Application server instances form the basis of an application deployment. Each instance belongs to a single domain and has its own directory structure, configuration, and deployed applications. Each server instance also includes the J2EE platform web and EJB containers. Every new server instance must contain a reference to a node agent name defining the machine on which the instance will reside.
There are three types of server instances that can be created. Each server instance can only be of one type:
In the stand-alone server instance the configuration is not shared by any other server instance or clusters.
In the shared server instance the configuration is shared with other instances or clusters.
In the clustered server instance the configuration is shared with other instances in the cluster.
A cluster is a group of server instances sharing the same set of applications, resources, and configuration information. A server instance can belong to only one cluster. Among other things, the cluster is used to facilitate load balancing, through distribution of a load across multiple machines, and high availability, through instance level failover.
Figure 1–2 shows an application server instance in detail. The application server instance is a building block in the clustering, load balancing, and session persistence features of the Application Server Enterprise Edition.