What is a Cache hierarchy?
It is an arrangement of cache servers in a tree structure usually remote from each other. So you could have a large server or array at head office, some smaller ones in the regional centres, and some babies in each branch for example. Each one uses the next higher one as a parent and requests it to fetch data when necessary on its behalf. This maximizes effectiveness and we have examples where this structure improves cache hit rates by over 10% in comparison to single large installations. This also has a number of other benefits. Redundancy is good because you can go direct if the server above you is down or to another server in the same layer. If employed in a large WAN environment it reduces traffic by 30 to 40% across the expensive links.
To reduce wide area network bandwidth demand and to reduce the load on Internet information servers, caches resolve misses through other caches higher in a hierarchy. In addition to the parent-child relationships, squid supports the notion of siblings: caches at the same level in the hierarchy, provided to distribute cache server load. Each cache in the hierarchy independently decides whether to fetch the reference from the object’s home site or from parent or sibling caches, using a a simple resolution protocol.
