A Functional System

This entry is part 3 of 8 in the series Design Principles

Focus: Functionality
Description: Make sure the system meets the stated requirements!

Surprisingly, when discussing design, it’s easy to forget about perhaps the most important design criteria, namely that the system should be able to provide the required functionality! I actually didn’t include this in my first draft of important design criteria.

Perhaps it is thought that the necessary functionality is expressed solely in the requirements specification, and then realized by the implementation, and that the design is in some way orthogonal to this. However, a sloppy system design can ruin even the most well-specified requirements. If nothing else, it might set some unfortunate decisions in stone (well, code), and make other requirements hard or impossible to meet.


That quick and dirty hack you did a couple of months ago – you know, the one where you remove obviously invalid customer email addresses on the incoming orders from the new web shop (because those caused issues at your end, and the consultants had far more pressing issues than improving email validation rules).

You know the hack doesn’t catch all possible errors, but it clearly improved the situation, and has been running OK since day one. Even if that hack had no other of the desirable properties we are discussing here, it was functional, and provides value, or you would’ve removed it from production.

And that new web shop, if it isn’t functional – if it does not even provide the basic shopping functionality that your company wanted it to have – then it’s in effect worthless! This holds even if it would have a whole host of other properties (including some less desirable properties such as costing too much).

Obviously, a more complex system can provide value, even though it does not do everything exactly the way the customer wanted. It may have bugs, or a huge misunderstanding that makes one module clearly less useful than intended, but it can still be functional as a whole.

How to achieve functionality by design

The most important tools for achieving functionality are:

  • A good requirements specification. (Hah! as if that’ll ever happen!)
  • Good communication with the stakeholders (the customer, the users etc.)
  • A good understanding of the problem domain. (Domain knowledge.)
  • General development experience. (The more, the merrier!)
  • Several releases of the system. (Since you will never get it right the first time!)

It is important to note that a requirements specification, or even communication with the customer, will never in itself give the whole picture. At least, I’ve never been on such a project, and I’m pretty sure I never will. This is because the customer won’t be able to get every tiny detail down on paper, and won’t even be able to tell you everything during meetings.

The more complex the system, the less likely you are to get the whole picture up front before starting development. In all likelihood, no one (including the customer) will even understand the full system completely at that point, so understandably they cannot give you all the details.

It is also important to realize that it is very common that many quite important requirements are not actually stated explicitly in the requirements specification! There is a whole category of requirements that the customer often leave implicit and unstated, but that we should nevertheless understand if we want to build a great system. More on this in the next post!

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