Making code reviews easier with Review Board

Reviewing code is a thankless, but very important, task for any software project. For free software projects, the "many eyes make all bugs shallow" aphorism only works if the eyes actually focus on the code in question. Review Board is a web-based application that helps reviewers examine the code, while making it easier for a developer to track those reviews.

Borne out of frustration with the process of code reviews at VMware, Review Board has made a great deal of progress since being released last May. The idea behind it is to centralize all of the pieces that need to come together for a review: code diffs, screenshots of UI functionality, comments by other developers, etc. On many projects, reviews are handled by email, but that can be difficult to use; various pieces of the puzzle are spread around in multiple messages and locations.

Often a reviewer needs to see more context than a simple email diff provides or wants to comment on a related section of code that is not contained in the diff; each requires a reviewer to do more work. In a complicated set of changes, ensuring that the developer and any other reviewers can follow what code the comments pertain to can also be difficult. It is these kinds of problems that Review Board is meant to solve.

Review Board presents a side-by-side diff view, shown at right, with lots of extras, many of which will be familiar to users of other graphical diff tools. Changed lines are highlighted in different colors based on whether they are additions, deletions, or changes. Changes on a particular line are highlighted in a slightly darker color so that they can be distinguished more easily as well. The numbered tabs along the left edge provide a link to a reviewer's comments about that section of the code. This is where Review Board shows that it is much more than just a diff viewer.

Using AJAX techniques, Review Board allows a reviewer to interact very naturally with the code. They can highlight a certain section, which will pop up a text widget that records comments associated with that section of code. When other reviewers or the developer read those comments, the code snippet is included, with a link back to the code in the diff view. Each of these comments can then be commented upon which allows for a conversation about the code to develop.

It is not just code that can be annotated; screenshots of application functionality or bugs can be attached to reviews, as well. Sections of the screenshot can be highlighted and commented upon, as shown at left. This feature is an excellent example of where a web-based tool can shine; doing the same task in text-based email would be painful. Not all projects need it, but those that do will find it quite useful as anyone who has spent time trying to describe a UI problem in email will attest.

Inter-diffs is another useful feature that Review Board provides. Often in the code review process, several revisions of the original patch are made. It can be tedious to wade through a large diff, most of which has been uncontroversial (or resolved earlier) to get to the changes in the area of interest. Review Board has the ability to see changes between any two revisions of the patch, which should reduce much of the hassle.

Another thing that Review Board does is to assist in managing code reviews. When a developer posts something for review, various reviewers can be notified via email. Review Board keeps track of that information, presenting users with a "dashboard" view of their pending reviews, both those they submitted and those that others have asked them to do. This high-level overview is the first screen the user sees when they log on to the system, shown at right. This makes keeping track of work that needs to be done – or who to prod to get a review moving again – much easier.

Currently, Review Board best supports the Subversion and Perforce version control systems (VCS), but support for others, including distributed VCS Mercurial and git, are being actively developed and are usable in their current states. Released under an MIT license, Review Board is written in Python, using the Django web framework. Development is hosted at Google Code; the developers, unsurprisingly, uses the software for internal code reviews.

Other systems to assist in the code review process do exist. Codestriker is a Perl based web application that has similar aspirations to Review Board. Also of interest is Python founder Guido van Rossum's first project at Google: a code review system he calls "Mondrian". It is closely tied to Google proprietary code, though, so it seems unlikely to be released as free software – though it might make an appearance as a tool for Google Code projects to use.

Code reviews are very powerful, but generally painful to perform; any tool that claims that "Code reviews are fun again! ...almost.", as Review Board does, will be welcomed by many. It will be interesting to see whether a code review tracker becomes a standard part of newer free software projects. Over the last few years, we have seen the rise of distributed VCS, bug trackers, and wikis to assist in distributed development. Will Review Board – or something like it – be the next tool to be added?