Watchman is a file watching service, with clients for a command line, NodeJS and more. It records any changes and triggers assigned actions. In the example with the Facebook app, Watchman would have made sure that only files that are changed or affected by our developers get updated. We previously shared how this service reduced build time by 60% in some cases!
Beyond the build system, Watchman has been updated to support other use cases. It can log changes, trigger test suites, transform files and more. Watchman was also updated to support a wide range of operating systems, including Windows.
Watchman was first open sourced by Facebook in 2013. Ever since then, Watchman has been used daily by our developers at Facebook. It was also integrated with many other open source projects like React Native and Buck.
To learn more about Watchman, visit their website. The project has an excellent starter and advanced documentation. The project has an active community chat on the Freenode and StackOverflow. If you were to encounter an issue, there is also a troubleshooting page available on the project’s site.
In a series of short videos (~1 min in length), one of our Developer Advocates on the Facebook Open Source team explains a Facebook open source project in a way that is easy to understand and use.
We will write an accompanying blog post (like the one you're reading right now) for each of these videos, which you can find on our YouTube channel.
Interested in working with open source at Facebook? Check out our open source-related job postings on our career page by taking this quick survey.