Gradual Refactoring

We’re right now in the middle of something that feels a little like an unplanned experiment in code management. Unplanned in the sense that I didn’t expect us to work the way we do, not so much in the sense that I think it is particularly risky. It started not quite a year ago with my colleague Mateusz suggesting that we should dedicate a fixed percentage of the story points in each sprint to what he called maintenance or technical stories. That soon crystallised into a backlog owned by me, where we schedule stories that are aimed at somehow improving our productivity as opposed to our product. So Paul, the ‘normal’ product owner, is in charge of the main backlog that deals with improving the products, and I am the owner of the maintenance backlog, where we improve the efficiency of how we work by improving processes, tools and removing technical debt. We’re aiming at spending about 15% of our time on technical backlog stories, and we more or less do.

Typical examples of stories that have gone onto our technical backlog are:

  • A tool that allows QA to specify that outgoing service calls matching certain regular expressions should return mock data specified in a file rather actually calling the service. This makes us a lot more productive with regard to verifying site behaviour in certain hard-to-recreate and data-dependent cases.
  • Improvements to our performance monitoring systems and tools that make it easier for us to figure out where we have performance problems when we do.
  • Auditing and optimising the QA server allocation in order to speed up especially our automated test scripts.
  • Various refactoring stories that clean up code where functional evolution has led to the original design no longer being suitable.

That has worked out pretty much as expected: we’ve gained benefits from the productivity improvements and we continue to spend 5-6 times more effort on money-making product improvements than on engineering driven platform-building.

The unexpected thing that has happened is that we’re heading towards a situation where we have different design generations that solve similar problems. As an example, the original pattern we used to create Spring MVC controllers has broken down, so we’ve come up with a new one that is better though not yet perfect. In order to have stories small enough to complete within one iteration, we’ve had to apply this pattern on a controller-by-controller basis – each refactoring has taken about 2 weeks of calendar time so far, except the first one which took about 4, so the effort isn’t trivial. There are nearly 40 controller implementations in our site code and about 95% of the incoming traffic is handled by 4 of those. We’re now at a stage where we’ve refactored three of those four controllers. Given that the other 35 or so controllers a) don’t serve a lot of traffic, so don’t have a lot of business value and b) don’t get changed a lot because the functions they provide aren’t ones that we need to innovate in, I don’t feel like refactoring all of them. In fact, the next refactoring story in the backlog is aimed at a different area in the site, where a repeated pattern has broken down in a similar way.

My initial gut reaction was that we should apply any new pattern for a commonly occurring situation across the board, then tackle the next similar situation. That keeps the code clean and makes it easy to find your way around. But the point of refactoring is that it must be an investment that you can recoup, and if we haven’t spent more than 4 hours working on a particular controller in the last year, what are the chances of ever recouping an investment of a man-week? We would probably have to keep using the same controller for at least 20 years, assuming the refactoring made us twice as productive, and that doesn’t seem likely to happen. Given that, it seems like the best option is to focus refactoring efforts where they give return on investment, which is those parts of the code that you do most of your work in.

I think we’re more or less permanently stuck with two different generations of controller patterns. But it will be interesting to see what will happen over the next year or two – can this super-pattern of having annual growth rings of standardised solution patterns handle more than two generations? I’m not sure, but I believe it can.

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