Hidden features of HTTP

Question

What hidden features of HTTP do you think are worth mentioning?

By hidden features I mean features that already are part of the standard but widely rath

Answer 1

In Dynamic content use Last_Modified or ETag header

At times you have dynamic content that can be large and/or costly to generate and that may not change from request to request. You can add a Last_Modified or ETag header to the your generated response.

At the top of your expensive dynamic code you can use the If_Modified_Since or the If_None_Match to determine if the content requestor already has is still current. If it is change the response status to "304 Unmodified" and end the request.

Some server-side technologies provide such features formally but you can do the above even in lowly ASP-Classic.

Note this differs from setting Cache-Control, Expires headers in that it ensures the client always has the latest info on request.