[xwiki-devs] New development strategy

Hi devs, Since we're moving to git / GitHub, it's time to re-evaluate the development / git usage strategy. = Short version: 1/ Where do developers commit: A. Always in the master branch B. In feature branches in the official repo, merged into the master branch when ready C. In their personal forks, requesting pulls in the official repo with a mandatory code review from another developer 2/ How to move code from development to release: A. Commit and release from the master branch B. Develop in feature branches, merge them in the master when ready, release from master C. Develop in feature branches, merge them in a development branch (master) for polishing, merge them in a release branch when done-done D. Develop in feature branches, merge them often in the development branch (master) for snapshot testing, move into the stabilization (pre-release) branch for polishing, move into the release branch when done-done. E. Develop in feature branches, merge them in a development branch for snapshot testing, move into a stabilization branch for the Next release, which becomes the Current release branch after the previous release is done, and which becomes a Maintenance branch after the new release is performed. Only bugfixes go in the Current release branch. I vote 1C and 2D. = Long version The common practice with Subversion is to have as few branches as possible, usually a trunk and a few maintenance branches, or development+stable+maintenance. This is a consequence of the perceived difficulty of merging changes between branches in svn, and the high cost of keeping multiple branches checked out. On the other hand, the git philosophy is to use branches as much as possible. Two core elements are "feature branches" and "forks". A feature branch is a branch where one feature is being developed, separated from the trunk and all the other features. While working on it, the developer "rebases" the branch on top of the trunk to keep his branch up to date with the trunk, and at the end "merges" the feature branch into the trunk. This way in-development features are kept out of the main trunk, but still allowing changes to be committed someplace public (no local uncommitted code anymore). A central element of GitHub is the ability to "fork" a repository. This means that a user clones a project in a personal repository where he can commit changes. He can later ask the maintainer of the original to "pull" those changes back into the original repository. This is the preferred way of contributing patches on GitHub. == Commit/Development-related strategies A. One central repository, one trunk (subversion-like) Developers clone the official trunks repository, prepare commits locally, then push back to the official repository. It's the same strategy that we're using now, except that we can also have an offline local repository. B. One central repository, feature branches Developers clone the official trunks repository, prepare commits locally in feature branches, then push back to the official repository in feature branches as well. When a feature is considered stable, it is merged into the master branch. Small bugfixes and improvements go directly in the master branch. B1. Also use specific helper branches Security fixes also go into a "security" branch so that users can cherry-pick them into older tags to build a custom patched version. Retired features can go into a "retired" branch so that users can re-include that feature in a custom build if they need it. C. One aggregated repository, pulling from developers Developers fork the official repositories, work on their fork (in feature branches as well), then make pull requests for integrating their work into the trunk. The rule would be that another developer has to do the pull after a code review (mandatory code reviews). This means more bugs spotted before committing, but also more work/time needed from the committers. We can relax the rule so that obvious bugfixes can be pulled by the same developer making the pull request. Personally, I prefer C, since it ensures better quality since at least two eyes see each line of code. == Integration/Release-related strategies Currently, we're developing on the trunk, and we're releasing from it during short breaks from live development. This is highly dangerous, and imposes a certain rhythm, with fast bursts of development right after a release, and imposed slowdown as the next release approaches (no work on new features after the last milestone). Short releases from a development branch is inline with agile development, but personally I find it too dangerous. Most big projects always keep the main development at least one branch away from the release branch. One example is the Linux kernel. While a kernel release lasts about 3 months, like our own releases, almost all of the code that goes into a release has been developed before the merge window opens. This means that after a kernel version is released, Linus opens a two-weeks merge window during which he accepts pull requests for existing, working, complete code. The next ~10 weeks are spent testing the new kernel and integrating bugfixes, while developers prepare the features for the next kernel version. This ensures that a released kernel has as few bugs as possible. They can afford to do that since there are hundreds or thousands of contributors. Still, this is entirely opposite to our way of working: after a release we barely start writing the code to go in the new release, and we get code in at the last minute (especially me). Another example that I'd like to present is the new proposed strategy for Mozilla Firefox: http://mozilla.github.com/process-releases/draft/development_overview/ Basically, the propose using 4 branches, from development to release, where code enters on the lowest branch, and moves up towards a release as it stabilizes and becomes release-ready. They use 6-weeks release cycles, and only stable-enough features get promoted from one branch to the next when a new cycle starts. This process ensures quality as well. I'd like to move closer to one of these two strategies, so that our releases are more polished. The mechanism for ensuring quality that we're currently using is to have an "investigation" phase during the previous release, which is supposed to help define the exact goals, so that during the current release the development should go smoothly towards that "idea goal". Unfortunately, this doesn't work that well. Without the code in place, investigations may miss important details/limitations that will shift the development in another direction. Or it can happen that the time is too short to fully implement something, so we can either release a very "in progress" feature, or decide near the end that it's not enough time to implement everything and focus on polishing what's already available to have a "partial" feature, but polished enough not to reek of low quality. The main problem here is that we're mixing feature- and time-based releases, with mandatory features that must find their way into a release, and a fixed deadline to make the release. This means that features have 8-10 weeks to be fully implemented, polished, tested, validated. And that doesn't always happen. So, here are some possible integration strategies: A. Master development (like now) All development is done in the master branch, from which we branch a few hours/days before the release, so that the master remains clear for development. B. Feature branches All new feature development is done in a separate branch for each feature, and we merge it in the trunk once it's considered done (or very close). When a release date comes, we release with the completed features, whatever those are. We don't force a merge of an incomplete feature just because it's in the roadmap if it's not stable enough. C. Feature+Development+Release branches All development is done in feature branches, but they get merged on the master branch more often to have test builds; the release branch is separate and it integrates features when they are considered ready. This has the advantage over B. that automated builds expose all the development features. D. Feature+Development+Stable+Release branches This is similar to the new Mozilla strategy. Developers merge their work in the Development branch very often. Users and other developers can contribute here as well, and preview the upcoming features. When they are close to finalization, they are also merged to the stable branch, where UX, QA and feature owners can test and improve the feature, preparing it for release. Once it's considered ready, it is merged into the release branch, where QA does a final thorough test. Releases happen from this branch. E. Feature+Development+Next+Release This is similar to D, with the exception that done features go into the next release, while the current release is staging. When the release is done, Release moves into Bugfix, Next becomes Release, and we create a new Next branch and start pulling in it. This would work well if we had very short release cycles (2 weeks), but it's not worth the effort for our current 3-month releases, since a feature would stagnate too much before being released. And it would also work if we had more beta testers. We can also impose windows, like 2-4 weeks for a feature to move into the next branch. We could also make faster releases, skipping milestones. and going to 6 week releases. This means that it would take longer for a feature to make its way from idea to release. One release for investigation, one or more for the main development, and one for integration and stabilization. But this also means that releases will be more solid, polished, with less bugs, and closer to the user needs. Personally I prefer option D, although it's a bit too much overkill with our current limited manpower. We need more contributors and committers! As for a change strategy, we can continue the way we're doing now, gradually switching to feature branches and release/pre-release branches during the following release. -- Sergiu Dumitriu http://purl.org/net/sergiu/