Server groups

The server_group statement identifies the set of IP addresses to update in a name server that represents a cluster of equivalent servers in the sysplex. ADNR updates the name server with the intersection between the IP addresses configured to ADNR in the server_group statement and the IP addresses on which each server in the group can be reached in the sysplex. The DNS names dynamically added to the name server take the form server_group_name.domain_suffix, where server_group_name is the ADNR administrator-defined name of the group of servers being registered to the GWM, and domain_suffix is the domain suffix name specified in a zone parameter on a dns statement. To construct the DNS name, the value of the server_group_name keyword of the server_group statement is used, followed by the value of the domain_suffix keyword of the zone referenced in the server_group statement. The intervening dot is supplied by ADNR; do not explicitly code the dot.

ADNR can also update name servers with DNS names representing individual server instances within the sysplex, using the member keyword. ADNR updates name servers with the intersection between the IP addresses configured for the member and the set of IP addresses on which the server can be reached on a particular host in the sysplex. The DNS names dynamically added to the name server take the form server_name.server_group_name.domain_suffix, where server_name is the ADNR administrator-defined name of the member being registered to the GWM, server_group_name is the ADNR administrator-defined name of the group of servers being registered to the GWM, and domain_suffix is the domain suffix name specified in a zone parameter on a dns statement. To construct the DNS name, the value of the server_name parameter of the server_group statement is used, followed by the value of the server_name keyword of the server_group statement, followed by the value of the domain_suffix keyword of the zone referenced in the host_group statement. The intervening dot is supplied by ADNR; do not explicitly code the dot. If the server_group statement contains a member keyword containing an optional server_name parameter, the name server is updated with these types of DNS names.

A server_group statement can have multiple member keywords within it. One of the coded member keywords does not have to have a server_name parameter. The member without a server_name parameter is sometimes referred to as the unnamed member, while members with a server_name parameter are sometimes referred to as named members. The IP addresses associated with the unnamed member are ones that can be dynamically added to the name server with the DNS name of the form server_group_name.domain_suffix. Configuring an unnamed member is not required. However, a virtual unnamed member is created for you if one is not configured.

Result: The IP addresses associated with the unnamed member are the union of all IP addresses explicitly configured in the unnamed member (if configured), and all IP addresses configured to all named members in that server group. In contrast, only the IP addresses explicitly configured in a named member are associated with the named member.

Typically, if affinity to a particular server instance is important, you configure one named member per server instance in the sysplex. If affinity to a particular server instance is not important, configuring only an unnamed member suffices.

Guideline: For high availability to the target applications to prevent reachability problems in the event of interface failure, IP addresses in server_group statements should either be static or dynamic VIPAs. If the target application is enabled to move from one TCP/IP stack to another, DVIPAs are appropriate for the server group. Otherwise, static VIPAs are appropriate.