It seams open

May 4 2009

i can clearly remember when i first discussed DIP and OCP with others: it was two years ago, during my apprenticeship as an XPer. to me, it was nothing new, i already had studied all the principles that now come under the SOLID ancronym. but, probably, i hadn’t digested them enough: something that just came later with experience.

Dependency Inversion was for sure my favourite since then, for its multiple implications depending on what “high level” and “low level” mean; in my view, there are at least two meanings: abstraction (high level policies vs. low level details) and layering (close-to-user layer such as GUI vs. infrastructure layer). much more, i loved its love-hate with Dependency Injection (maybe more on this in a separate post).

Open-Closed is harder to understand, at first. how could you “change without modifying”? abstraction is the key! let modules depend on abstraction, then provide new implementations when behaviour has to change, without the need to modify existing code. in other words, always depend on abstraction: which, in the end, is DIP itself. OCP and DIP are such “yin and yang” in software design: they help achieve each other.

then, when i first discussed OCP with team-mates, i pushed for an analogy with Feather’s Seam Model. it’s discussed in “Working Effectively with Legacy Code” book: use seams to let legacy (which means untested) code be tested. to be fair, my analogy was not welcomed too much! i had to force my thesis a bit, and in the end not everybody was convinced.

two years later, it happened again! indeed, a few months ago we had a study group on OCP. i was in charge of preparing material to study on, and i chose a few corollary articles: first chapter from GoF Design Patterns book, which focuses on “design targeting interfaces”, and WELC chapter 4 “the Seam Model”. this time i was more convincing, and the analogy between OCP and the Seam Model became clear during our study session! and now, i want to tell you too.

after reading the bunch of articles i prepared, i asked a colleague to state in a few words what OCP was about. he said “change behaviour without modifying code”. great! then, i asked him again to state what the Seam Model was about, and he said “let code behave in a different way without modifying it”. well.. nothing left to say!

abstraction is the key, that’s true. but what about code which doesn’t follow OCP/DIP? it doesn’t depend on abstraction. we can modify it, refactoring, but we need an automatic test suite, in order to guarantee no behavioural change. and that’s exactly what Feather’s model is about: change code a little bit putting seams, to test it in isolation.

on the other side: what seams can you use to test OCP-compliant code? of course, already existing abstractions: you just have to change enabling points (in a test, usually in fixture setup).

in the report for our study session, on our internal wiki, we wrote:

We then discussed what OCP and Feather’s Seam Model have in common:

  • they seem the same idea, applied to reach different goals
    • OCP: put abstractions to isolate from future source code changes
    • Seam: put abstractions to test applications without changing its source code
  • to recap
    • closure/abstraction = seam
    • “main() routine”/factories = enabling point

to be precise, Feather’s model is about three different techniques, useful for testing legacy code written in any language, not just object-oriented ones. he talks about preprocessing seam, linking seam and object seam. so, the analogy with OCP is just between abstractions and object seams, even if sometime linking techniques are also used to achieve abstraction (such as reflection or some configuration-based IoC tool).

so, when i watched Misko‘s clean-code-talks videos, i was surprised to hear him use the term “seam” while talking about DI and SOLID principles: he confirmed my analogy have sense!

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One Response to “It seams open”

  1. Lyo Says:

    Nice topic! I never had a chance or the courage to discuss such matters of standardization and the true inspiration for such ideas (infact “authors” usually do not publish the source of their ideas).

    If I could just leave a feedback, could you please rafactor in order to include capital letters after each full stop or at the beginning of the paragraph or subjects? That was quite annoying. Still, very nice blog post!


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