Regardless of which methodology you use to develop software, the following technical allocation chain must occur to arrive at working source code from some form of requirements:
The figure below shows a 2/6/13 end result of the allocation chain for a hypothetical example project. How the 2/6/13 combo was arrived at is person and domain-specific. Given the same set of requirements to N different, domain-knowledgeable people, N different designs will no doubt be generated. Person A may create a 3/6/9 design and person B may conjure up 4/8/16 design.
Given a set of static or evolving requirements, how should one allocate components to namespaces and libraries? The figure below shows extreme 1/1/13 and 13/13/13 cases for our hypothetical 13 component example.
As the number of components, N, in the system design gets larger, the mindless N/N/N strategy becomes unscalable because of an increasing dependency management nightmare. In addition to deciding which K logical components to use in their application, library users must link all K physical libraries with their application code. In the mindless 1/1/N strategy, only one library must be linked with the application code, but because of the single namespace, the design may be harder to logically comprehend.
Expectedly, the solution to the allocation problem lies somewhere in between the two extremes. Arriving at an elegant architecture/design requires a proactive effort with some upfront design thinking. Domain knowledge and skillful application of the coupling-cohesion heuristic can do the trick. For large scale systems, letting a design emerge won’t.
Emergent design works in nature because evolution has had the luxury of millions of years to get it “right“. Even so, according to angry atheist Richard Dawkins, approximately 99% of all “deployed” species have gone extinct – that’s a lot of failed projects. In software development efforts, we don’t have the luxury of million year schedules or the patience for endless, random tinkering.
“The connections between modules are the assumptions which the modules make about each other.” – David Parnas
Before you unquestioningly accept the gospel of the “evolutionary architecture” and “emergent design” priesthood, please at least pause to consider these admonitions:
Give me six hours to chop down a tree and I will spend the first four sharpening the axe – Abe Lincoln
Measure twice, cut once – Unknown
If I had an hour to save the world, I would spend 59 minutes defining the problem and one minute finding solutions – Albert Einstein
100% test coverage is insufficient. 35% of the faults are missing logic paths – Robert Glass
Comprehensiveness is the enemy of comprehensibility – Martin Fowler
Martin’s quote may be the main reason why this preference was written into the Agile Manifesto…
Working software over comprehensive documentation
Obviously, it doesn’t say “Working software and no documentation“. I’d bet my house that Martin and his fellow colleagues who conjured up the manifesto intentionally stuck the word “comprehensive” in there for a reason. And the reason is that “good” documentation reduces costs in both the short and long runs. In addition, check out what the Grade-ster has to say:
The code tells the story, but not the whole story – Grady Booch
Now that the context for this post has been set, I’d like to put in a plug for Simon Brown’s terrific work on the subject of lightweight software architecture documentation. In tribute to Simon, I decided to hoist a few of his slides that resonate with me.
Note that the last graphic is my (and perhaps Simon’s?) way of promoting standardized UML-sketching for recording and communicating software architectures. Of course, if you don’t record and communicate your software architectures, then reading this post was a waste of your time; and I’m sorry for that.
Alright, before we go on, let’s first get something out of the way so that we can start from the same context. This post is not about Small Scale Development (SSD) projects. As the following figure shows, on SDD projects one can successfully write code directly from a list of requirements (with some iterative back-and-forth of course) or set of use cases or (hopefully not) both.
Now that we’ve gotten that out of the way, let’s talk about the real subject of this post: Large Scale Development (LSD <- appropriate acronym, no?) projects. On hallucinogenic LSD efforts, one or possibly two additional activities are required to secure any chance at timely success. As the next figure shows, these two activities are “System Design” and “Software Design“.
So, what’s the difference between “system design” and “software design“? Well, if you’re developing software-intensive products for a specialized business domain (e.g. avionics, radar, sonar, medical, taxes), then you’re gonna need domain experts to bridge the GOHI (Gulf Of Human Intellect) between the higher level requirements and the lower level software design…..
Most specialized domain experts don’t know enough about general software design (object-oriented, structured, functional) and most software experts don’t know enough about domain-specific design to allow for successfully skipping the system design phase/stage. But that hasn’t stopped orgs from doing this….