A couple of days ago we announced Watchman on the Facebook Engineering blog.

Watchman watches files and records information about them as they change. You can arrange to trigger build or test steps in response to changes in matching files, but the main the reason that we built it was so that we can instantaneously query file status for a set of files.

Watchman maintains a view of the filesystem that is kept in sync using kernel filesystem notification facilities. This view is indexed so that we can quickly return information about the watched portions of the filesystem and also query the set of files that changed since a given point in time.

The since index is the heart of this; we maintain a linked list of file nodes, with the head representing the most recently changed file. As new files are observed they are inserted at the head of the list. As existing file changes are observed they are unlinked from their former position and inserted at the head of the list. This keeps the list sorted in time order without having to employ a traditional sorting algorithm; it's very fast to update the list and this is important in the face of a high volume of change notifications.

Finding the set of files that changed since we last looked is then a simple walk from the head until we reach the time in question. The cost of the search is proportional to the number of changed files since we last asked.

We use both the triggering and the querying features to accelerate our build processes for the main Facebook web site. They are important to us because we have such a large number of files that it is impractical to manually maintain a traditional static build recipe; we need to crawl the tree to determine the nature of the build. Since the tree is so large, and due to other factors in our dev environments, we also run into issues with the filesystem cache going cold; we need to minimize our I/O profile so that we only visit the files that we truly need to in order for our builds to run fast.

Watchman @ GitHub