Introducing Group Services
Group Services is a distributed subsystem of the IBM® Reliable Scalable Cluster Technology (RSCT) software.
RSCT software provides a set of services that support high availability on your system. Another service included with the RSCT software is the Topology Services distributed subsystem. The Topology Services subsystem is described in The Topology Services subsystem.
The function of the Group Services subsystem is to provide other subsystems with a distributed coordination and synchronization service. These other subsystems that depend upon Group Services are called client subsystems. Each client subsystem forms one or more groups by having its processes connect to the Group Services subsystem and use the various Group Services interfaces. A process of a client subsystem is called a GS client.
- The list of processes that have joined the group, called the group membership list.
- A client-specified group state value.
Group Services guarantees that all processes that are joined to a group see the same values for the group information, and that they see all changes to the group information in the same order. In addition, the processes may initiate changes to the group information via protocols that are controlled by Group Services.
A GS client that has joined a group is called a provider. A GS client that wants only to monitor a group, without being able to initiate changes in the group, is called a subscriber.
Once a GS client has initialized its connection to Group Services, it can join a group and become a provider. All other GS clients that have already joined the group (those that have already become providers) are told as part of a join protocol about the new providers that want to join. The existing providers can either accept new joiners unconditionally (by establishing a one-phase join protocol) or vote on the protocol (by establishing an n-phase protocol). During a vote, they can choose to approve the protocol and accept the new providers into the group, or reject the protocol and refuse to allow the new providers to join.
Group Services monitors the status of all the processes that are joined to a group. If either the process or the node on which a process is executing fails, Group Services initiates a failure protocol that informs the remaining providers in the group that one or more providers have been lost.
Join and failure protocols are used to modify the membership list of the group. Any provider in the group may also propose protocols to modify the state value of the group. All protocols are either unconditional (one-phase) protocols, which are automatically approved and not voted on, or conditional (n-phase) protocols, which are voted on by the providers.
During each phase of an n-phase protocol, each provider can take application-specific action and must vote to approve, reject, or continue the protocol. The protocol completes when it is either approved (the proposed changes become established in the group), or rejected (the proposed changes are dropped).