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Appendix A Caching Solutions and Performance 129
Advantages of server acceleration
Server acceleration advantages are similar to Web proxy caching:
✔ The appliance is optimized for speed and multiple user connections and can
be deployed close to users.
✔ Serving cached documents saves network bandwidth.
Server acceleration offers the following server advantages:
✔ Web servers can be off-loaded, providing overload insurance. An appliance
cluster dynamically mirrors content from heavily loaded Web servers.
✔ Web administration is centralized. Administrators maintain the Web server(s)
being accelerated, and the appliances do the job of distributing content.
Note Server acceleration described here applies to HTTP requests.
How server acceleration works
When a browser makes a request, it normally sends that request directly to the
origin server. When the appliance is in reverse proxy mode, it must intercept the
request for that origin server.
Interception occurs by setting up the DNS entry for the origin server (the origin
server’s advertised host name) to resolve to the appliance’s IP address. If the
appliance is clustered, using a virtual IP address provides added reliability (if a
node fails, another node takes on the virtual IP address of the failed node).
When the appliance is set up as the Web server, the browser connects to it rather
than to the origin server (see Figure 6). The origin server cannot have the same
name as the advertised host name, or there would be a DNS conflict.
Retrieving requested documents
Because the appliance is advertised as the origin server, it needs to act as a Web
server rather than a proxy server, meaning that it receives server requests, not
proxy requests. In this case, the appliance constructs a proxy request from the
server request and then satisfies the proxy request.
In HTTP, server requests differ from proxy requests. The main difference is that
server requests don’t specify the entire URL, just the path. A server request might
look like this:
GET /index.html HTTP/1.0
HOST: real.janes_books.com