The feature sets of Rietveld and Review Board are strikingly similar, which is not surprising as they both used Mondrian as a model. van Rossum originally wanted to turn Mondrian into a free software project, but it was too tied to "proprietary Google infrastructure", so he started over, with Rietveld as the result. Both tools are implemented in Python using the Django framework, but one major difference is that Rietveld is written to use Google App Engine.

There are multiple ways to get a set of patches into the Rietveld system to create an "issue"—the term used for a patch set undergoing review—from an upload of a unified diff to using a python script to retrieve the patches from a repository. Currently Rietveld only supports Subversion, but van Rossum would like to see support added for other version control systems over time. Review Board has a bit of a head start in this area, so it supports Mercurial, Git, Bazaar, Perforce, Subversion and CVS.

Once an issue has been created in the system, reviewers can then be invited to comment on the changes. Navigating through the diff is straightforward, with Javascript being used liberally to give an interactive "local application" feel to the interface. Double-clicking on a line brings up a comment box that a reviewer can fill in to attach some comments to that line. All comments are held as "drafts" until the reviewer is satisfied with their review at which point they "publish" the comments for the author and other reviewers to see.

The Rietveld project is free software, released under the Apache 2.0 license, while the application itself runs via the Google App Engine. Anyone can browse the system, but folks who have a Google account can add issues, comments, and conduct reviews using the tool. Because it uses App Engine, people wanting to try it out on their code need not find a server to install and run the application—as would be required with Review Board—they can just upload a set of patches, invite some reviewers, and proceed.

This kind of simplified deployment is one of the benefits that Google App Engine is meant to provide. For free software projects, where code review is purposely done in the open, Rietveld provides a way to quickly try the application out. Those who wish to keep their source code secret may want to install their own instance of Review Board or another tool. It may be possible to install Rietveld in a different environment by replacing the App Engine-specific pieces, but that clearly is not where it is targeted.

While Rietveld does not provide much in the way of additional functionality from Review Board—in fact it lags Review Board in some areas—it does provide a very nice introduction to the Google App Engine interface. Developers will undoubtedly be using the code as a template for their own ideas once Google makes more App Engine accounts available. Given the shared history, language, and framework, it isn't impossible to imagine that Review Board and Rietveld might join forces one day. Even if they don't, some cross-pollination is inevitable which will result in both getting better. Hopefully, with more projects using one or both, better code for the community is the result.