Wednesday, 4 March 2009

Report on UK/Europe Sun SPOT Symposium

The first UK/Europe Sun SPOT Symposium was held on 24th February 2009 at the British Computer Society’s excellent London HQ. The event was sponsored by the BCS’s Software Practice Advancement Specialist Group, to whom many thanks are due.

A total of 28 people attended the Symposium. The programme can be seen at

First up was a presentation by Robert Taylor from Manchester University on The Yggdrasil Data Collection Framework. This was a good overview of the data collection projects using SPOTs and of some of the planned future work but we could have done with more technical details about Yggdrasil and perhaps a demonstration.

Bart Braem, University of Antwerp, explained how SPOTs are used within their Masters course as part of the Computer Networks and Distributed Systems option. His presentation can be found here. Although the use of SPOTs is in its early days they have been popular and well-received by students, and a number of interesting projects have already been undertaken.

The replacement radio stack developed at Universit├Ąt Karlsruhe was the subject of the talk by Markus Bestehorn. His presentation can be found here. This stack completely replaces all the pieces above the MAC layer but provides some backwards compatibility at the application API level with the standard stack. Marcus presented a very convincing demo and statistics to illustrate the unreliability of the standard stack when transferring large data sets (e.g. library suites) across multiple hops. He also demonstrated just how resilient the KSN stack is when faced with changing network topology.

Most of the afternoon was devoted to ad hoc Open Space-style sessions.

There were four lightning talks. John Nolan, in many ways the instigator of the SPOT project, talked about the original objectives and urged participants to focus on novel applications rather than just improvements to the SDK. Kurt Smolderen used his talk to request more general interfaces for network routing, arguing that the current interfaces were biased towards AODV. Daniel Van Den Akker described his project to rework the lower part of the radio stack to make it practical to replace either the MAC layer or the physical radio. John Daniels talked about some of the add-on boards created in Sun Labs, the upcoming revision 7 main board, and gave a demonstration of the “mega SPOT”.

There were several sessions in the longer breakout slots:
  • A demonstration of the KSN management application including OTA deployment by Stephan Kessler.
  • A technical question and answer session hosted by Dave Cleal and John Daniels.
  • A demonstration of the use of Wireshark to analyse the output of his SPOT radio traffic packet sniffer by Kurt Smolderen.
  • A demonstration of radio communication between a SPOT and a Telos mote by Daniel Van Den Akker.
  • A discussion of possible thesis projects led by Bart Braem.
The day finished with a video call link up with Roger Meike, the head of the SPOTs project in Sun Labs. Roger took questions from the Symposium participants, mainly concerning the availability of SPOTs and the process by which contributions can be made to the code base.

Everyone I spoke to enjoyed the day and thought it worthwhile. Hopefully another Symposium will be held later in the year.

Tuesday, 3 March 2009

Why OCL shouldn't die

It perhaps won’t come as much of a surprise to those of you who have barely heard of OCL to discover that some people think it should die.

In his blog article OCL is a dead language. Universities: stop wasting time teaching it Jason Gorman argues that universities are wasting their time teaching students the Object Constraint Language, a somewhat obscure part of the Unified Modeling Language, since it has nearly no users in the real world. He’d prefer that universities spent their time on “writing better unit tests, or learning to automate acceptance tests.” I agree those things are important, but I’m going to argue here that the OCL has its uses, and that those uses are important enough to justify its inclusion – albeit in a minor way – in any serious software design class.

Of course, I would say that wouldn’t I? I’m one of the people Jason describes as “one degree of separation from the chap who invented OCL in the first place.” I’ve written books that use OCL or its predecessor extensively. So I’m prejudiced. Or perhaps I can just see the situation more clearly.

I’m going to start by assuming that UML itself is still alive. You see it being used less these days now that Big Design Up Front has been exposed as a charlatan. But I still see people using it extensively in an ad hoc fashion on whiteboards and to capture high-level abstractions. I don’t see anyone arguing that having a graphical language in which to describe the structure and behaviour of systems is a bad thing, so I’ll assume UML is sticking around.

Sometimes we want to describe systems that are not wholly – or even partially – implemented in software. It’s useless to argue that “the model is in the code” in situations like this, and UML certainly has a role to play there.

So now we come to my claims for OCL:

OCL is very helpful in defining and explaining the meaning of UML diagrams

Lots of people draw UML diagrams with only a slender understanding of how the diagrams should be interpreted. Although not a topic for beginners, any serious user of UML will need to understand precisely what the diagrams mean, and OCL is invaluable for this. The UML specification is itself partially written in OCL.

OCL allows you to say things that can’t be said in UML diagrams

Diagrams have their limitations – the graphical notations can’t express everything you might want to say. You don’t really want to clutter up UML diagrams with lots of text annotations but sparing use of OCL to express key rules that would otherwise be missing can be very valuable.

OCL is (or should be) the language of Model-Driven Engineering

MDE is the generic term for software development processes that focus on producing high-level models that can then be executed. The best known approach is MDA™, promoted by the OMG™. MDE isn’t as popular right now as it was, but many people still see it as the obvious long term goal for the software industry.

The most popular language for MDE is UML. For models to be executable they must contain complete behavioural specifications and the only way to write such specifications in UML is to use OCL. So OCL had better not die.

OCL helps you appreciate the value of precision and abstraction

For most software developers the only tool they have that takes a firm position on precision is the compiler. But not every program that compiles is correct, and my belief is that people who understand how to express their designs precisely yet abstractly in UML/OCL write better programs, even if the programs aren’t explicitly designed using those tools.

So ultimately I think it is worth teaching university students OCL – provided it’s done as part of a sensible approach to modelling – even if prospective employers aren’t asking for it. Students who get that education will think about their software at a level of abstraction higher than the code and will be better developers.