It is a declarative programming paradigm to build asynchronous processing pipelines.

• user events: When a user clicks a button.
• I/O Responses

1. Request comes into the server. Server takes a thread from the thread pool to handle the request.
2. that thread does the processing
3. finally return the response to that waiting client.

• handle high data volume
• high-responsive and optimized usage
• optimize hardware resource to reduce cloud hosting cost