The two-phase iteration

Developing software for medical devices implies two special aspects:

  • Documentation becomes important. Regulations require a ton of documentation, and a failure to do so might hamper the market access of your medical device. Conclusion: documentation becomes a part of the definition of done and you have to get some distance from the second principle of the agile manifesto (favoring working software over comprehensive documentation). The problem is, developers in general hate documentation (but have all kind of justifications to make it appear as a conscious choice, such as “documentation is always outdated” or “just read the code”). But when you have to do something that you don’t like and there’s no way to get around it, I believe you should get at it and do it on a regular basis. Don’t postpone documentation until the end of the project, when people leave or are assigned to new endeavors, and three quarters of the knowledge has evaporated.
  • Leaving bugs is not an option. And an excellent technique for finding bugs is manual testing. In my best-effort projects as far as automated testing is concerned, we still find 7 bugs a day through manual testing. And some tests are just too complicated to be automated (example: recovery after a power outage, intrusion testing). Manual testing has a very direct consequence: at some point, you have to deliver a version, testers will test it and it will take time, and when they put their hands on the next version, they don’t want it crippled by regressions due to new developments. Manual testers don’t have the patience of a continuous integration system.


A good way to adjust to these constraints is to use two-phase iterations.Phase 1: build. Phase 2: stabilize.

Build & stabilize
Build & stabilize

To elaborate a bit more:

  • Phase 1: construction.
    • Build a product increment. In this phase, you take risks. You write that ambitious new feature. You refactor that awfully complex engine. You change the build system.
    • You maintain quality (automated tests, bug fixing), but it’s not your main concern. You might let bugs and broken tests pile up a little (but not too much).
    • Little manual testing occurs (only functional challenges by those who wrote the specs, weekly general regression testing), but testers prepare for the next phase by honing test procedures and test strategies (according to impact analysis).
    • The final days of the construction phase will be a little constrained. There has to be a feeling of deadline around this date. Everybody works hard to be on time.
    • The most concrete consequence of the end of the construction phase is that a stabilization branch for this iteration is created.
  • Phase 2: stabilization.
    • Finish the product increment.
      • Testers test.
      • Developers fix bugs and broken tests. They are not allowed to take risks on the stabilization branch.
      • Several versions are issued and tested until the last one meets quality standards (I like to set a low total known bugs threshold – more on this in a dedicated post).
    • Write the documentation. It’s a good time: after the rush, and while things are still fresh in everyone’s head. Write the mandatory one-shot documentation for the iteration (test report, formal reviews…). Update long-term documentation (e.g. architecture documents).
    • Prepare for the next iteration. Select the candidate user stories. Have functional people explain them to developers. Extract the requirements related to the user stories, make sure that evil details are taken into account (remember, at that time, specs should be ready. The months of talking and studying and analyzing features are over. If the spec is not ready, then the feature is not mature enough for this iteration. If you can’t write it down, you can’t code it). Developers and architects should throw their first design ideas one whiteboards and start negotiating solutions. Then formal planning occurs (detailed planning poker with tasks). Product Owners compare task estimates with personnel availability and estimated throughput, then choose which user stories will be part of the iteration, and which will not.


Two-phase iteration
Detailed activities inside the two-phase iteration

Practical considerations

  • Iteration size. Although Scrum advocates for 2 to 4 weeks iterations, for this kind of process, I’ve experimented with values between 4 and 8 and settled for 6 weeks. This seems to me a good compromise: big enough so that content matches iteration overhead (documentation, planning, manual test campaign), small enough to be manageable. Of course, this value works in my environment, you should try several and see what works for you.
  • Phase size. My biggest project started with a 2/3-1/3 proportion (that is, four weeks of construction and two weeks of stabilization). Then, as maintenance cost increased, I increased stabilization level to 12 days (leaving 18 for construction) and plan to increase it again soon. It may seem long, but in my usual environment it takes about 4 deliveries to get a software version of good enough quality. This is a key feature of the two-phase iteration: by adjusting the relative size of construction and stabilization, you have a built-in mechanism for regulating speed and quality. More on this in a dedicated article.
  • Be tough with construction end date. If a user story is not ready, then it’s going to ship in the next iteration (Of course, if everybody at system level waits for it, you may want to be (or be forced to be) a little flexible. But if that user story is so important, why did it slip until the end of the construction phase? Couldn’t it have been planned earlier? You should always have a buffer of user stories or technical tasks of lesser importance ready to be sacrificed if something important goes out of hand).
  • Be tough with quality at the end of stabilization. If the product is buggy, it’s not shippable. Immediate course of action is to fix it and ship it. The following construction phase will be smaller than usual, with less features. That’s a smaller problem than a buggy software – your users are more important than your bosses.
  • On the day stabilization phase begins, a stab branch should be created in the repository (named, for example, iteration_XX_stab). Why?
    • Dev on trunk/master: sometimes it is reasonable to allow a developer to start construction N+1 during stabilization N. Example: a huge refactoring with lots of impact, that should rather be performed when the change level on the code is lower (during stab). That developer should work on the trunk/master while everybody remains on the stab branch.
    • Psychological reasons: the fact that they have to switch from trunk/master to stab branch helps developers materialize the fact that activities will be different. The fact that it’s possible to perform minor tasks in the trunk helps keep the stab branch clean (bug fixing only!). For example: fix that bug in the stab branch the quick and dirty way to avoid unnecessary regressions, but merge the fix at once in the trunk, and then refactor it until the design doesn’t make you blush anymore.
    • Version maintenance. Imagine you have to fix a bug for the software version of iteration N is 3 months or 3 years from now: pick stab branch N just where you left it.

3 thoughts on “The two-phase iteration

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