The first web servers were developed to fulfill a simple role - they responded to requests for HTML documents that were (hopefully) located in their physical storage by streaming the contents of those documents to the client.

Request-Response Pattern Request-Response Pattern

This is embodied in our request-response pattern. The client requests a resource (such as a HTML document), and receives either a status 200 response (containing the document), or an error status code explaining why it couldn’t be retrieved.