For background information, see the description of cache management.
The HTTP protocol provides for headers that can indicate to downstream proxy caches, browser caches, and dedicated caches that certain documents and images are cacheable. Most images, for example, can safely be cached for a long time. Anonymous visits to most primary pages can be cached as well.
An accelerated HTTP cache manager lets you control the headers that get sent with the responses to requests so that downstream caches will know what to cache and for how long. This allows you to reduce the traffic to your site and handle larger loads than otherwise possible. You can associate accelerated HTTP cache managers with any kind of cacheable object that can be viewed through the web.
The main risk in using an accelerated HTTP cache manager involves a part of a page setting headers that apply to the whole response. If, for example, your home page contains three parts that are cacheable and one of those parts is associated with an accelerated HTTP cache manager, Zope will return the headers set by the part of the page, making downstream caches think that the whole page should be cached.
The workaround is simple: don't use an accelerated HTTP cache manager with objects that make up parts of a page unless you really know what you're doing.
There are some parameters available for accelerated HTTP cache managers. The interval is the number of seconds the downstream caches should cache the object. 3600 seconds, or one hour, is a good default. If you find that some objects need one interval and other objects should be set to another interval, use multiple cache managers.
If you set the cache anonymous connections only checkbox, you will reduce the possibility of caching private data.
The notify URLs parameter allows you to specify the URLs of
specific downstream caches so they can receive invalidation messages
PURGE directives. Dedicated HTTP cache software such
as Squid will clear cached data for a given URL when receiving the
PURGE directive. (More details below.)
Simple statistics are provided. Remember that the only time Zope receives a request that goes through an HTTP cache is when the HTTP cache had a miss. So the hits seen by Zope correspond to misses seen by the HTTP cache. To do traffic analysis, you should consult the downstream HTTP caches.
When testing the accelerated HTTP cache manager, keep in mind that
the reload button on most browsers causes the
header to be sent, forcing HTTP caches to reload the page as well.
Try using telnet, netcat, or tcpwatch to observe the headers.
To allow Zope to execute the Squid PURGE directive, make sure the
following lines or the equivalent are in squid.conf (changing
localhost to the correct host name if Squid is on a different
acl PURGE method purge http_access allow localhost http_access allow purge localhost http_access deny purge http_access deny all