Petri Nets

I’m trying to build bridges between mathematics and practical subjects like ecology and engineering — subjects that might help us save the planet.

As part of this, I have a project to explain how some ideas from electrical engineering, control theory, systems ecology, systems biology and the like can be formalized — and to some extent unified — using “symmetric monoidal categories”. Whenever you have a setup with:

• abstract gadgets that have ‘input wires’ and ‘output wires’,

and you can

• hook up these gadgets by connecting outputs to inputs,

and

• the wires can cross over each other, and

• it doesn’t matter whether one wire crosses over or under another

then you’ve probably got a symmetric monoidal category! For a precise definition, and lots of examples, try:

• John Baez and Mike Stay, Physics, topology, logic and computation: a Rosetta Stone, in New Structures for Physics, ed. Bob Coecke, Lecture Notes in Physics vol. 813, Springer, Berlin, 2011, pp. 95-174.

Back then I was excited about examples from ‘topological quantum field theory’, and ‘linear logic’, and ‘programming semantics’, and ‘quantum circuit theory’. But now I’m trying to come down to earth and think about examples of a more everyday nature. And they turn out to be everywhere!

For example, when you see these diagrams:

you may see a 10-watt amplifier with bass boost, and 16-volt power supply with power-on indicator light. But I see morphisms in symmetric monoidal categories!

Back in week296 I explained a symmetric monoidal category where the morphisms are electrical circuits made only from linear resistors. It’s easy to generalize this to ‘passive linear circuits’ where we include linear inductors and capacitors… and we can keep going further in this direction. I’m writing a paper on this stuff now.

But today I want to head in another direction, and tell you about something I’m beginning to work on with Jacob Biamonte: something called ‘Petri nets’.

Before I do, I have to answer a question that I can see forming in your forehead: what’s the use of this stuff?

I don’t actually think electrical engineers are going to embrace category theory — at least not until it’s routinely taught as part of college math. And I don’t even think they should! I don’t claim that category theory is going to help them do anything they want to do.

What it will do is help organize our understanding of systems made of parts.

In mathematics, whenever you see a pattern that keeps showing up in different places, you should try to precisely define it and study it — and whenever you see it showing up somewhere, you should note that fact. Eventually, over time, your store of patterns grows, and you start seeing connections that weren’t obvious before. And eventually, really cool things will happen!

It’s hard to say what these things will be before they happen. It’s almost not worth trying. For example, the first people who started trying to compute the perimeter of an ellipse could never have guessed that the resulting math would, a century or so later, be great for cryptography. The necessary chain of ideas was too long and twisty to possibly envision ahead of time.

But in the long run, having mathematicians develop and investigate a deep repertoire of patterns tends to pay off.

Petri nets – the definition

So: what’s a Petri net? Here’s my quick explanation, using graphics that David Tweed kindly provided for this article:

• Petri net, Azimuth Project.

A Petri net is a kind of diagram for the describing processes that arise in systems with many components. They were invented in 1939 by Carl Adam Petri — when he was 13 years old — in order to describe chemical reactions. They are a widely used model of concurrent processes in theoretical computer science. They are also used to describe interactions between organisms (e.g. predation, death, and birth), manufacturing processes, supply chains, and so on.

Here is an example from chemistry:

The circles in a Petri net denote so-called states, which in this example are chemical compounds. The rectangular boxes denote transitions, which in this example are chemical reactions. Every transition has a finite set of input states, with wires coming in from those, and a finite set of output states, with wires going out. All this information can equally well be captured by the usual notation for chemical reactions, as follows:

C + O₂ → CO₂

CO₂ + NaOH → NaHCO₃

NaHCO₃ + HCl → H₂O + NaCl + CO₂

One advantage of Petri nets is that we can also label each state by some number (0,1,2,3,…) of black dots, called tokens. In our example, each black dot represents a molecule of a given kind. Thus

describes a situation where one atom of carbon, one molecule of oxygen, one molecule of sodium hydroxide, and one molecule of hydrochloric acid are present. No molecules of any other kind are present at this time.

We can then describe processes that occur with the passage of time by moving the tokens around. If the carbon reacts with oxygen we obtain:

If the carbon dioxide combines with the sodium hydroxide to form sodium bicarbonate, we then obtain:

Finally, if the sodium bicarbonate and hydrochloric acid react, we get:

Note that in each case the following rule holds: for any given transition, we can delete one token for each of its input states and simultaneously add one token for each of its output states.

Here is a simpler example, taken from this article:

• Petri net, Wikipedia.

Here the transitions are denoted by black rectangles. This example is somewhat degenerate, because there no transitions with more than one input or more than one output. However, it illustrates the possibility of having more than one token in a given state. It also illustrates the possibility of a transition with no inputs, or no outputs. In chemistry this is useful for modelling a process where molecules of a given sort are added to or removed from the environment.

Symmetric monoidal categories

Now, what about symmetric monoidal categories? If you really want the definition, either read theRosetta Stone paper or the nLab article. For now, you can probably fake it.

A symmetric monoidal category is, very roughly, a category with a tensor product. If we ignore the tokens, a Petri net is a way of specifying a symmetric monoidal category by giving a set of objects and a set of morphisms between tensor products of these objects, for example

where denotes the tensor product of no objects. For example, the objects might be molecules and the morphisms might be chemical reactions; in this case the tensor product symbol is written ‘‘ rather than ‘‘.

The kind of symmetric monoidal category we get this way is a ‘free strict symmetric monoidal category’. It’s ‘free’ in the sense that it’s ‘freely generated’ from some objects and morphisms, without any relations. It’s ‘strict’ because we’re assuming the tensor product is precisely associative, not just associative ‘up to isomorphism’.

For more on how Petri nets describe free symmetric monoidal categories, see:

• Vladimiro Sassone, On the category of Petri net computations, 6th International Conference on Theory and Practice of Software Development, Proceedings of TAPSOFT ’95, Lecture Notes in Computer Science 915, Springer, Berlin, pp. 334-348.

Here Sassone describes a category of Petri nets and sketches the description of a functor from that category to the category of strict symmetric monoidal categories. Sassone also has some other papers on Petri nets and category theory.

As I mentioned, Petri nets have been put to work in many ways. I won’t even start trying to explain that today — for that, try the references on the Azimuth Project page. My only goal today was to convince you that Petri nets are a pathetically simple idea, nothing to be scared of. And if you happen to be a category theorist, it should also be pathetically simple to see how they describe free strict symmetric monoidal categories. If you’re not… well, never mind!