Server preferences are controlled by giving each server a preference rank number. Clients search for NIS+ servers in order of numeric preference, querying servers with lower preference rank numbers before seeking servers with higher numbers.
Thus, a client will first try to obtain namespace information from NIS+ servers with a preference of zero. If there are no preference=0 servers available, then the client will query servers whose preference=1. If no 1's are available, it will try to find a 2, and then a 3, and so on until it either gets the information it needs or runs out of servers.
Preference rank numbers are assigned to servers with the nisprefadm command as described in Specifying NIS+ Global Server Preferences.
Server preference numbers are stored in client_info tables and files. If a machine has its own /var/nis/client_info file, it uses the preference numbers stored in that file. If a machine does not have its own client_info file, it uses the preference numbers stored in the domain's client_info.org_dir table. These client_info tables and files are called “preferred server lists” or simply server lists.
You customize server usage by controlling the server preferences of each client. For example, suppose a domain has a client machine named mailer that makes heavy use of namespace information and the domain has both a master server (nismaster) and a replica server (replica1). You could assign a preference number of 1 to nismaster and a number of 0 to replica1 for the mailer machine. The mailer machine would then always try to obtain namespace information from replica1 before trying nismaster. You could then specify that for all the other machines on the subnet the nismaster server had a preference number of zero and replica1 the number 1. This would cause the other machine to always try nismaster first.
You can give the same preference number to more than one server in a domain. For example, you could assign both nismaster1 and replica2 a preference number of 0, and assign replica3, replica4, and replica5 a preference number of 1.
If there is no client_info file or table, the cache manager automatically assigns all servers on the local subnet a default preference number of zero (0) and all servers outside the local subnet a preference of infinite. The purpose of nisprefadm is to change these default preference numbers to what you want them to be.
A client must seek all servers with a given preference number before searching for servers with the next higher number. It requires 5 or more seconds for a client to search for all the servers with a given preference number. This means that if you have a master server and 4 replicas in a domain, and you give each one a different preference number from 0 to 4, it could take a client more than 25 seconds to run through all of those preference levels.
To maximize performance, you should not use more than two or three levels of server preference. For example, in the case described above, it is better to give one of those five servers a preference=0 and all the others a preference of 1, or give two of them a preference of 1 and the remaining three a preference of 2.