Why we do Feature freeze
Yesterday we entered the Icehouse development cycle Feature Freeze. But with the incredible growth of the OpenStack development community (508 different contributors over the last 30 days, including 101 new ones !), I hear a lot of questions about it. I've explained it on various forums in the past, but I figured it couldn't hurt to write something a bit more definitive about it.
Those are valid questions. Why freeze features ? That sounds very anti-agile. Isn't our test-centric development model supposed to protect us from regressions anyway ? Let's start with what feature freeze is not. Feature freeze should only affect the integrated OpenStack release. If you don't release (i.e. if you don't special-case certain moments in the development), then feature freezing makes little sense. It's also not a way to punish people who failed to meet a deadline. There are multiple reasons that a given feature will miss a deadline, and most of those are not the fault of the original author of the feature. We do time-based releases, so some features and some developers will necessarily be caught on the wrong side of the fence at some point and need to wait for the next boat. It's an artifact of open innovation projects.
Feature freeze (also known as "FF") is, obviously, about stopping adding new features. You may think of it as artificially blocking your progress, but this has a different effect on other people:
As was evidenced by the Icehouse cycle, good code reviewers are a scarce resource. The first effect of feature freeze is that it limits the quantity of code reviews and make them all about bugfixes. This lets reviewers concentrate on getting as many bugfixes in as possible before the "release". It also helps developers spend time on bugfixes. As long as they can work on features, their natural inclination (or their employer orders) might conflict with the project interest at this time in the cycle, which is to make that point in time we call the "release" as bug-free as possible.
From a QA perspective, stopping the addition of features means you can spend useful time testing "in real life" how OpenStack behaves. There is only so much our automated testing will catch. And it's highly frustrating to spend time testing software that constantly changes under you.
QA is not the only group that needs to catch up. For the documentation team, the I18N team, feature freeze is essential. It's difficult to write documentation if you don't know what will be in the end product. It's frustrating to translate strings that are removed or changed the next day.
And then you have all the downstream consumers of the release that can use time to prepare it. Packagers need software that doesn't constantly change and add dependencies, so that they can prepare packages for OpenStack projects that are released as close to our release date as possible. The marketing team needs time to look into what was produced over the cycle and arrange it in key messages to communicate to the outside world at release time.
Finally, for release management, feature freeze is a tool to reduce risk. The end goal is to avoid introducing an embarassing regression just before release. By gradually limiting the impact of what we accept in the release branch (using feature freeze, but also using the RC dance that comes next), we try our best to prevent that.
For all these groups, it's critical that we stop adding features, changing behavior, adding new configuration options, or changing translatable strings as early as possible. Of course, it's a trade-off. There might be things that are essential to the success of the release, or things that are obviously risk-limited. That's why we have an exception process: the Feature Freeze exceptions ("FFEs").
Feature freeze exceptions may be granted by the PTL (with the friendly but strong advice from the release management team). The idea is to weigh the raw benefit of having that feature in the release, against the complexity of the code that is being brought in, its risk of causing a regression, and how deep we are in feature freeze already. A self-contained change that is ready to merge a few days after feature freeze is a lot more likely to get an exception than a refactoring of a key layer that still needs some significant work to land. It also depends on how many exceptions were already granted on that project, because at some point adding anything more just causes too much disruption.
It's a difficult call to make, and the release management team is here to help the PTLs make it. If your feature gets denied, don't take it personally. As you saw there are a large number of factors involved. Our common goal is to raise the quality of the end release, and every feature freeze exception we grant is a step away from that. We just can't take that many steps back and still guaranteeing we'll win the race.