System Requirements - Novell LINUX ENTERPRISE SERVER 10 - INSTALLATION AND ADMINISTRATION 08-05-2008 Installation Manual

HIT code if the object was detected or a MISS if it was not. If multiple HIT responses
were found, the proxy server decides from which server to download, depending on
factors such as which cache sent the fastest answer or which one is closer. If no satis-
factory responses are received, the request is sent to the parent cache.
TIP
To avoid duplication of objects in different caches in the network, other ICP
protocols are used, such as CARP (cache array routing protocol) or HTCP (hy-
pertext cache protocol). The more objects maintained in the network, the
greater the possibility of finding the desired one.
41.1.3 Caching Internet Objects
Not all objects available in the network are static. There are a lot of dynamically gener-
ated CGI pages, visitor counters, and encrypted SSL content documents. Objects like
this are not cached because they change each time they are accessed.
The question remains as to how long all the other objects stored in the cache should
stay there. To determine this, all objects in the cache are assigned one of various possible
states. Web and proxy servers find out the status of an object by adding headers to these
objects, such as "Last modified" or "Expires" and the corresponding date. Other headers
specifying that objects must not be cached are used as well.
Objects in the cache are normally replaced, due to a lack of free hard disk space, using
algorithms such as LRU (last recently used). Basically this means that the proxy ex-
punges the objects that have not been requested for the longest time.

41.2 System Requirements

The most important thing is to determine the maximum network load the system must
bear. It is, therefore, important to pay more attention to the load peaks, because these
might be more than four times the day's average. When in doubt, it would be better to
overestimate the system's requirements, because having Squid working close to the
limit of its capabilities could lead to a severe loss in the quality of the service. The fol-
lowing sections point to the system factors in order of significance.
The Proxy Server Squid 783