Bladerunner: Stream Processing at Scale for a Live View of Backend Data Mutations at the Edge

Consider a social media platform with hundreds of millions of online users at any time, utilizing a social graph that has many billions of nodes and edges. The problem this paper addresses is how to provide each user a continuously fresh, up-to-date view of the parts of the social graph they are currently interested in, so as to provide a positive interactive user experience. The problem is challenging because the social graph mutates at a high rate, users change their focus of interest frequently, and some mutations are of interest to many online users. We describe Bladerunner, a system we use at Facebook to deliver relevant social graph updates to user devices efficiently and quickly. The heart of Bladerunner is a set of back-end stream processors that obtain streams of social graph updates and process them on a per application and per-user basis before pushing selected updates to user devices. Separate stream processors are used for each application to enable application-specific customization, complex filtering, aggregation and other message delivery operations on a per-user basis. This strategy minimizes device processing overhead and last-mile bandwidth usage, which are critical given that users are mostly on mobile devices.