The Cluster Node Manager

Servers outside the cluster may do a variety of other things as well, such as:

• Act as license servers for applications running inside the cluster.

• Act as the central user account database.

• Provide other services such as DNS and email to all cluster nodes.

• Monitor the health of the cluster.

Again, the applications running on these servers outside the cluster should not need to be modified to support the nodes inside the cluster. In other words, the applications offering services to clients that are cluster nodes need not be cluster aware.

These functions can be combined on a high-availability server pair that we will call the cluster node manager. The cluster node manager holds the master user account database (and uses NIS or LDAP to distribute user accounts to all nodes^[5]). The cluster node manager monitors the health of the cluster (using the Mon and Ganglia packages that will be discussed in Part 4); it is also a central print spooler (using LPRng). The cluster node manager can also offer a variety of other services to the cluster using the classic client-server model, such as the ability to send faxes.^[6]

^[5]The Webmin and OSCAR packages both have methods of distributing user accounts to all cluster nodes that do not rely on NIS or LDAP, but these methods can still use the cluster node manager as the central user account database.

^[6]A brief discussion of Hylfax is included near the end of Chapter 17.