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The redirection status codes either tell clients to use alternate locations for the resources they're interested in or provide an alternate response instead of the content. If a resource has moved, a redirection status code and an optional Location header can be sent to tell the client that the resource has moved and where it can now be found (see Figure 3-14). This allows browsers to go to the new location transparently, without bothering their human users.
Figure 3-14. Redirected request to new location
Some of the redirection status codes can be used to validate an application's local copy of a resource with the origin server. For example, an HTTP application can check if the local copy of its resource is still up-to-date or if the resource has been modified on the origin server. Figure 3-15 shows an example of this. The client sends a special If-Modified-Since header saying to get the document only if it has been modified since October 1997. The document has not changed since this date, so the server replies with a 304 status code instead of the contents.
Figure 3-15. Request redirected to use local copy
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Created: January 29, 2003
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