RESTful Java Web Services - Third Edition

HTTP conditional requests

The conditional requests feature offered by HTTP allows clients to ask the server whether it has an updated copy of the resource by including some extra header parameters in the request. The server will return the resource back to the client only if the resource that the client has is not up to date.

Conditional requests can be implemented either by comparing the hash value of resource contents (via the ETag header) or by checking the time at which the resource is modified (via the Last-Modified header). Here is the quick summary of the request and response header parameters used in conditional requests:

• ETag: When requesting for a resource, the client adds an ETag header, containing a hash or checksum of the resource ...