Skip to main content

Semantics of BPMN

Recently I have been involved with the standardization of BPMN. I find BPMN fascinating because it lies on the precise boundary between the IT world and the business world. Boundaries tend to be uncomfortable places to be around and BPMN is no exception.

My focus has been primarily on the semantics of BPMN. There have been many issues concerning the semantics of BPMN features. From a strictly programming language point of view BPMN seems to throw away most of the hard learned lessons of programming in the last 40 years: it is unstructured, full of gotos and no objects in sight.

But that is to forget the true purpose of BPMN, it is not a programming language. It is not even a scripting language, it is a language to help humans to manage their own activities. Even when BPMN is supported by automation, it is still primarily humans doing the work.

Anyway, the traditional approach to the semantics of languages like BPMN is Petri Nets. The merit of Petri nets is that they are quite simple to understand and quite simple to implement too.

However, BPMN has a bunch of features that are quite difficult to capture in petri nets without getting quite twisted. These are mostly due to the rich variety of merge behaviors supported by BPMN.

A classic one is the pass-through merge: where two streams are merged into one and all the tokens are forwarded. Simple, but combined with parallelism can get you very quickly into trouble:


This diagram, which shows an and-split forking off into three parallel branches, two of which are merged by a pass-through merge and the resulting pair merged back by an and-merge.

Interpreted as a petri net, three tokens originate from the split, two are merged onto the same 'wire' and the and-merge merges tokens on its two wires. This will result in a deadlock after the first successful firing of the and-merge because there is still a token left hanging about that is not collected by the and-merge.

The instinctive reaction of an IT person would be to say "don't do that"; but we are expecting business analysts to use BPMN, not programmers. And, this kind of situation can show up as easily in a large 400+ node diagram as a 3-node diagram.

One 'fix' is to allow annotations on the wires that go into an and-merge: specifically how many tokens that wire is supposed to accept. In my view, that is even worse because there may be no way of setting that count reliably, and further, it is low-level hacking of the worst kind: programming by numbers.

Here is another approach: instead of using petri nets, use graph rewriting semantics. In a graph rewriting approach, there are three kinds of nodes and wires: dormant, active and expired. The BPMN diagram becomes the initial graph which is rewritten until the entire graph is expired (or until only all the end-event nodes are active). This is how the pass-through is evaluated using graph rewriting:


The six stages show the evolution of the graph from a single active and-split ending up at the single active and-merge. The green is active, and the gray/purple signifies expired. (Normally you would remove expired entities but I have kept them for illustration.

Essentially, the pass-through node becomes a kind of 'cloner', and the dead-lock problem disappears! Neat!

I think that this may well end up to be the way that BPMN's semantics is defined. Given the experience of the G-machine (used in combinator approaches to functional programming languages) it's also not a bad way to implement BPMN automation.

Unless, of course, some problem shows up to make it unsuitable after all.

Popular posts from this blog

Comments Should be Meaningless

This is something of a counterintuitive idea: Comments should be meaningless What, I hear you ask, are you talking about? Comments should communicate to the reader! At least that is the received conventional wisdom handed does over the last few centuries (decades at least). Well, certainly, if you are programming in Assembler, or C, then yes, comments should convey meaning because the programming language cannot So, conversely, as a comment on the programming language itself, anytime the programmer feels the imperative to write a meaningful comment it is because the language is not able to convey the intent of the programmer. I have already noticed that I write far fewer comments in my Java programs than in my C programs.  That is because Java is able to capture more of my meaning and comments would be superfluous. So, if a language were able to capture all of my intentions, I would never need to write a comment. Hence the title of this blog.

Another thought about Turing and Brooks

Rodney Brooks once wrote that robots would be human when treating them as though they were human was the most efficient way of interacting with them. (Not a precise quote.) This is an interesting variation on the Turing test. It assumes that we decide the smartness of machines in the context of frequent interactions with them. It also builds on an interesting idea: that in order to deal with another entity, be it human, animal or mineral, we naturally build an internal model of the entity: how it behaves, what it can do, how it is likely to react to stimuli etc. That model exists for all entities that we interact with; a rock is not likely to kick you back, your word processor will likely crash before you can save the document etc. When the most effective way to predict the behavior of a machine is to assume that it has similar internal structure to ourselves, then it will, for all intents and purposes, be human. So, here is another thought: how do we know that another human is human?

What is an Ontology for?

I am sure that everyone who has ever dabbled in the area of Ontology has been asked that question. Personally, I have never heard a truly convincing response; even though I strongly feel that Ontologies are quite important. I recently listened to a radio segment in which someone in Algeria (I think) was complaining about the new law that required all teaching to be done in Arabic. It seems that most university-level education is in French, and that many parents try to send their kids to schools that teach in French. The issue was that Arabic simply does not have the vocabulary demanded by a modern high-tech education. Arabic is not alone in this dilemma: French itself is littered with Les mots Anglais; and English is a true hodge-podge of Anglo-Saxon, French, German, Hindu, Japanese, and many other languages. It often happens that when a culture acquires a set of concepts, it does so in the language of the originators of those concepts. It is often considerably easier to import wholes