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Educational, Investigative, and Absurd Writings by M. R. Côté

A Vision for Engineering Workflow at Mozilla (Part Three)

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This is the last post in a three-part series on A Vision for Engineering Workflow at Mozilla. The first post in this series provided some background, while the second introduced the first four points of our nine-point vision.

The Engineering Workflow Vision (continued)

5. Reviews are straightforward and streamlined

The Engineering Workflow team has spent a lot of time over the last few years on review tools, starting with Splinter, moving into MozReview, and now onto Phabricator. In particular, MozReview was a grand experiment; its time may be over, but we learned a lot from the project and are incorporating these lessons not just into our new tools but also into how we work.

There are a lot of aspects to the code-review process. First and foremost is, of course, the tool that is used to actually leave reviews. One important meta-aspect of review-tool choice is that there should only be one. Mozilla has suffered from the problems caused by multiple review tools for quite a long time. Even before MozReview, users had the choice of raw diffs versus Splinter. Admittedly, the difference there is fairly minimal, but if you look at reviews conducted with Splinter, you will see the effect of having two systems: initial reviews are done in Splinter, but follow ups are almost always done as comments left directly in the bug. The Splinter UI rarely shows any sort of conversation. We didn’t even use this simple tool entirely effectively.

Preferences for features and look and feel in review tools vary widely. One of the sole characteristics that is uncontroversial is that it should be fast—but of course even this is a trade off, as nothing is faster than commenting directly on a diff and pasting it as a comment into Bugzilla. However, at a minimum the chosen tool should not feel slow and cumbersome, regardless of features.

Other aspects that are more difficult but nice to have include

  • Differentiating between intentional changes made by the patch author versus those from the patch being rebased
  • Clear and effective interdiff support
  • Good VCS integration

For the record, while not perfect, we believe Phabricator, our chosen review tool for the foreseeable future, fares pretty well against all of these requirements, while also being relatively intuitive and visually pleasing.

There are other parts of code review that can be automated to ease the whole process. Given that they are fairly specific to the way Mozilla works, they will likely need to be custom solutions, but the work and maintenance involved should easily be paid off in terms of efficiency gains. These include

  • Automated reviews to catch all errors that don’t require human judgement, e.g., linting. Even better would be the tool fixing such errors automatically, which would eliminate an extra review cycle. This feedback should ideally be available both locally and after review submission.
  • Reviewers are intelligently suggested. At the minimum, our module system should be reflected in the tool, but we can do better by calculating metrics based on file history, reviewer load and velocity, and other such markers.
  • Similarly, code owners should be clearly identified and enforced; it should be made clear if the appropriate reviewers have not signed off on a change, and landing should be prevented.

This last point segues into the next item in the vision.

6. Code is landed automatically

Mozilla has had an autoland feature as part of MozReview for about 2.5 years now, and we recently launched Lando as our new automatic-landing tool integrated with Phabricator. Lando has incorporated some of the lessons we learned from MozReview (not the least of which is “don’t build your custom tools directly into your third-party tools”), but there is much we can do past our simple click-to-land system.

One feature that will unlock a lot of improvements is purely automatic landings, that is, landings that are initiated automatically after the necessary reviews are granted. This relies on the system understanding which reviews are necessary (see above), but beyond that it needs just a simple checkbox to signal the author’s intent to land (so we avoid accidentally landing patches that are works in progress). Further, as opposed to Try runs for testing, developers don’t tend to care too much about the time to land a completed patch as long as a whole series lands together, so this feature could be used to schedule landings over time to better distribute load on the CI systems.

Automatic landings also provide opportunities to reduce manual involvement in other processes, including backouts, uplifts, and merges. Using a single tool also provides a central place for record-keeping, to both generate metrics and follow how patches move through the trains. More on this in future sections.

7. Bug handling is easy, fast, and friendly

Particularly at Mozilla, bug tracking is a huge topic, greater than code review. For better or worse, Bugzilla has been a major part of the central nervous system of Mozilla engineering since its earliest days; indeed, Bugzilla turns 20 in just a couple months! Discussing Bugzilla’s past, present, and future roles at Mozilla would take many blog posts, if not a book, so I’ll be a bit broad in my comments here.

First, and probably most obviously, Mozilla’s bug tracker should prioritize usability and user experience (yes they’re different). Mozilla engages not just full-time engineer employees but also a very large community with diverse backgrounds and skill sets. Allowing an engineer to be productive while encouraging users without technical backgrounds to submit bug reports is quite a challenge, and one that most high-tech organizations never have to face.

Another topic that has come up in the past is search functionality. Developers frequently need to find bugs they’ve seen previously, or want to find possible duplicates of recently filed bugs. The ideal search feature would be fast, of course, but also accurate and relevant. I think about these two aspects are slightly differently: accuracy pertains to finding a specific bug, whereas relevancy is important when searching for a class of bugs matching some given attributes.

Over the past couple years we have been trying to move certain use cases out of Bugzilla, so that we can focus specifically on engineering. This is part of a grander effort to consolidate workflows, which has a host of benefits ranging from simpler, more intuitive interfaces to reduced maintenance burden. However this means we need to understand specific use cases within engineering and implement features to support them, in addition to the more general concerns above. A recent example is the refinement of triage processes, which is helped along by specific improvements to Bugzilla.

8. Metrics are comprehensive, discoverable, and understandable

The value of data about one’s products and processes is not something that needs much justification today. Mozilla has already invested heavily in a data-driven approach to developing Firefox and other applications. The Engineering Workflow team is starting to do the same, thanks to infrastructure built for Firefox telemetry.

The list of data we could benefit from collecting is endless, but a few examples include * backout rates and causes * build times * test-run times * patch-review times * tool adoption

We’re already gathering and visualizing some of these stats:

Naturally such data is even more valuable if shared so other teams can analyze it for their benefit.

9. Information on “code flow” is clear and discoverable

This item builds on the former. It is the most nebulous, but to me it is one of the most interesting.

Code changes (patches, commits, changesets, whatever you want to call them) have a life cycle:

  1. A developer writes one or more patches to solve a problem. Sometimes the patches are in response to a bug report; sometimes a bug report is filed just for tracking.

  2. The patches are often sent to Try for testing, sometimes multiple times.

  3. The patches are reviewed by one or more developers, sometimes through multiple cycles.

  4. The patches are landed, usually on an integration branch, then merged to mozilla-central.

  5. Occasionally, the patches are backed out, in which case flow returns to step 1.

  6. The patches are periodically merged to the next channel branch, or occasionally uplifted directly to one or more branches.

  7. The patches are included in a specific channel build.

  8. Repeat 6. and 7. until the patch ends up in the mozilla-release branch and is included in a Release build.

There’s currently no way to easily follow a code change through these stages, and thus no metrics on how flow is affected by the various aspects of a change (size, area of code, author, reviewer(s), etc.). Further, tracking this information could provide clear indicators of flow problems, such as commits that are ready to land but have merge conflicts, or commits that have been waiting on review for an extended period. Collecting and visualizing this information could help improve various engineering processes, as well as just the simple thrill of literally watching your change progress to release.

This is a grand idea that needs a lot more thought, but many of the previous items feed naturally into it.


This vision is just a starting point. We’re building a road map for short-to-medium-term map, while we think about a larger 2-to-3-year plan. Figuring out how to prioritize by a combination of impact, feasibility, risk, and effort is no small feat, and something that we’ll likely have to course-correct over time. Overall, the creation of this vision has been inspiring for my team, as we can now envision a better world for Mozilla engineering and understand our part in it. I hope the window this provides into the work of the Engineering Workflow team is valuable to other teams both within and outside of Mozilla.