# Why Distributed GLC is different from Programming with Chemical Reaction Networks

I use the occasion to bookmark the post at Azimuth Programming with Chemical Reaction Networks, most of all because of the beautiful bibliography which contains links to great articles which can be freely downloaded. Thank you John Baez for putting in one place such an essential list of articles.

Also, I want to explain very briefly why CRNs are not used in Distributed GLC.

Recall that Distributed GLC  is a distributed computing model which is based on an artificial chemistry called chemlambda, itself a variant (slightly different) of graphic lambda calculus, or GLC.

There are two stages of the computation:

1. define the initial participants at the computation, each one called an “actor”. Each actor is in charge of a chemlambda molecule. Molecules of different actors may be connected, each such connection being interpreted as a connection between actors.  If we put together all molecules of all actors then we can glue them into one big molecule. Imagine this big molecule as a map of the world and actors as countries, each coloured with a different colour.  Neighbouring countries correspond to connected actors. This big molecule is a graph in the chemlambda formalism. The graph which has the actors as nodes and neighbouring relations as edges is called the “actors diagram” and is a different graph than the big molecule graph.
2. Each actor has a name (like a mail address, or like the colour of a country). Each actor knows only the names of neighbouring actors. Moreover, each actor will behave only as a function of the molecule it has and according to the knowledge of his neighbouring actors behaviour. From this point, the proper part of the computation, each actor is by itself. So, from this point on we use the way of seeing of the Actor Model of Carl Hewitt.  Not the point of view of Process Algebra. (See  Actor model and process calculi.)  OK, each actor has 5 behaviours, most of them consisting into graph rewrites of it’s own molecule or between molecules of neighbouring actors. These graph rewrites are like chemical reactions between molecules and enzymes, one enzyme per graph rewrite. Finally, the connections between actors (may) change as a result of these graph rewrites.

That is the Distributed GLC model, very briefly.

It is different from Programming with CRN because of several reasons.

1.  Participants at the computation are individual molecules. This may be unrealistic for real chemistry and lab measurements of chemical reactions, but this is not the point, because the artificial chemistry chemlambda is designed to be used on the Internet. (However, see the research project on  single molecule quantum computer).

2. There is no explicit stochastic behaviour. Each actor in charge of it’s molecule behaves deterministically. (Or not, there is nothing which stops the model to be modified by introducing some randomness into the behaviour of each actor, but that is not the point here). There are not huge numbers of actors, or some average behaviour of those.

That is because of point 1. (we stay at the level of individual molecules and their puppeteers, their actors) and also because we use the Actor Model style, and not Process Algebra.

So, there is an implicit randomness, coming from the fact that the computation is designed Actor Model style, i.e. such that it may work differently, depending on the particular physical  timing of messages which are sent between actors.

3.  The computation is purely local. It is also decentralized. There is no corporate point of view of counting the number of identical molecules, or their proportion in a global space – global time solution.  This is something reasonable from the point of view of a distributed computation over the Internet.

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All this being said,  of course that it would be interesting to see what happens with CRNs of reactions of molecules in chemlambda.  May be very instructive, but this would be a different model.

That is why Distributed GLC does not use the CRN point of view.

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