Monday, June 25, 2018

Thinking about Software Architecture & Design : Part 13


Harold Abelson, co-author of the seminal tome Structure and Interpretation Of Computer Programs (SICP) said that “programs must be written for people to read, and only incidentally for machines to execute.”

The importance of human-to-human communication over human-to-machine is even more true in Software Architectures, where there is typically another layer or two of resolution before machines can interpret the required architecture.

Human-to-human communications is always fraught with potential for miscommunications and the reasons for this run very deep indeed. Dig into this subject and it is easy to be amazed that anything can be communicated perfectly at all. It is a heady mix of linguistics, semiotics, epistemology and psychology. I have written before (for example, in the “What is Law Series - http://seanmcgrath.blogspot.com/2017/06/what-is-law-part-14.html) about the first three of these, but here I want to talk about the fourth – psychology.

I had the good fortune many years ago to stumble upon the book Inevitable Illusions by Massimo Piattelli-Palmarini and it opened my mind to the idea that there are mental concepts we are all prone to develop, that are objectively incorrect – yet unavoidable. Think of your favorite optical illusion. At first you were amazed and incredulous. Then you read/discovered how it works. You proved to your own satisfaction that your eyes were deceiving you. And yet, every time you look at the optical illusion, your brain has another go at selling you on the illusion. You cannot switch it off. No amount of knowing how you are being deceived by your eyes will get your eyes to change their minds, so to speak.

I have learned over the years that some illusions about computing are so strong that it is often best to incorporate them into architectures rather than try to remove them. For example, there is the “send illusion”. Most of the time when there is an arrow between A and B in a software architecture, there is a send illusion lurking. The reason being, it is not possible to send digital bits. They don't move through space. Instead they are replicated. Thus every implied “send” in an architecture can never be a truly simple send operation and it involves at the very least, a copy followed by a delete. 

Another example is the idea of a finite limit to the complexity of business rules. This is the very (very!) appealing idea that with enough refinement, it is possible to arrive at a full expression of the business rules that express some desirable computation. This is sometimes true (especially in text books) which adds to the power of the inevitable illusion. However, in many cases this is only true if you can freeze requirements – a tough proposition – and often is impossible even then. For example in systems where there is a feedback loop between the business rules and the data creating a sort of “fractal boundary” that the corpus of business rules can never fully cover.

I do not let these concerns stop me from using concepts like “send” and “business rule repository” in my architectures because I know how powerfully these concepts are locked into all our minds. However, I do try to conceptualize them as analogies and remain conscious of the tricks my mind plays with them. I then seek to ensure that the implementation addresses the unavoidable delta between the inevitable illusion in my head and the reality in the machine.


No comments: