UML by Ivar Jacobson

October 30, 2003

The largest conference on object technology is OOPSLA. The first OOPSLA conference was in 1986 and I have attended all of them but one. And, I have presented papers, or given tutorials or been on panels each time. Last year I decided not to go there. I wanted to break the rule (to go to OOPSLA) so that I wouldn't become a slave under it. This year the conference took place in Los Angeles, or more precisely in Anaheim - the home of Disneyland.

OOPSLA is a meeting-place for pioneers in software technology from all over the world. Everywhere there are friends from many years back, and we really get together and discuss what we are doing, what we want to do, what we think is hot and what we need to change.

This year I participated in two panels, one on Reuse and Repositories, another on Model Driven Architecture. The most interesting and passionate one was the one on MDA. Much thanks to my friend Dave Thomas (also on the advisory board of Jaczone) who dared to question the UML as well as MDA, as envisioned by OMG, in a very direct way.

Dave: "UML is currently a language with no conventional syntax, and not even a comprehensive standard way for tool interchange (XMI is both awkward and inconsistently implemented). UML was designed by committee, most of whom are methodologists and hence have little experience with language design. UML lacks a simple semantic account as an executable language. There is no published semantic account although proponents claim it is well defined via action semantics."

In general (not only from Dave) I understand the critique against UML as follows. UML is designed by people who have no experience from traditional language design, which means that the language is a mess. Even worse, they have no experience of real software development but they come from the methodology camp with no real software product background. These people have never developed any real software.

Of course, in a setting like OOPSLA Dave's words fell in very good soil. It was a very popular standpoint.

While I am not completely positive to everything in UML, I heard nothing really new in Dave's position. To me it was a dejà vue. I have heard the same critique for more than thirty years. I heard it when I introduced component-based development with visual modelling back in 1967. I heard it when we developed SDL that became a world standard in 1976 within telecommunications. And now I hear it again.

I was originally very critical to how we worked with UML 1.0. Some people wanted a very intuitive approach and define the language with basically just a notation, some meta-model diagrams and plain English text. UML 1.0 became hopeless to understand. I advocated that we should apply classical language design techniques. Starting with an abstract syntax, build a static semantic model on top of the syntax, introduce dynamic semantic domains. I didn't suggest that we should define the operational semantics formally. I made this compromise primarily because most people wouldn't understand it and very few people would care. Moreover this wouldn't be different in approach from most other language designs. I have personally done a lot of language design work when we defined SDL and CHILL, I also did it when working with my doctoral thesis. Thus from a practical point of view I couldn't recommend it. Also, I felt comfortable we could add it later.

During the work with UML 1.1 my approach was adopted. We didn't formalize the dynamic semantic domains. We defined the operational semantics textually only.

Thus it is not correct to say that language-design amateurs defined UML. My own thesis was a definition of a modelling language using classical language design techniques.

It is also not true that UML was designed by people with no practical experience from software development. I was not the only one with many years of hard work in real software development. The UML team was also backed by 10,000's of developers working behind the scenes and applying the technologies being proposed. We had reference groups all around the world that helped us understand the needs to satisfy.

This was UML 1.1.

With UML 2.0 the situation changed. A lot of new people entered the scene, and a committee took over. With "a fresh look", they changed everything they didn't like. And they added what their "fresh look" told them should be added. Personally, I was so bored working in a committee so I stayed out of the work almost completely (I had done language designs three times before, so I couldn't feel any excitement). I said "almost" because I had to get involved on one occasion. That was when the committee decided to remove use cases from UML. Well, they removed the kind of use cases we have had, and replaced them with something different…and then they called this new thing use cases. They were so arrogant, that they told me, without a blink, that I had never understood use cases, but now they had got them right. They voted in the committee and agreed to remove our use cases. Luckily, people outside the committee set things straight and put use cases back into UML.

Many members of that committee had never produced any useful software. Some members were really competent, but the politics became a burden. Now the committee took a lot of time, they were delayed by years, and the resulting proposal UML 2.0 is very complex. Still, I am optimistic. The interest for UML is so high and there are so many people that are willing to fix what potentially is broken. Thus, I am convinced without a single doubt that UML will continue to be successful. It represents very much what a good part of the software community wants to have.

However, this part is not in majority. It may today be 20% of the software community. The remaining 80% have not adopted visual modelling at all. Twenty years ago I believe that it was less than 5% that used visual modelling.

Thus I believe the reason UML is criticized have other roots than bad language designs. The critique represents symptoms not the root cause. I believe that the roots of the critique are that some people don't believe in visual modelling as such but that textual languages work better.

The critique is healthy. UML will most certainly become better defined. We will get better and better tools. The tools will make software development less complex, we will be more productive, get higher quality, and get software much faster --- very much thanks to UML and how you can apply UML.

Anyway, it was a very good panel. People love a good fight. Dave was the clear winner, but that was a given. He was playing on home ground.

During the aftermath of the heated discussion, someone said that whether you are for or against UML or MDA rests on religious grounds. That is something I can wholeheartedly agree with. On the other hand, this is what I heard when I introduced components, use cases, … I have been very honest about it. I have never been able to prove that my approach is better than anyone else's. At best, I have been able to make it believable.

With UML and MDA we have a similar situation. It is very much about gut feeling. Either your gut feeling tells you that UML and MDA are good, or it does not. Your way of working will be coloured by your feelings.

Regarding UML, I have absolutely not a single doubt about its long-term success. About MDA, I am very sympathetic. I know that MDD (model-driven development) has been successful and will become even more successful in the future. Whether MDA will be successful is a question of execution, i.e., how well OMG and its members succeed in implementing the vision. I will support them, and I wish them very good luck.  

1 Comment
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