Web Caching and Replication

5.1. Conditional Requests

An HTTP feature central for caching and replication is support for conditional requests. These requests specify certain conditions in their headers. A server executes a conditional request and responds with the message body only if the condition is true. Otherwise, the server simply responds with a special status code, either "304 Not Modified" or "412 Precondition Failed," which indicate that the condition does not hold. The headers that specify the conditions are called conditional headers.

Clients formulate conditional requests using metainformation about the objects. Servers provide this information together with the objects in last-modified and ETag headers of their responses. The last-modified header contains the ...