High Performance Web Sites

Chapter 15. Rule 13: Configure ETags

Reducing the number of HTTP requests necessary to render your page is the best way to accelerate the user experience. You can achieve this by maximizing the browser's ability to cache your components, but the ETag header thwarts caching when a web site is hosted on more than one server. In this chapter, I explain what ETags are and how their default implementation slows down web pages.

What's an ETag?

Entity tags (ETags) are a mechanism that web servers and browsers use to validate cached components. Before jumping into the details of ETags, let's review how components are cached and validated.

Expires Header

As the browser downloads components, it stores them in its cache. On subsequent page views, if the cached component is "fresh," the browser reads it from disk and avoids making an HTTP request. A component is fresh if it hasn't expired, based on the value in the Expires header. Let's look at an example.

When a component is requested, the server of origin has the option to send an Expires header back in the response:

Expires: Thu, 15 Apr 2010 20:00:00 GMT

Chapter 5 recommends setting an expiration date in the far future. How far is "far" depends on the component in question. An ad image might have to expire daily, whereas a company logo could expire in 10 years. The HTTP specification (http://www.w3.org/protocols/rfc2616/rfc2616-sec14.html#sec14.21) suggests servers should not set an Expires date more than one year in the future, but this is ...