This document discusses stream processing at LinkedIn and summarizes two case studies:
1. LinkedIn's communications platform processes over a trillion events per day using Apache Samza for real-time notifications, security monitoring, and other applications.
2. Activity tracking in the News Feed uses stream processing to track metrics like ads clicked and session durations in real-time for hundreds of applications across LinkedIn.
3. The document also outlines some of the hard problems in stream processing like state management, remote I/O, and deployment models that Samza addresses through techniques such as local state caching, asynchronous remote access, and running on YARN or embedded.