>

Qpid python client has only blocking interface, so it’s impossible to write code like that. Fortunately there’s a nonblocking client in Barry’s library. There’s even an example:

./nbdemo_receive.py

The bad

The demo doesn’t work correctly when there are more messages in the queue and fails not very nicely with this traceback:

Traceback (most recent call last):
[...]
File "build/bdist.linux-i686/egg/amqplib/nbclient_0_8.py", line 74, in write
P' read_buf='&!amq.ctag-FC3ET7kTcqy/A93gJYQWqw==6!amq.ctag-FC3ET7kTcqy/A93gJYQWqw==myfan2

Above bug is triggered by basic_ack. So I removed that command - I stopped sending message acknowledges. Now the order of delivering messages got broken:

['1', '1']
['2', '1', '2']
['3', '1', '2', '3']
['4', '1', '2', '3', '4']

If the problem is with acknowledges, then one could say to use no_ack on a channel and just disable acknowledges. Sorry, this also doesn’t work.

I send ten messages:
['a_0', 'a_1', 'a_2', 'a_3', 'a_4', 'a_5', 'a_6', 'a_7', 'a_8', 'a_9']
Then another ten:
['b_0', 'b_1', 'b_2', 'b_3', 'b_4', 'b_5', 'b_6', 'b_7', 'b_8', 'b_9']
I receive:
['b_0', 'b_1', 'b_2', 'b_3', 'b_4', 'b_5', 'b_6', 'b_7', 'b_8', 'b_9',
'a_0', 'a_1', 'a_2', 'a_3', 'a_4', 'a_5', 'a_6', 'a_7', 'a_8', 'a_9',
'b_0', 'b_1', 'b_2', 'b_3', 'b_4', 'b_5', 'b_6', 'b_7', 'b_8', 'b_9']

While I was working at this issue I was worried that my Python process is eating more and more memory. After some debugging I discovered that amqplib has very nice memory leaks (okay, reference cycles).

The ugly

After few days I finally managed to create nonblocking code that I needed. But it’s really ugly:

def my_nb_callback(ch):
  raise MException
conn = nbamqp.NonBlockingConnection('localhost',
                   userid='guest', password='guest',
                   nb_callback=my_nb_callback, nb_sleep=0.0)

ch = conn.channel()
ch.access_request('/data', active=True, read=True)

ch.exchange_declare('myfans', 'fanout', auto_delete=True)
qname, _, _ = ch.queue_declare()
ch.queue_bind(qname, 'myfans')

msgs = []
def callback(msg):
   msgs.append( msg )

ch.connection.sock.sock.setblocking(False)
ch.basic_consume(qname, callback=callback)
   while True:
       msgs = []
       <magically_wait_for_data_on_ch.connection.sock.sock>
       try:
           nbamqp.nbloop([ch])
       except MException:
           pass
       unique_msgs_filter = {}
       unique_msgs = []
       for msg in msgs:
           msg.channel.basic_ack(msg.delivery_tag)
           if msg.body not in unique_msgs_filter:
               unique_msgs_filter[msg.body] = True
               unique_msgs.append(msg.body)
       print '%r ' % (unique_msgs)

Back to RabbitMQ

For this simple research Rabbit works perfectly just out of the box. For a moment I even forgot that python library is not all of the software involved. But when I wanted to see some more details about it, things became messy:

$ rabbitmqctl --help
Password:

Which password would you like? Even if I know that, I’m not going to give any passwords away to see the help message, sorry. Next try:

$ rabbitmq-server --help
/usr/sbin/rabbitmq-server: 44: cannot create /var/log/rabbitmq/rabbit.log.bak: Permission denied
/usr/sbin/rabbitmq-server: 45: cannot create /var/log/rabbitmq/rabbit-sasl.log.bak: Permission denied
{error_logger,{{2008,11,21},{0,26,12}},"Protocol: ~p: register/listen error: ~p~n",["inet_tcp",einval]}
{error_logger,{{2008,11,21},{0,26,12}},crash_report,[[{pid,<0.22.0>},{registered_name,net_kernel},{error_info,{error,badarg}},{initial_call,{gen,init_it,[gen_server,<0.19.0>,<0.19.0>,{local,net_kernel},net_kernel,{rabbit,shortnames,15000},[]]}},{ancestors,[net_sup,kernel_sup,<0.9.0>]},{messages,[]},{links,[<0.19.0>]},{dictionary,[{longnames,false}]},{trap_exit,true},{status,running},{heap_size,377},{stack_size,21},{reductions,309}],[]]}

Conclusion
Even if the AMQP protocol is great, AMQP user stack still needs a lot of polish in my opinion.

Update #1:
rabbitmq-discuss thread

How to escape from blocked unactivated Vista

Few times I was pissed off by this screen:
It means that I haven't activated Vista on time and Microsoft stopped liking me. The problem is that at least two times I was caught by this screen when I haven't got access to the net. I had password to WiFi, but haven't entered it yet. When Vista is blocked you don't have access to any networking settings so the password is useless. Yet another time, because of bug in Vista I needed to reenter my windows key. Of course the easiest way to get the key is to use KeyFinder rather than waste time on searching the key. Which, in the end, would probably be somewhere online - like in MSDNAA.

Fortunetely you can easily escape from the blocked Vista. This is how you can do it. After clicking "Activate now" this screen appears:

Just choose "buy a new product key online", which would spawn a new browser window. I had Opera set as default browser, but any browser should work. The trick is to enter "c:\windows\system32\cmd.exe" as the url.
We want to run this magic program.
Next, the well known black cmd dialog should appear. We're back home :) Just enter "explorer" to run the usual windows Start menu and task bar.
That's it. You now have normal access to your computer. You can then configure wireless networks or do whatever you like.

The only problem is to remember this instructions in case you won't have access to the internet.

Tracing Python memory leaks

Tracing Python memory leaks.