While we've touched upon client side caching in our series on Web performance, we haven't discussed how client caching has grown more rich and useful over the years. In the initial days of the Web and the HTTP/1.0 protocol, caching was mostly limited to a handful of headers, including Expires, If-Modified-Since, and Pragma: no-cache. Since then, client caching has evolved to embrace greater granularity. Some new technologies even permit the deployment of offline-aware, browser-based applications.

Browser Request Caching

The most common and oldest type of client-side caching on the client is browser request caching. Built into the HTTP protocol standard, browser request caching allows the server to control how often the browser requests new copies of files from the server. We discussed the major aspects of browser request caching in part 1 of our series. Over time, Webmasters have taken to using different headers to improve caching on their site, including:

Pragma: no-cache. This old directive is used mostly by HTTP/1.0 servers, and instructs a client that a specific response's contents should never be cached. It is used for highly dynamic content that is apt to change from request to request.

Expires. Supported since HTTP/1.0, this header specifies an explicit expiration date for cached content. It can be superseded by the value of the Cache-Control header. For example, if Cache-Control: no-cache is sent in a response, this will take precedence over any value of the Expires header.

If-Modified-Since: Since the HTTP/1.0 protocol, clients have been able to use this header to request that the server only send data if the resource has been changed since the specified date. If there have been no changed, the server returns an HTTP 304 Not Modified response.