Activity Tracking: real-time publish-subscribe feeds in domains of page views, searches, and other user interactions. Metrics: operational monitoring data processing. Log Aggregation: collecting physical log files and store them for further processing. Stream Processing: multi-stage data processing pipelines. Event Sourcing: support of apps built with stored event sequences that can be replayed and applied again for deriving a consistent system state. Commit Log: the type of data stored in distributed system that ensures the re-syncing mechanism. 5 . 1