"Pattern Languages as Autocatalytic Systems”

1) Patterns

As defined by Alexander "Each pattern is a three-part rule, which expresses a relation between a certain context, a problem, and a solution."

He also says that:

"The pattern is, in short, at the same time a thing, which happens in the world, and the rule which tells us how to create that thing, and when we must create it. It is both a process and a thing; both a description of a thing which is alive, and a description of the process which will generate that thing (p. 247)."

From this perspective, patterns are similar to the genes encoded in the DNA that are waiting to be activated by an appropriate proteinic context. And as with genes, when the correct context is found, patterns are triggered and generate structure.

As in genetics, part of the resulting context, and initial context of other patterns, is the structure generated by the pattern. In biological systems the resulting context/initial context for other genes to be active is the existing generated structure and the generated proteinic substrate.

The main difference from organic systems is that in systems where there are humans that enforce the patterns (rules), or in software systems where the patterns are enforced by the software, the patterns can be enforced at any level of structure, not only at the equivalent "cellular level".

Also, most systems created and/or maintained by humans do not have the same level of dynamic complexity as organic systems do. For example, there is limited or no self-organization, autocatalysis, metabolic (self-regulation), or autonimic functions.

However, it would be perhaps desirable to have the above dynamic characteristics present in man-made systems to more closely resemble the properties of living systems.

2) Pattern Languages

Pattern languages are "concurrent rule systems" that generate structure by the application of one or more sequences of patterns.

In this regard, pattern languages play the same role upon systems that DNA plays in organic systems.

In organic systems one or more genes may be active in a single cell, and other cells will also have independent and different activations.

Similarly, in systems where human intervention is required to enforce the patterns in the pattern language, all of the rules' contexts need to be periodically and concurrently evaluated, to guarantee that all triggered patterns are executed at any one time.

This requirement makes applying pattern languages hard because in a system there might be one or more concurrent processes that evaluate patterns simultaneously. Yet, at any one time, the existing local generated structure must be part of the initial context for every pattern. (This is another way of saying “the solution introduces new problems” i.e. the BART experiment.)

The executed patterns in turn generate the correct structure for the system - adapting and providing self-organizing mechanisms for the system as time goes by.

3) Sequences

Sequences of patterns are formed by the execution of rules linked in a network of "contexts" and "resulting contexts".

In general, and depending on pattern context:

a) patterns execute in *any order* that conforms to the structure of "context and "resulting contexts" dictated by the pattern language. i.e. a sequence may be started in a low, medium or high level pattern.

However, most pattern languages tend to form a proto-skeleton and then fill in the details, just as an embryo emerges into a living form.

b) a pattern, or a subsequence or patterns, may be executed many times (in iteration or recursion). More on this in Self-Organization below.

c) different patterns may be triggered given external parameters or conditions. The pattern language, through its "morphological completeness" clause will provide paths to generate different structures, akin to the generation of different cell types. Also, similar patterns need not be contained exactly in one another. There might be overlapping structure.

(A city is not a tree.)

d) several sequences might be executing for the same system at any one time.

e) as in rule systems, a sequence may be partially executed and then wait for a temporal or structural condition that may activate the execution of the sequence.

Etc.

4) Emergent Structure

As patterns in a pattern language generate structure, the emergent structure together with other local conditions defines the context of where other patterns apply.

This is akin to morphogenesis -- the embryo _and_ the localized proteinic substrate determines what proteins are next generated by the ribosomes as guided by the triggering of active genes, and this in turns determines cell differentiation over time.

5) Self-Organization

In dynamical systems some patterns can form sequences of patterns that form autocatalytic chains, akin to the organic proteinic self-organizing autocatalytic chains in cellular processes and/or metabolic functions.

These self-organizing chains contribute in many ways to the system:

a) it provides dynamical inertia to the system. More on this on

Stability below.

b) contributes to the adaptation of the system, as per Stu Kauffman’s conclusions: adaptation depends on self-organization and natural selection. See “The Origin of Order”.

6) Stability

The stability of the generated system strongly depends on the amount of structure controlled by the pattern language, and in the strength of the pattern sequences in either autocatalytic or simple sequence form.

This is akin to how a living system is able to maintain metabolism and continued structure embedded in a soup of bacteria, viruses, other living organisms, and chemicals that do not directly contribute to the living form.

7) Adaptation

An important adaptive feature is the introduction of new patterns into the system. This provides the ability to generate new structures that allow the system to adapt to new situations, but in addition, it may also contribute to the formation of new autocatalytic (self-organizing) mechanisms, that stabilize the system over time.