For some time I've been using Spread as a messaging bus between processes and machines using Python bindings, but there are a few things that make it not quite ideal for what I've been trying to do.
- There's no access control
- Messages are non-persistent - so if a receiver daemon is down and some important message comes through, it's SOL
- The wire protocol is not documented, the docs basically just say use the C client library.
- The Python bindings to the C library have a glitch of some sort when used in py-exim-localscan, I had to resort to a small ctypes wrapper to get around this.
I ran across the Advanced Message Queuing Protocol(AMQP), with RabbitMQ as one implementation of the protocol, that looks like a better fit for my needs.
There's a Python client library available named QPID, but there are a few issues with that:
- Relies on threading, which is trouble when Python is embedded in something else, or if you want to try using it in Stackless Python
- Lacking documentation
- Has to load a big AMQP XML Spec file, which takes a few seconds.
I decided to take a whack at my own AMQP client, partially as a learning excercise to learn more about the protocol. I wrote a program to take the AMQP 0-8 spec file and statically generate the skeleton of a Python module, and then fleshed it out by hand. The generator is able to put lots of documentation from the spec file into Python docstrings, so the pydoc of this module is fairly decent. Because the module is statically generated, it should be easier to debug than QPID which generates lots of stuff on-the-fly. It's also much faster at making the first connection because it's not parsing the spec file. I also thew in SSL support in since it wasn't too difficult.
It has a ways to go, and some parts are probably naively conceived, but it does seem to work.
The first thing I've used it for is a syslog->AMQP bridge. I've setup my FreeBSD syslogd to feed all
info or higher events to a Python daemon, which extracts the date, time, program name, host name, etc and reformats as an AMQP message and published to a 'syslog' topic exchange with the program name as the routing key.
My plan is then to write other daemons that subscribe to the 'sshd' topic for example, and then generate higher-level messages that say things like: 'block IP address xx.xx.xx.xx' in case of failed login attempts. Then i just need one daemon to listen for these firewall control message and make changes to the PF tables.
It's fun stuff. The only weak part is that there's no way to tell if the original syslog message was spoofed, but after that point, AMQP access controls should keep things trustworthy.
See py-amqplib for a Mercurial repository and eventual downloads.