Hierarchies

An important feature of Netra Proxy Cache Server is the ability it gives you to create hierarchies of proxy cache servers or, a related feature, create collections of sibling servers. You can create hierarchies simply by pointing proxy cache servers to succeeding proxy cache servers as you proceed toward a firewall. Alternatively, you can take advantage of Netra Proxy Cache software's support for the Inter Cache Protocol (ICP) to build sibling and parent relationships among proxy cache servers.

When you configure a set of Netra Proxy Cache Servers as an array, automatically, those servers become ICP siblings, so that the cache is extended over all of the machines in the Netra Proxy Cache Array.

Figure 1-1 illustrates a simple hierarchy of proxy cache servers.

Figure 1-1 Simple Hierarchy

Referring to Figure 1-1, assume the client browser requests a web object that originated somewhere in the Internet and is, at the moment, not in Netra Proxy Cache Server A's cache. The following sequence ensues:

1. Machine A checks with its parent, machine B.

2. Likewise, B does not have the object in its cache and checks its parent, machine C. If C does not have the object, it goes out through the firewall to the web server to obtain it.

3. Machine C returns the object--obtained from a remote web server or its local cache--to machine B.

4. Machine B returns the object to machine A.

5. Machine A then returns the object to the requesting client.

If the object is cacheable, each proxy stores a copy upon receipt. Note that communication between parent proxies is over TCP connections.

Netra Proxy Cache software also supports a variation of the preceding scenario. This variation is illustrated in Figure 1-2.

Figure 1-2 Multiple Parent Proxies

Referring to Figure 1-2, if a client requests an object of its proxy server, machine A, that is not in A's cache, machine A relays the request to its two parents, machines B and C. If one of the parents has the object, it returns the object to A. If neither has the object, machine A forwards the request to the parent that responds faster, assuming that machine to be less loaded and/or have a better network connection.

If you configure multiple parents, the Netra Proxy Cache software allows you to give greater weight to one or the other, or set up one as the default. When no parent (of multiple parents) has a requested object, the "child" proxy always forwards the request to the default parent.

In addition to supporting hierarchies of parent proxies, the Netra Proxy Cache Server supports sibling proxies. The sibling scenario is illustrated in Figure 1-3.

Figure 1-3 Sibling Proxies

Referring to Figure 1-3, assume a client browser requests an object that is, at the moment, not in Netra Proxy Cache Server A's cache. The following sequence ensues:

1. Machine A checks with its parent, machine B. Machine A has no awareness of machines C and D.

2. Likewise, B does not have the object in its cache. Using ICP over UDP, machine B checks its siblings, machines C and D. If either of those machines has the object, it returns the object to machine B, which returns it to machine A.

If none of B, C, and D have the object, the request is forwarded to B's parent, machine E.

It is important to note that queries among siblings are over the relatively lightweight ICP exchanges, while communication among parents and transfer of web objects occurs over the relatively more resource-intensive TCP connections.

As indicated in Figure 1-3, the siblings (B, C, and D) form a Netra Proxy Cache Array. The establishment of sibling relationships occur automatically when you configure the array.

Note that you can add one or more Netra Proxy Cache Servers to any single proxy server shown in Figure 1-1, Figure 1-2, or Figure 1-3 to form a Netra Proxy Cache Array. For a proxy server that relies on a parent, pointing to single machine is no different from pointing to a Netra Proxy Cache Array. That is, the child proxy requires no additional configuration and needs no awareness that the proxy service is provided by multiple hosts.