Google Mondrian: web-based code review and storage

Google Mondrian logo

Guido van Rossum unveiled his first Google project, Mondrian, tonight during a Python tech talk at the Google campus in Mountain View. Mondrian is a web-based code review system built on top of a Perforce and BigTable backend with a Python-powered front-end. Mondrian is a pretty impressive system and is currently in use across Google.

Shared Development Environment

Google uses a company-wide Perforce depot with almost no developer branches. Each developer has their own NFS workspace readable by anyone in the company, including automated processes. An administrative process takes snapshots of each developer workspace including local development environments accessed over SSH. Files within these snapshots can be compared to checked-in data, encrypted, and archived.

Previous methods of review

Previous to Mondrian code review was conducted largely over e-mail using Google command-line wrappers built on top of Perforce. A developer could initiate a code review from within the g4 mail tool, which would fire off an e-mail and begin a review thread. When the developer receives a response of “looks good to me,” or lgtm for short, they could proceed to checkin. Changes could be compared using tkdiff.

Design-level reviews are often conducted by e-mailing around Word documents or editing a team wiki. Recently some design reviews have moved onto an internal version of Google Docs.

Web-based collaboration meets code review

Mondrian code review

The Mondrian tool creates a much better workflow by creating task-specific dashboards, in-line commenting, well-tracked statistics, and more. The application is built on top of Python open source libraries such as the Django framework, mail service, and the wsgiref web server software.

Code reviews can be initiated and completed from within the Mondrian interface. A developer requests a review from another user or a group of users to kick off the process. Each invited reviewer can add comments directly underneath a line of code or reference the entire file. You can request and diff the file against previous versions as well. It’s a pretty slick interface, lightly highlighting each line of code as you hover, and popping open a comment box in response to a double-click. Comments can be saved as a draft and shared at a later time.

Putting the entire code review process online means you never have to worry about referencing the most recent version of a file or losing e-mails. Mondrian captures every outgoing e-mail related to the workflow, looks for key data such as revision numbers, and updates a to-do list accordingly.

More on BigTable

Mondrian uses BigTable as backend storage for user data. More specifically, it’s used to store:

  • Change metadata such as a description or list of files
  • Comments entered through the web interface or via e-mail
  • Encrypted file snapshots taken from user workspaces
  • Per-user data such as active changes or last view dates


The Mondrian web code review system is pretty impressive. Guido estimates he has spent about 25% of his work time on the project since joining Google in December 2005. Mondrian served as Guido’s introduction to Google technologies and processes with the help of a few other Googlers treating it as a side-project. The application is so deeply intertwined with Google technologies it’s not likely to be available as open source until Subversion and a backend such as SQLite can be supported.

Guido’s full talk, including a demo of Mondrian, is available on Google Video.