Standard use cases for stream processing are: 1. Enriching event streams. Say you have a stream of log records with an IP address field. You want to enrich wit

Standard use cases for stream processing are: 1. Enriching event streams. Say you have a stream of log records with an IP address field. You want to enrich with a geo-location before sending the logs to Elasticsearch. 2. Windowed aggregation. Maybe you have an application that is emitting "login" events and you want to to detect login attempts from different IP addresses within X minutes of each other. 3. Joining multiple event streams. You have multiple different event streams and you want to join them together using some common join key (maybe session ID or something like that) to compute a metric that aggregates all of them. There are plenty of more esoteric use cases as well.