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:


And-Deadlock-1



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:


And-Graph-2


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

Minimum Viable Product

When was the last time you complained about the food in a restaurant? I thought so. Most people will complain if they are offended by the quality or service; but if the food and/or service is just underwhelming then they won't complain, they will simply not return to the restaurant. The same applies to software products, or to products of any kind. You will only get negative feedback from customers if they care enough to make the effort. In the meantime you are both losing out on opportunities and failing your core professional obligation. Minimum Viable Product speaks to a desire to make your customers design your product for you. But, to me, it represents a combination of an implicit insult and negligence. The insult is implicit in the term minimum. The image is one of laziness and contempt: just throw some mud on the wall and see if it sticks. Who cares about whether it meets a real need, or whether the customer is actually served. The negligence is more subtle but, in the end,

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.

In Praise of Crappy Code

Not all code needs to be perfect! This is pretty heretical thinking for a software engineer. The issue is simple: how do you go about developing software for a small fixed budget. Imagine that you have $500 to implement a solution to a problem. If you spend more than that you will never recoup the extra that you spent. This comes up a lot in systems integration scenarios and also in customization efforts. Someone wants you to 'tweak' an application that they are using; you know that no-one else would want that feature and that if you spend more than what the customer will pay you will end up losing money. From the customer's perspective, the common 'time and materials' approach to quoting for software development is a nightmare. Being able to offer a fixed price contract for a task is a big benefit for the customer. But, how much do you quote for? Too much and you scare the customer away. Too little and you lose money. This is where 'crappy code' com