Tag Archives: software architecture

stomp: more with less. Publishing for the masses, rather than doing everything yourself

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")
while True:
     stomp.put("fake sensor1: %d" % random.randint(0,50), destination="/topic/karlnet")

Tada! And likewise for any receiver…

stomp = Client("hostname")
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.

karlnet – a wireless sensor network that hopefully doesn’t suck.

I’ve been thinking about wireless sensor networks a lot, and for a while now. It’s something I’ve always been interested in, accumulating vast amounts of data and making pretty charts. But not just sensing, also control. I home brew beer, and also make cheese from time to time, both of which benefit from temperature control at the very least. Custom controllers for brewing or cheese making are quite common, and if you add the words “arduino” it even becomes ?cool?

So, why can’t these be the same devices? And why are all these controllers implemented completely locally? Things like HABS are cool and all, very neat bits of engineering, but why on earth do I want the times of my hop additions to be stored in EEPROM on device itself? And why are all these wireless sensors so expensive? I can get a nice weather station, max, min, humidity, indoor outdoor, but no data logging. The minute you add wireless datalogging the price jump is rather more than I feel it should. And heaven help you if you think “this has been done in industry, surely they don’t pay so much?” Well, seems they do.

So, enough rambling background, what am I going to do about it? Well, my basic idea was to have a bunch of nodes (I think “mote” is the word du jour) that are totally dumb. They can read some local sensor values, and if told to, turn on or off some outputs. But mostly, they just send raw unprocessed data back to base. And then instead of doing application programming for a brewery control system in embedded C and counting bytes and memory accesses, we can use any language we like, with any user interface we like, using nice tools that we like.

Seems simple right? Why isn’t it done? I’m not sure, and I really haven’t done as much research as I could have, but suffice to say, I can’t go down to the store and get 4-5 temperature/humidity monitoring units, a single base station, dumping data in any open format that I know of. At least, not cheap.

And, of course, certainly not as much fun as reinventing the wheel in the name of training. :)

Which brings us to the hardware. (The software can always be fixed and shipped later right?) I’ve gone with Xbee’s, or plain 802.15.4 for the wireless layer. These are pretty nice, reasonable throughput, low power, good enough range for reliable in home monitoring, security built in, and cheapish. At least, I thought they were a steal when I first found them. All this in a drop in module for only $US20 woot! Bluetooth or wifi cost double or triple that, and though you can get some FM modules, and 400MHz modules, you then have a lot more problems with interference, and you miss out on the built in addressing of 802.15.4.

I’ve since found things like Hope RF’s RFM12b and friends, which seem to be a pretty good compromise in between. Anyway, if the general hardware and system design is ok, it shouldn’t be a problem to have multiple radio types in the network right? As long as the base station can understand them all, you should be fine.

So, Xbee’s for now. For microcontrollers, AVRs win hands down. Being able to use plain old gcc, with plain old C, on plain old chips is a huge plus. Also, given how plentiful flash and ram is on even the tiniest little 8 bit microprocessor these days, you don’t even have to use ugly plain old C. The rise of physical computing, the trendy new thing for artists to be involved in, has brought about some very welcome changes in the world of the embedded hobbyist. The arduino platform, simplifying and hiding a lot of the ugly guts has done magnificent work, and has led to wonderfully low prices on high quality dev boards. Have a look at Modern Device or JeeLabs or pjrc’s Teensy for some good quality development and hobby boards. The dev boards I cut my teeth on in university cost nigh on $100, were 5 times the size, and had a fraction of the power and capability. Technology, plus a massive surge in demand, (I’ll even shout out to Make for some of it) has really brought out some great designs, at great prices.

There are some other great microprocessor options, looking strictly at a features/price point, but you need to be prepared for custom tool suites, or worse yet, paying for tools. No Fun.

And that’s the general idea. A bunch of dumb (yet very smart and overly powerful) AVR micros, hooked up to a bunch of simple, easy, cheap(ish) xbee radio modules, just sending sensor data back to a base station, and being told what to do. Put all the smarts up in an environment where it’s easy.

Over the next few articles/days/weeks I’ll be expanding on this, with what I’ve done so far, and where I’m at in my own personal implementation of this :)