Introducing Dynamic Clusters in WebLogic Server 12.1.2

Introducing Dynamic Clusters in WebLogic Server 12.1.2 

In WebLogic 12.1.2, we added Dynamic Clusters, which is a set of new cluster configuration options that make it really easy to initially roll out a new cluster, and then make it really easy to scale out that cluster.
With Dynamic Clusters, you define how many servers you want in the cluster and you define a server template to use as the base configuration for servers in the cluster. Machine mappings, listen addresses, and port mappings can be mapped auto-magically. You can also use the WebLogic Diagnostics Framework to drive automation around expanding a cluster based on any metric you choose.

These features rely on some new and extended MBeans and configuration objects:

As shown in this diagram, the Cluster MBean has a new child: DynamicServers. That MBean handles the configuration aspects of the cluster that enable the cluster to be expanded dynamically, including a link to the Server Template, machine mapping settings, Managed Server Name prefix, the number of Dynamic Servers to create, and so forth.
The number of Dynamic Servers that you define in the Dynamic Servers configuration spurs the system to create a ServerLifeCycleRuntime MBean, which is a lightweight bean that reserves a Managed Server name and provides methods for starting and stopping the servers with the Node Manager.
The Server Template is a new configuration object that can store the same configuration attributes that a Server configuration contains. A Server Template is used as the basis for other Managed Servers, both configured servers and Dynamic Servers. And when you make a change to a Server Template, all servers that are based on that template inherit the changes dynamically. That makes changing configuration much easier when you have a bunch of servers to update.
 At runtime, a cluster is still a cluster: the runtime behavior for clusters has not changed. It is only the configuration and scaling that we have made easier. And just to be clear: configured clusters are still fully available and supported, so your existing WLST scripts that you use to configure a cluster are still valid.

http://docs.oracle.com/middleware/1212/wls/CLUST/dynamic_clusters.htm