Topology Services is a distributed subsystem of the RS/6000 Cluster Technology (RSCT) software that can run on any |IBM pSeries or RS/6000 system. The RSCT software provides a set of high availability services to the PSSP software. The other services in the RSCT software are the Event Management and Group Services distributed subsystems. These three distributed subsystems operate within a domain. A domain is a set of |IBM pSeries or RS/6000 nodes where the RSCT components execute and, exclusively of other nodes, provide their services. On the SP system, a domain is an SP system partition. Note that a node may be in more than one RSCT domain; the control workstation is a member of each system partition and, therefore, a member of each RSCT domain. When a node is a member of more than one domain, there is an executing copy of each RSCT component for each domain.
Topology Services provides other high availability subsystems with network adapter status, node connectivity information, and a reliable messaging service. The adapter status and node connectivity information is provided to the Group Services subsystem upon request, Group Services then makes it available to its client subsystems. The Reliable Messaging Service, which takes advantage of node connectivity information to reliably deliver a message to a destination node, is available to the other high availability subsystems.
This adapter status and node connectivity information is discovered by an instance of the subsystem on one node, participating in concert with instances of the subsystem on other nodes, to form a ring of cooperating subsystem instances. This ring is known as a heartbeat ring, because each node sends a heartbeat message to one of its neighbors and expects to receive a heartbeat from its other neighbor. Actually each subsystem instance can form multiple rings, one for each network it is monitoring. Usually, each subsystem monitors two rings; the SP Ethernet and the SP switch. This system of heartbeat messages enables each member to monitor one of its neighbors and to report to the heartbeat ring leader, called the Group Leader, if it stops responding. The Group Leader, in turn, forms a new heartbeat ring based on such reports and requests for new adapters to join the membership. Every time a new group is formed, it lists which adapters are present and which adapters are absent, making up the adapter status notification that is sent to Group Services.
In addition to the heartbeat messages, connectivity messages are sent around all rings. Connectivity messages for each ring will forward its messages to other rings, so that all nodes can construct a connectivity graph. It is this graph that determines node connectivity and defines a route that Reliable Messaging would use to send a message between any pair of nodes that have connectivity.
For more detail on maintaining the heartbeat ring and determining node connectivity, see Topology Services components.