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Chapter 2 # Concepts Used in the Foundation Services This chapter describes the concepts on which the Foundation Services are built. Make sure that you are familiar with the concepts described in this chapter before installing and using the Foundation Services. This chapter includes the following topics: Cluster Model Reliability, Serviceability, Redundancy, and Availability Service Models Fault Management Models ## Cluster Model This section describes the cluster environment and the types of nodes in a cluster. A cluster is a set of interconnected nodes that collaborate to provide highly available services. The recommended cluster configuration is to have: A master node to coordinate the cluster membership information and to provide the highly available services A vice-master node that backs up the master node Replication of data between the master node and the vice-master node A redundant network between all nodes for reliable inter-node communication In addition to the master node and the vice-master node, a cluster can contain other nodes. The following figure is an example of the nodes inside and outside a cluster. Figure 2-1 Example of Nodes Inside and Outside a Cluster The types of nodes in Figure 2-1 are described in the following sections. ### Peer Nodes and Nonpeer Nodes Nodes that are configured as members of a cluster are called peer nodes. Peer nodes can run the Foundation Services and communicate with each other on the same network. Peer nodes can be master-eligible nodes or master-ineligible nodes. Nodes that are not configured as members of a cluster are called nonpeer nodes. A nonpeer node communicates with one or more peer nodes to access resources or services provided by the cluster. In Figure 2-1, the nonpeer node is connected to both of the redundant network links. For information about the options for connecting nonpeer nodes to a cluster, see Chapter 5, External Addressing. ### Master-Eligible Nodes A cluster contains two master-eligible nodes. A master-eligible node is a peer node that can be elected as the master node or the vice-master node. The master node is the node that coordinates the cluster membership information. The master node generates its view of the cluster configuration. It communicates this view to the other peer nodes. The master node provides the Reliable Boot Service and Reliable NFS to the cluster. The vice-master node backs up the master node. The vice-master node has a copy of all of the cluster management information that is on the master node. It can transparently take control of the cluster if required. You must take care that any tasks you run on the vice-master node either have a very low-level of load, or are designed in such a way that the tasks can be interrupted if the vice-master needs to become master. The vice-master must always be available to take over the master node's load if the current master is no longer able to continue in the master role. Each master-eligible node must be a diskfull node. A diskfull node has at least one disk on which information can be permanently stored. A master-eligible node must be configured as master-eligible at the time of installation and configuration. The master node and vice-master node are the only nodes that are configured as diskfull in a Foundation Services cluster. ### Master-Ineligible Nodes A cluster can contain only two master-eligible nodes, the master node and the vice-master node. All other peer nodes are master-ineligible. In a Foundation Services cluster, master-ineligible nodes are either diskless nodes or dataless nodes. For examples of supported cluster configurations, see the Netra High Availability Suite Foundation Services 2.1 6/03 Hardware Guide. A diskless node either does not have a local disk or is configured not to use its local disk. Diskless nodes boot through the network, using the master node as a boot server. A dataless node has a local disk from which it boots, but it cannot store data permanently on its disk. A dataless node accesses the Foundation Services through the cluster network. Data generated on diskless nodes and dataless nodes is sent to the master node. ## Reliability, Serviceability, Redundancy, and Availability This section defines the concepts of reliability, serviceability, redundancy, and availability. These concepts use the mechanisms of failover and switchover, described in Failover and Switchover. ### Reliability Reliability is a measure of continuous system uptime. The Foundation Services include Reliable NFS, CGTP, and the Reliable Boot Service to increase the reliability of your system. ### Serviceability Serviceability is the probability that a service can be restored within a specified period of time following a service failure. The Foundation Services have a high degree of serviceability through node switchover. Switchover ensures a transfer of services and data from a node requiring maintenance to a backup node. ### Redundancy Redundancy increases the availability of a service by providing a backup to take over in the event of failure. The Foundation Services use the 2N redundancy model. The master node is backed up by the vice-master node. If the master node fails, there is a transparent transfer of services to the vice-master node. In the Foundation Services, the instance of the service running on the master node is the primary instance. The instance of the service running on the vice-master node is the secondary instance. The Foundation Services provide file system redundancy. The file system on the master node is replicated on the vice-master node. The Foundation Services also provide transport redundancy. All cluster transport is duplicated over dual, redundant Ethernet links. If a link fails, a copy of a data packet can still reach its destination through the other link. ### Availability Availability is the probability that a service is available for use at any given time. Availability is a function of system reliability and serviceability, supported by redundancy. ### Failover and Switchover Failover and switchover are the mechanisms that ensure the high availability of a cluster. Failover occurs if the master node fails, or if a vital service running on the master node fails. The services on the master node fail over to the vice-master node. The vice-master node has all of the necessary state information to take over from the master node. The vice-master node expects no cooperation or coordination from the failed master node. Switchover is the planned transfer of services from the master node to the vice-master node. Switchover is orchestrated by the system or by an operator so that a node can be maintained without affecting system performance. Switchover is not linked to node failure. As in the case of a failover, the backup must have all of the necessary state information to take over at the moment of the switchover. Unlike failover, in switchover the master node can help the vice-master node by, for example, flushing caches for shared files. Only the master node and vice-master node take part in failover and switchover. If a diskless node or dataless node fails, there is no failover. If a diskless node or dataless node is the only node running an application, the application fails. If there are other diskless nodes or dataless nodes running the application, the application will continue to run on these other nodes. ## Service Models The Foundation Services can be divided into two categories of service: highly available services and distributed services. Highly available services run on the master node and vice-master node only. The Reliable Boot Service and Reliable NFS are highly available services. If the master node or one of these services on the master node fails, a failover occurs. Distributed services are services that run on all peer nodes. The distributed services include the Cluster Membership Manager, the Node State Manager, the Node Management Agent, the Daemon Monitor, and the Watchdog Timer. If a distributed service fails and cannot be restarted, the node running the service is removed from the cluster. If the node is the master node, a failover occurs.
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