Determining Server Roles

Within an SNA Server subdomain, you must determine the role of each SNA Server computer. Unlike their Windows NT Server namesakes, the primary, backup, or member SNA Server designation only identifies which SNA Server computers will have a copy of the SNA Server configuration for the SNA Server subdomain. SNA Server clients can connect to any SNA Server computer in the subdomain, regardless of its configuration role.

An SNA Server computer can be assigned one of three roles:

• Primary—This is the computer that provides host connectivity services to your users and contains the master copy of the configuration file. Only one computer can be designated as the primary SNA Server in a subdomain.

• Backup—This computer contains a read-only copy of the configuration file maintained by the primary server. Backup servers can be promoted to a primary role if the primary server fails. An SNA Server subdomain can contain up to 14 backup servers.

• Member—This computer does not contain a copy of the configuration file.
  An SNA Server subdomain can contain up to 14 member servers.

The primary SNA Server does not have to be the primary domain controller for the Windows NT domain, although it can be. Nor do backup servers need to be backup domain controllers in the domain. For smaller environments, however, a single Windows NT Server can be the primary domain controller for the Windows NT domain and the primary server in an SNA Server subdomain. You should decide which Windows NT Server computers to designate as primary or backup SNA Server computers separately from deciding which servers to designate as primary or backup Windows NT domain controllers.

The primary SNA Server computer should be the first server you install in an SNA Server subdomain. If possible, you should have one or more backup servers in the subdomain, as well, to maintain copies of the SNA Server configuration file in case the primary server fails. If security is an issue, the backup servers should be kept physically secure, since each one contains a copy of the SNA Server configuration file.

After you have designated one primary server and several backup servers, the remaining servers can be designated as member servers (SNA Server computers without a configuration file). As long as the primary server is running, you can administer SNA Server from a member server just as you would from any other server. You can also manage SNA Server from a Windows NT – based client that has SNA Server Manager installed. You can limit the ability of users or groups to administer SNA Server by setting up SNA Server permissions.

In situations in which the primary SNA Server computer has failed and must be replaced, a backup server can be promoted to a primary role. If the former primary server can still run (say, only a network card has failed), you can change its role to backup and enable a backup server to become the primary server. Following this sequence of steps ensures that there is only one primary SNA Server computer at any given time. In situations in which the primary SNA Server computer has completely failed, a backup server can simply be promoted to a primary role to ensure that service is maintained.

Table 4.1 summarizes the different SNA Server roles:

Table 4.1 Summary of SNA server roles.

Note    The concept of a backup SNA Server is different from the concept of hot backup. Hot backup is the capability of computers in an SNA Server subdomain to work together to provide session support even when a server or a connection is not working. When SNA Server computers provide hot backup, it means that LUs and other resources are configured so that they can automatically fill in and support sessions required by other servers in the subdomain, even if another, similar resource is not available.