In this article, you’ve seen how you can use the explicit content expiration model of the HTTP/1.1 caching architecture to improve the responsiveness of your web applications and reduce the server load. The explicit content expiration by itself is suitable for static or slowly-changing content that’s not tailored to individual users.
To declare the content expiration date, you can set the Expires or Cache-Control HTTP headers. Most server-side scripting environments give you control over these headers; the actual methods vary from language to language. In all cases, you have to set the HTTP headers before sending the first HTML output to the client.
The next article in this series will explain the Last-Modified header and its role in conditional GETs to greatly reduce the page-download time experienced by your visitors.