Basics of Cluster

What Is a WebLogic Server Cluster?

A WebLogic Server cluster consists of multiple WebLogic Server server instances running simultaneously and working together to provide increased scalability and reliability. A cluster appears to clients to be a single WebLogic Server instance. The server instances that constitute a cluster can run on the same machine, or be located on different machines.
You can increase a cluster's capacity by adding additional server instances to the cluster on an existing machine, or you can add machines to the cluster to host the incremental server instances. Each server instance in a cluster must run the same version of WebLogic Server.

A cluster is part of a particular WebLogic Server domain.
A domain can include multiple clusters. Clustered WebLogic Server instances behave similarly to non-clustered instances, except that they provide failover and load balancing.

What Are the Benefits of Clustering?

A WebLogic Server cluster provides these benefits:

* Scalability

The capacity of an application deployed on a WebLogic Server cluster can be increased dynamically to meet demand. You can add server instances to a cluster without interruption of service—the application continues to run without impact to clients and end users.

* High-Availability

In a WebLogic Server cluster, application processing can continue when a server instance fails. You "cluster" application components by deploying them on multiple server instances in the cluster—so, if a server instance on which a component is running fails, another server instance on which that component is deployed can continue application processing.


What Are the Key Capabilities of a Cluster?

# Application Failover

Simply put, failover means that when an application component (typically referred to as an "object" in the following sections) doing a particular "job"—some set of processing tasks—becomes unavailable for any reason, a copy of the failed object finishes the job.

WebLogic Server uses standards-based communication techniques and facilities—multicast, IP sockets, and the Java Naming and Directory Interface (JNDI)—to share and maintain information about the availability of objects in a cluster.

# Server Migration
WebLogic Server supports automatic and manual migration of a clustered server instance from one machine to another.A Managed Server configured for automatic migration will be automatically migrated to another machine in the even of failure.

# Load Balancing

Load balancing is the even distribution of jobs and associated communications across the computing and networking resources in your environment. For load balancing to occur:

* There must be multiple copies of an object that can do a particular job.
* Information about the location and operational status of all objects must be available.

WebLogic Server allows objects to be clustered—deployed on multiple server instances—so that there are alternative objects to do the same job. WebLogic Server shares and maintains the availability and location of deployed objects using multicast, IP sockets, and JNDI.