August 10, 2015
A Hierarchy Of Software Design NeedsDesign is not a binary proposition. There is no clear dividing line between a good software design and bad software design, and even the best designs are compromises that seek to balance competing forces like performance, readability, testability, reuse and so on.
When I refactor a design, it can sometimes introduce side-effects - namely, other code smells - that I deem less bad than what was there before. For example, maybe I have a business object that renders itself as HTML - bad, bad, bad! Right?
The HTML format is likely to change more often than the object's data schema, and we might want to render it to other formats. So it makes sense to split out the rendering part to a separate object. But in doing so, we end up creating "feature envy" - an unhealthy high coupling between our renderer and the business object so it can get the data in needs - in the process.
I consider the new feature envy less bad than the dual responsibility, so I live with it.
In fact, there tends to be a hierarchy of needs in software design, where one design issue will take precedence over another. It's useful, when starting out, to know what that hierarchy of needs is.
Now, the needs may differ depending on the requirements of our design - e.g., on a small-memory device, memory footprint matters way more than it does for desktop software usually - but there is a fairly consistent pattern that appears over and over in the majority of applications.
There are, of course, a universe of qualities we may need to balance. But let's deal with the top six to get you thinking:
1. The Code Must Work
Doesn't matter how good you think the design is if it doesn't do what the customer needs. Good design always comes back to "yes, but does it pass the acceptance tests?" If it doesn't, it's de facto a bad design, regardless.
2. The Code Must Be Easy To Understand
By far the biggest factor in the maintainability of code is whether or not programmers can understand it. I will gladly sacrifice less vital design goals to make code more readable. Put more effort into this. And then put even more effort into it. However much attention you're paying to readability, it's almost certainly not enough. C'mon, you've read code. You know it's true.
But if the code is totally readable, but doesn't work, then spend more time on 1.
3. The Code Must Be As Simple As We Can Make It
Less code generally means a lower cost of maintenance. But beware; you can take simplicity too far. I've seen some very compact code that was almost intractable to human eyes. Readability trumps simplicity. And, yes, functional programmers, I'm particularly looking at you.
4. The Code Must Not Repeat Itself
The opposite of duplication is reuse. Yes it is: don't argue!
Duplication in our code can often give us useful clues about generalisations and abstractions that may be lurking in there that need bringing out through refactoring. That's why "removing duplication" is a particular focus of the refactoring step in Test-driven Development.
Having said that, code can get too abstract and too general at the expense of readability. Not everything has to eventually turn into the Interpreter pattern, and the goal of most projects isn't to develop yet another MVC framework.
In the Refuctoring Challenge we do on the TDD workshops, over-abstracting often proves to be a sure-fire way of making code harder to change.
5. Code Should Tell, Not Ask
"Tell, Don't Ask" is a core pillar of good modular -notice I didn't say "object oriented" - code. Another way of framing it is to say "put the work where the knowledge is". That way, we end up with modules where more dependencies are contained and fewer dependencies are shared between modules. So if a module knows the customer's date of birth, it should be responsible for doing the work of calculating the customer's current age. That way, other modules don't have to ask for the date of birth to do that calculation, and modules know a little bit less about each other.
It goes by many names: "encapsulation", "information hiding" etc. But the bottom line is that modules should interact with each other as little as possible. This leads to modules that are more cohesive and loosely coupled, so when we make a change to one, it's less likely to affect the others.
But it's not always possible, and I've seen some awful fudges when programmers apply Tell, Don't Ask at the expense of higher needs like simplicity and readability. Remember simply this: sometimes the best way is to use a getter.
6. Code Should Be S.O.L.I.D.
You may be surprised to hear that I put OO design principles so far down my hierarchy of needs. But that's partly because I'm an old programmer, and can vaguely recall writing well-designed applications in non-OO languages. "Tell, Don't Ask", for example, is as do-able in FORTRAN as it is in Smalltalk.
Don't believe me? Then read the chapter in Bertrand Meyer's Object Oriented Software Construction that deals with writing OO code in non-OO languages.
From my own experiments, I've learned that coupling and cohesion have a bigger impact on the cost of changing code. A secondary factor is substitutability of dependencies - the ability to insert a new implementation in the slot of an old one without affecting the client code. That's mostly what S.O.L.I.D. is all about.
This is the stuff that we can really only do in OO languages that directly support polymorphism. And it's important, for sure. But not as important as coupling and cohesion, lack of duplication, simplicity, readability and whether or not the code actually works.
Luckily, apart from the "S" in S.O.L.I.D. (Single Responsibility), the O.L.I.D. is fairly orthogonal to these other concerns. We don't need to trade off between substitutability and Tell, Don't Ask, for example. They're quite compatible, as are the other design needs - if you do it right.
In this sense, the trade off is more about how much time I devote t thinking about S.O.L.I.D. compared to other more pressing concerns. Think about it: yes. Obsess about it: no.
Like I said, there are many, many more things that concern us in our designs - and they vary depending on the kind of software we're creating - but I tend to find these 6 are usually at the top of the hierarchy.
So... What's your hierarchy of design needs?
Posted 2 years, 11 months ago on August 10, 2015