Project Conduit

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In 2017, Engineering Productivity is starting on a new project that we’re calling “Conduit”, which will improve the automation around submitting, testing, reviewing, and landing commits. In many ways, Conduit is an evolution and course correction of the work on MozReview we’ve done in the last couple years.

Before I get into what Conduit is exactly, I want to first clarify that the MozReview team has not really been working on a review tool per se, aside from some customizations requested by users (line support for inline diff comments). Rather, most of our work was building a whole pipeline of automation related to getting code landed. This is where we’ve had the most success: allowing developers to push commits up to a review tool and to easily land them on try or mozilla-central. Unfortunately, by naming the project “MozReview” we put the emphasis on the review tool (Review Board) instead of the other pieces of automation, which are the parts unique to Firefox’s engineering processes. In fact, the review tool should be a replaceable part of our whole system, which I’ll get to shortly.

We originally selected Review Board as our new review tool for a few reasons:

  • The back end is Python/Django, and our team has a lot of experience working with both.

  • The diff viewer has a number of fancy features, like clearly indicating moved code blocks and indentation changes.

  • A few people at Mozilla had previously contributed to the Review Board project and thus knew its internals fairly well.

However, we’ve since realized that Review Board has some big downsides, at least with regards to Mozilla’s engineering needs:

  • The UI can be confusing, particularly in how it separates the Diff and the Reviews views. The Reviews view in particular has some usability issues.

  • Loading large diffs is slow, but also conversely it’s unable to depaginate, so long diffs are always split across pages. This restricts the ability to search within diffs. Also, it’s impossible to view diffs file by file.

  • Bugs in interdiffs and even occasionally in the diffs themselves.

  • No offline support.

In addition, the direction that the upstream project is taking is not really in line with what our users are looking for in a review tool.

So, we’re taking a step back and evaluating our review-tool requirements, and whether they would be best met with another tool or by a small set of focussed improvements to Review Board. Meanwhile, we need to decouple some pieces of MozReview so that we can accelerate improvements to our productivity services, like Autoland, and ensure that they will be useful no matter what review tool we go with. Project Conduit is all about building a flexible set of services that will let us focus on improving the overall experience of submitting code to Firefox (and some other projects) and unburden us from the restrictions of working within Review Board’s extension system.

In order to prove that our system can be independent of review tool, and to give developers who aren’t happy with Review Board access to Autoland, our first milestone will be hooking the commit repo (the push-to-review feature) and Autoland up to BMO. Developers will be able to push a series of one or more commits to the review repo, and reviewers will be able to choose to review them either in BMO or Review Board. The Autoland UI will be split off into its own service and linked to from both BMO and Review Board.

(There’s one caveat: if there are multiple reviewers, the first one gets to choose, in order to limit complexity. Not ideal, but the problem quickly gets much more difficult if we fork the reviews out to several tools.)

As with MozReview, the push-to-BMO feature won’t support confidential bugs right away, but we have been working on a design to support them. Implementating that will be a priority right after we finish BMO integration.

We have an aggressive plan for Q1, so stay tuned for updates.

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