About hierarchies

What is a hierarchy?

The NetCache hierarchy feature imposes a logical structure on top of a physical network. This structure enables a NetCache appliance to forward requests it cannot resolve to other NetCache appliances, third-party proxy-cache servers, and firewalls. Hierarchies are commonly used for Web cache and streaming media cache configurations, and are typically configured to

Distribute content over multiple caches
Provide load balancing and failover capability
Provide backup for single hosts
Note
The NetCache hierarchy feature does not apply to news cache configurations.

Purpose of a hierarchy

Hierarchy configurations affect traffic outbound from the NetCache appliance on which you configure the hierarchy. Hierarchies enable you to define what occurs when the NetCache appliance you are configuring cannot resolve a request, that is, what happens when a cache miss occurs. A cache miss occurs in the following situations:

A NetCache appliance does not have the requested cacheable object in its cache.
The requested object is noncacheable. For example, the requested object contains secure data, personal Web pages, or any HTTP objects (URL substrings, MIME types, and object extensions) that you do not want the NetCache appliance to cache.

With no hierarchy configured, the NetCache appliance forwards the request to the Internet and attempts to retrieve the requested object from the origin server.

With a hierarchy configured, the NetCache appliance forwards the request to the next level up in the hierarchy, to its parent cache. If the parent cache has the object, it returns the object to the NetCache appliance, the child cache. If the parent cache does not have the object, it retrieves the object from the Internet on behalf of the child cache.

Logical roles

Each member of a hierarchy is assigned a logical role, and each member is configured according to protocol, port, and any requirements for authentication. A hierarchy member can be logically identified as a parent, child, or neighbor.

Logical role	Description
Parent	NetCache appliances, third-party proxy-cache servers, and firewalls can be identified as a parent. Parents can fetch objects directly from the Internet on behalf of the NetCache appliance you are configuring.
Child	When you identify a parent or firewall as a member of the hierarchy, the NetCache appliance you are configuring is the child of that parent or firewall.
Neighbor	NetCache appliances and third-party proxy-cache servers can be identified as a neighbor. Neighbors can accept a query for a requested object from another NetCache appliance or from a third-party proxy-cache server. Neighbors cannot fetch objects directly from the Internet on behalf of another cache. Note Neighbors cannot be assigned in streaming media cache configurations.

Logical role

Description

Parent

NetCache appliances, third-party proxy-cache servers, and firewalls can be identified as a parent. Parents can fetch objects directly from the Internet on behalf of the NetCache appliance you are configuring.

Child

When you identify a parent or firewall as a member of the hierarchy, the NetCache appliance you are configuring is the child of that parent or firewall.

Neighbor

NetCache appliances and third-party proxy-cache servers can be identified as a neighbor. Neighbors can accept a query for a requested object from another NetCache appliance or from a third-party proxy-cache server. Neighbors cannot fetch objects directly from the Internet on behalf of another cache.

Note
Neighbors cannot be assigned in streaming media cache configurations.

Firewalls as hierarchy members

Firewalls filter client requests and prevent a company's network devices from sending traffic directly to the Internet; they are responsible for connecting to origin servers. Because some requests must be sent to origin servers on the Internet, a firewall must be explicitly identified as a hierarchy member.

Note
Transparent firewalls give the appearance that users are connecting directly to origin servers. You do not need to identify transparent firewalls in NetCache appliance hierarchies because requests are routed to the Internet through the NetCache appliance default gateway.