Google Web Accelerator

Prefetching takes advantage of idle time when a user is viewing a page to accelerate the links which he or she is likely to follow next by “prefetching” those pages. This speeds up users' online experience by making many pages load faster. You can learn more about prefetching on the Mozilla website.

2. How does Google Web Accelerator decide what to prefetch?

Google Web Accelerator decides what links should be prefetched based on aggregate usage statistics as well as the user’s mouse movements. But Google Web Accelerator also allows websites to specify which links should be prefetched (for example, the top story on a news website), so that webmasters can prioritize links which they expect their users to be interested in.

3. Which links are, and aren’t, prefetchable?

Google Web Accelerator only prefetches links that should have no side effects. According to the HTTP 1.1 specification, the GET method is defined as a Safe Method which "SHOULD NOT have the significance of taking an action other than retrieval." In practice, Google Web Accelerator does not prefetch links which have query parameters (i.e. have a "?" in the URL) and encrypted pages (i.e. URL starting with https://).

4. How do I differentiate prefetch requests from regular user-initiated requests?

Prefetch requests include an "x-moz: prefetch" HTTP header. Websites can choose to filter out prefetch requests in their server statistics, as well as reject prefetch requests, which would cause the request to be sent only when a user actually clicks on a link. Please read the additional FAQ below for more on how to differentiate prefetch requests in server statistics.

5. What is the impact of prefetching on a website?

User experience: Prefetching can make the page download when a users clicks on a link close to instantaneous.
Traffic: Prefetching can increase the overall load on a website. However, thanks to the Google Web Accelerator cache, if a website's caching rules are well defined, this additional load can be minimal.
Statistics: If prefetch requests aren’t filtered out in the web logs due to the "x-moz: prefetch" header, prefetch requests may be perceived as actual user page view.

6. How do I use prefetching effectively?

Caching rules: When a website sets appropriate caching rules, it can benefit from the speedup of prefetching without incurring the increased load.
Prefetch hints: As a webmaster, you know best what links users are likely to click on, and if you specify links to prefetch, these will be sped up for users who have Google Web Accelerator installed. For each link you'd like Google Web Accelerator to prefetch, simply add the following snippet somewhere in your page's HTML source code: <link rel="prefetch" href="http://url/to/get/">. The href value should be the actual URL you want prefetched. Google will prefetch this page, and when your users click on this link, that page will load more quickly. You can learn more about the tag on the Mozilla website.

7. How can I ensure that Google Web Accelerator doesn’t prefetch links that might have side effects?

If you follow the HTTP specification to ensure that GET requests with no parameters don’t have side effects, Google Web Accelerator won’t prefetch links with side-effects.

8. Will Google Web Accelerator prefetch ads on my website and make them seem like clicks?

No, it wont; links to ads that are embedded in a web page generally include query parameters and therefore aren’t prefetched.

9. Will Google Web Accelerator execute Javascipt in prefetched pages before the page is explicitly requested by the user?

No, Google Web Accelerator doesn’t execute the Javascript in a preloaded page.

10. How many links can I specify for prefetching on a page?

Neither Google nor you want to overload your users' connections with too large a download, and if a large number of links on a page are marked for prefetching, overall performance may diminish. So you should only specify prefetching for the links your users are most likely to click on.

11. How can I ensure that prefetching doesn’t increase the load on my servers?

The most important way to control the load to your site due to prefetching is by setting effective caching rules. You can also configure your server to refuse prefetch requests by returning a 403 HTTP response code for requests which include an "x-moz: prefetch" header.

Caching	[back to top]

1. What is caching?

Caching allows content to be stored closer to end users so that when they request it, it can be retrieved from the "closest" source without going back to the origin server. Caching both reduces the load on the origin server and speeds up users' online experience. The two main types of caches are browser caches (which reside on the user's computer) and proxy caches (which are on the network and can serve one or many users). Properly tuning a website's caching rules is tricky, but also provides substantial benefits which make it worth your while.

2. How does Google Web Accelerator decide what to cache?

Google Web Accelerator acts as a caching proxy and complies with the HTTP 1.1 specification (RFC 2616).

3. What’s the best way to use the Google Web Accelerator cache?

The best way to effectively use the Google Web Accelerator cache is to follow the HTTP specification with respect to caching. The tutorials listed below provide some great tips on how to optimally configure your site to take advantage of caching while behaving as you intended it to.

4. My website makes users log in. How can I ensure that a user doesn’t see another user's cached page?

Pages that are behind a login and marked either as uncacheable, or private are not shared across users.

Resources	[back to top]

1. Caching

Tutorials:

Tools:

Google search on "Web Caching"

2. Prefetching

What if I have a question that you haven't answered here	[back to top]

1. What if I have a question that you haven't answered here?

We're happy to hear from you. Contact us at labs+webaccelerator@google.com.