In distributed applications, a stream of data (events) often acts as a shared data source that multiple downstream microservices read from.
Streaming System: Actions taken by users on an e-commerce site (Login, Update Name, Add to Cart, Update Cart, Checkout) are appended to an event stream. Separate services read from the stream to create audit logs and trigger suspicious behavior alerts for security, assign promotions and push notifications to eligible users.
Traditional System: Actions taken by users on an e-commerce site trigger transactions to a database, an application reads from the database periodically to scan for suspicious behavior, promotions and push notifications are created in the same application code that is updating the database.