Wednesday, September 06, 2006

Agile Methodologies - Anti-Reusability?

If you subscribe to the Agile notions of YAGNI (You aren't gonna need it) and DTSTTCPW (do the simplest thing that could possibly work), are you potentially writing code with limited reusablility?

The Purpose of a Routine

In Steve McConnell's Code Complete he identifies many reasons to create a routine. Avoiding duplicate code is the most popular, but there are other reasons too. One of them is 'Promoting code reuse'. In the Agile age then, is it still prudent to try to make code reusable? Or should developers simply abstract when the need arises? Developing for the immediate need, and not some uncertain future.

If we apply the Agile XP practices in their purest form, it could be argued that new routines should only be created when needed. Perhaps that's not quite right. Perhaps we should create a function or method when a refactoring requires it. This still means blocking out any thoughts of reusability. Making a method more 'reusable' than necessary still contradicts the spirit of agile.

For example, if my code always multiplies a number x by y and y is always 2 my function should look someting like,

public int multiplyXAxisBy2(x)
return x * 2;

If my multiply method took two arguments x and y and multiplied them it could be argued that I am 'building for the future'. Alright, I'll admit this is a contrived and extreme example. But I have seen developers argue for hard-coded values in their methods on the grounds that exposing those values as parameters violates the YAGNI principle.


Like most things in software, there is no simple answer. The best you can hope for is 'it depends'. In the case of agile design and reuse, the 'it depends' postulate seems to hold. YAGNI is perhaps a reaction to the 'modeling the world' design dreams of the past. It is a way to pull back on the programmers reigns and say 'Hey, the customer needs something real, today! stop dreaming and get on track'.

Achieving balance between YAGNI and REUSE means looking at you method interface and asking 'does it stand on its own, does it make sense?'. Constantly changing method names through refactorings is probably and indication of a poor interface. The method names should hardly change at all, so make them specific and understandable. The parameters should jive with the method name. For example, a method like SaveAttachment() should take a parameter like an Attachment object. It should not take parameters that leave the caller trying to understand the internals of the method. Something like SaveAttachment(UserLogin, Attachment, UrlLink, AttachmentType) is probably a sign of a bad object design (some of these parameters should probably be contained in the object itself).

So, in sum I have to say use your judgement and try to look at your interfaces in isolation, not as interconnected pieces, and hopefully you will create reusable code without straying too far from YAGNI.

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