The early prototype head end software for karlnet did all the decoding of the wireless frames, and also handled doing something with the data, in that case, sticking it all into RRD databases so that tools like Cacti could make me some pretty graphs of the temperature. Now, that sort of architecture doesn’t scale very well, and it was about to become quite a pig when I started adding more nodes.

I already had more things I wanted doing, I wanted webpages, I wanted to try out some different RRD graphing engines (cacti is a bit of a pig to configure, but it does make nice graphs)

What I really wanted was to put all the hard work of reliably slurping data off the xbee network into one place, and put all the user applications that use that data somewhere else. (Reliably reading off the xbees is the topic of another post)

I’d been very happy with python for the serial decoding so far, but I certainly didn’t want to restrict myself to just python. I also wanted to make sure that anyone who wanted to could write an application using the data, not just me. A message bus was exactly what I wanted, and although it’s a little bit of a “big enterprise buzzword” it really doesn’t have to be. Or at least, you don’t have to be scared by it.

Stomp is one of the simpler, lighter weight message bus protocols. With a suitable “broker” (the name of the magic blob of software that handles all the messages) you can quite easily publish a message, like, “node:1, sensorA: 19 degrees Celsius” and then _anyone_ else who had registered that they were interested in receiving temperature readings would be delivered a copy of the message. Whether there was one listener, or one hundred.

With the 5 second pub/sub overview out of the way, how did I do it?

ActiveMQ is a well established solid player in the message bus world, and although traditionally associated with JMS messaging, it also has built in stomp support. Surprisingly, there was no package for activemq for ubuntu 9.10, but the installation is straightforward (some new skills learnt will go in another post) and then the instructions for enabling stomp are very clear.

Now, trying it out! I use stompy for python support. I also tried stomp.py, but it did nasty things like swallowing keyboard interrupts, so you couldn’t kill your scripts with ^C. I don’t recommend it. I simply tried it first :(

Stompy was easy!

import random, time
from stompy.simple import Client
stomp = Client("hostname of activemq/stomp")
stomp.connect()
while True:
     stomp.put("fake sensor1: %d" % random.randint(0,50), destination="/topic/karlnet")
     time.sleep(5)

Tada! And likewise for any receiver…

....
stomp = Client("hostname")
stomp.connect()
stomp.subscribe("/topic/karlnet")
while True:
     message = stomp.get()
     print "hohoho, we got some sensor readings: " + message.body

Now that’s cool, but we’re only sending fixed strings there, we want real objects! We need to be careful not to use anything python specific here, because we can’t control (and don’t want to control) who’s listening at the other end. XML or JSON are probably the simplest choices here, though you could come up with your own custom scheme here, even just manual string parsing, if you were you were really masochistic.

I chose JSON, mostly because my day job has enough XML, and JSON seems like a perfectly reasonable alternative. The python standard library json might be enough for you, but my message format is an object, not a dictionary of key/values, so I had to use jsonpickle

So, the final shiny fake sender / receivers are in svn, and in the same consumers/producers directories, you can see the two real apps, the main producer, reading data from the base station Xbee, and a simple script that updates RRD databases. That code is beautifully simple now that it’s been extracted out of the nitty gritty of the sensor network code.

Finally, for an example of what can be done now that we live in the future, have a read of Stomp on WebSockets and perhaps even a look at consumers/simple_web in svn.