There has been a great exchange on the London Clojurians mailing list recently talking about the impact of a good REPL on development cycles. The conversation kicks into high-gear with this post from Malcolm Sparks although it is worth reading it from the start (membership might be required I can’t remember). In his post Malcolm talks about the cost of up-front quality. This, broadly speaking, is the cost of the testing required to put a feature live, it is essentially a way of looking at the cost that automated testing adds to the development process. As Malcolm says later: “I’m a strong proponent of testing, but only when testing has the effect of driving down the cost of change.”.

Once upon a time we had to fight to introduce unit-testing and automated integration builds and tests. Now it is a kind of given that this is a good thing, rather like a pendulum, the issue is going too far in the opposite direction. If you’ve ever had to scrap more than one feature because it failed to perform then the up-front quality cost is something you consider as closely as the cost of up-front design and production failure.

Now the London Clojurians list is at that perfect time in its lifespan where it is full of engaged and knowledgeable technologists so Steve Freeman drops into the thread and sensibly points out that Malcolm is also guilty of excess by valuing feature mutability to the point of wanting to be able to change a feature in-flight in production, something that is cool but is probably in excess of any actual requirements. Steve adds that there are other benefits to automated testing, particularly unit testing, beyond guaranteeing quality.

However Steve mentions the Forward approach, which I also subscribe to, of creating very small codebases. So then Paul Ingles gets involved and posts the best description I’ve read of how you can use solution structure, monitoring and restrained codebases to avoid dealing with a lot of the issues of software complexity. It’s hard to boil the argument down because the post deserves reading in full. I would try and summarise it as the external contact points of a service are what matters and if you fulfil the contract of the service you can write a replacement in any technology or stack and put the replacement alongside the original service.

One the powerful aspects of this approach is that is generalises the “throw one away” rule and allows you to say that the current codebase can be discarded whenever your knowledge of the domain or your available tools change sufficiently to make it possible to write an improved version of the service.

Steve then points out some of the other rules that make this work, being able to track and ideally change consumers as well. Its an argument for always using keys on API services, even internal ones, to help see what is calling your service. Something that is moving towards being a standard at the Guardian.

So to summarise, a little thread of pure gold and the kind of thing that can only happen when the right people have the time to talk and share experiences. And when it comes to testing, ask whether your tests are making it cheaper to change the software when the real functionality is discovered in production.