A data service is the combination of software and configuration files that enables an application to run without modification in a Sun Cluster configuration. When running in a Sun Cluster configuration, an application runs as a resource under the control of the Resource Group Manager (RGM). A data service enables you to configure an application such as Sun Java System Web Server or Oracle database to run on a cluster instead of on a single server.
The software of a data service provides implementations of Sun Cluster management methods that perform the following operations on the application:
Starting the application
Stopping the application
Monitoring faults in the application and recovering from these faults
The configuration files of a data service define the properties of the resource that represents the application to the RGM.
The RGM controls the disposition of the failover and scalable data services in the cluster. The RGM is responsible for starting and stopping the data services on selected nodes of the cluster in response to cluster membership changes. The RGM enables data service applications to utilize the cluster framework.
The RGM controls data services as resources. These implementations are either supplied by Sun or created by a developer who uses a generic data service template, the Data Service Development Library API (DSDL API), or the Resource Management API (RMAPI). The cluster administrator creates and manages resources in containers that are called resource groups. RGM and administrator actions cause resources and resource groups to move between online and offline states.
A resource type is a collection of properties that describe an application to the cluster. This collection includes information about how the application is to be started, stopped, and monitored on nodes of the cluster. A resource type also includes application-specific properties that need to be defined in order to use the application in the cluster. Sun Cluster data services has several predefined resource types. For example, Sun Cluster HA for Oracle is the resource type SUNW.oracle-server and Sun Cluster HA for Apache is the resource type SUNW.apache.
A resource is an instance of a resource type that is defined cluster wide. The resource type enables multiple instances of an application to be installed on the cluster. When you initialize a resource, the RGM assigns values to application-specific properties and the resource inherits any properties on the resource type level.
Data services utilize several types of resources. Applications such as Apache Web Server or Sun Java System Web Server utilize network addresses (logical hostnames and shared addresses) on which the applications depend. Application and network resources form a basic unit that is managed by the RGM.
Resources that are managed by the RGM are placed into resource groups so that they can be managed as a unit. A resource group is a set of related or interdependent resources. For example, a resource derived from a SUNW.LogicalHostname resource type might be placed in the same resource group as a resource derived from an Oracle database resource type. A resource group migrates as a unit if a failover or switchover is initiated on the resource group.
Data services enable applications to become highly available and scalable services help prevent significant application interruption after any single failure within the cluster.
When you configure a data service, you must configure the data service as one of the following data service types:
Failover data service
Scalable data service
Parallel data service
Failover is the process by which the cluster automatically relocates an application from a failed primary node to a designated redundant secondary node. Failover applications have the following characteristics:
Capable of running on only one node of the cluster
Dependent on the cluster framework for high availability
If the fault monitor detects an error, it either attempts to restart the instance on the same node, or to start the instance on another node (failover), depending on how the data service has been configured. Failover services use a failover resource group, which is a container for application instance resources and network resources (logical hostnames). Logical hostnames are IP addresses that can be configured up on one node, and later, automatically configured down on the original node and configured up on another node.
Clients might have a brief interruption in service and might need to reconnect after the failover has finished. However, clients are not aware of the change in the physical server that is providing the service.
The scalable data service enables application instances to run on multiple nodes simultaneously. Scalable services use two resource groups. The scalable resource group contains the application resources and the failover resource group contains the network resources (shared addresses) on which the scalable service depends. The scalable resource group can be online on multiple nodes, so multiple instances of the service can be running simultaneously. The failover resource group that hosts the shared address is online on only one node at a time. All nodes that host a scalable service use the same shared address to host the service.
The cluster receives service requests through a single network interface (the global interface). These requests are distributed to the nodes, based on one of several predefined algorithms that are set by the load-balancing policy. The cluster can use the load-balancing policy to balance the service load between several nodes.
Sun Cluster systems provide an environment that shares parallel execution of applications across all the nodes of the cluster by using parallel databases. Sun Cluster Support for Oracle Real Application Clusters is a set of packages that, when installed, enables Oracle Real Application Clusters to run on Sun Cluster nodes. This data service also enables Sun Cluster Support for Oracle Real Application Clusters to be managed by using Sun Cluster commands.
A parallel application has been instrumented to run in a cluster environment so that the application can be mastered by two or more nodes simultaneously. In an Oracle Real Application Clusters environment, multiple Oracle instances cooperate to provide access to the same shared database. The Oracle clients can use any of the instances to access the database. Thus, if one or more instances have failed, clients can connect to a surviving instance and continue to access the database.