Monday, May 28, 2018

Thinking about Software Architecture & Design : Part 10


Once the nouns and verbs I need in my architecture start to solidify, I look at organizing them across multiple dimensions. I tend to think of the noun/verb organization exercise in the physical terms of surface area and moving parts. By "surface area" I mean minimizing the sheer size of the model. I freely admin that page count is a crude-sounding measure for a software architecture, but I have found over the years that the total size of the document required to adequately explain the architecture is an excellent proxy for its total cost of ownership.

It is vital, for a good representation of a software architecture, that both the data side and the computation side are covered. I have seen many architectures where the data side is covered well but the computation side has many gaps. This is the infamous “and then magic happens” part of the software architecture world. It is most commonly seen when there is too much use of convenient real world analogies. i.e. thematic modules that just snap together like jigsaw/lego pieces, data layers that sit perfectly on top of each other like layers of a cake, objects that nest perfectly inside other objects like Russian Dolls etc.

When I have a document that I feel adequately reflects both the noun and the verb side of the architecture, I employ a variety of techniques to minimize its overall size. On the noun side, I can create type hierarchies to explore how nouns can be considered special cases of other nouns. I can create relational de-compositions to explore how partial nouns can be shared by other nouns. I will typically “jump levels” when I am doing this. i.e. I will switch between thinking of the nouns in purely abstract terms (“what is a widget really” to thinking about them in physical terms: “how best to create/read/update/delete widgets?”). I think of it as working downwards towards implementation an upwards towards abstraction at the same time. It is head hurting at times, but in my experience produces better practical results that the simpler step-wise refinement approach of moving incrementally downwards from abstraction to concrete implementation.

On the verb side, I tend to focus on the classic engineering concept of "moving parts". Just as in the physical world, it has been my experience that the smaller the number of independent moving parts in an architecture, the better. Giving a lot of thought to opportunities to reduce the total number of verbs required pays handsome dividends. I think of it in terms of combinatorics. What are the fundamental operators I need from which, all the other operators can be created by combinations of the fundamental operators? Getting to this set of fundamental operators is almost like finding the architecture inside the architecture.

I also think of verbs in terms of complexity generators. Here I am using the word “complexity” in the mathematical sense. Complexity is not a fundamentally bad thing! I would argue that all system behavior has a certain amount of complexity. The trick with complexity is to find ways to create the amount required but in a way that allows you to be in control of it. The compounding of verbs is the workhorse for complexity generation. I think of data as a resource that undergoes transformation over time. Most computation – even the simplest assignment of the value Y to be the value Y + 1 has an implicit time dimension. Assuming Y is a value that lives over a long period of time – i.e. is persisted in some storage system – then Y today is just the compounded result of the verbs applied to it from its date of creation.

There are two main things I watch for as I am looking into my verbs and how to compound them and apply them to my nouns. The first is to always include the ability to create an ad-hoc verb “by hand”. By which I mean, always having the ability to edit the data in nouns using purely interactive means. This is especially important in systems where down-time for the creation of new algorithmic verbs is not an option.

The second is watching out for feedback/recursion in verbs. Nothing generates complexity faster than feedback/recursion and when it is it used, it must be used with great care. I have a poster on my wall of a fractal with its simple mathematical formula written underneath it. It is incredible that such bottomless complexity can be derived from such a harmless looking feedback loop. Using it wisely can produce architectures capable of highly complex behaviors but with small surface areas and few moving parts. Used unwisely.....

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