There are a number of situations where it’s beneficial for caching proxies to talk to each other. There are different names for some different configurations. A cluster is a tightly coupled collection of caches, usually designed to appear as a single service. That is, even if there are seven systems in a cluster, to the outside world it looks like just one system. The members of a cluster are normally located together, both physically and topologically. As I explain in Chapter 9, many people like cache clusters because they provide scalability and reliability.
A loosely coupled collection of caches is called a hierarchy or mesh. If the arrangement is tree-like, with a clear distinction between upper- and lower-layer nodes, it is called a hierarchy. If the topology is flat or ill-defined, it is called a mesh. A hierarchy of caches make sense because the Internet itself is hierarchical. However, when a mesh or hierarchy spans multiple organizations, a number of issues arise. We’ll talk more about hierarchies in Chapter 7. Then, in Chapter 8, we’ll explore the various protocols and techniques that caches use to communicate with each other.
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