Kafka is a messaging system that was built at LinkedIn to serve as the foundation for our activity stream processing. Activity stream data is a normal part of any website for reporting on usage of the site. Activity data is things like page views, information about what content was shown, searches, etc. This kind of thing is usually handled by logging the activity out to some kind of file and then periodically aggregating these files for analysis. In recent years, however, activity data has become a critical part of the production features of websites, and a slightly more sophisticated set of infrastructure is needed.