Weblogic Server Clustering

Weblogic Server Clustering

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.

How Does a Cluster Relate to a Domain?

A cluster is part of a particular WebLogic Server domain.

A domain is an interrelated set of WebLogic Server resources that are managed as a unit. 

A domain includes one or more WebLogic Server instances, which can be clustered, non-clustered, or a combination of clustered and non-clustered instances. 

A domain can include multiple clusters.

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.

WebLogic Server Communication In a Cluster

WebLogic Server instances in a cluster communicate with one another using two basic network technologies:

IP sockets, which are the conduits for peer-to-peer communication between clustered       server instances.

IP unicast or multicast, which server instances use to broadcast availability of services and heartbeats that indicate continued availability.

Note : 

When creating a new cluster, it is recommended that you use unicast for messaging within a cluster. For backward compatibility with previous versions, WebLogic Server you must use multicast for communications between clusters.

Key Capabilities of a Cluster


The following sections define, in non-technical terms, the key clustering capabilities that enable scalability and high availability.

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.
For the new object to be able to take over for the failed object:

• There must be a copy of the failed object available to take over the job.
• There must be information, available to other objects and the program that manages failover, defining the location and operational status of all objects—so that it can be determined that the first object failed before finishing its job.
• There must be information, available to other objects and the program that manages failover, about the progress of jobs in process—so that an object taking over an interrupted job knows how much of the job was completed before the first object failed, for example, what data has been changed, and what steps in the process were completed.

WebLogic Server uses standards-based communication techniques and facilities— including IP sockets and the Java Naming and Directory Interface (JNDI)—to share and maintain information about the availability of objects in a cluster. These techniques allow WebLogic Server to determine that an object stopped before finishing its job, and where there is a copy of the object to complete the job that was interrupted.

Information about what has been done on a job is called state. 

WebLogic Server maintains information about state using techniques called session replication and replica-aware stubs. When a particular object unexpectedly stops doing its job, replication techniques enable a copy of the object to pick up where the failed object stopped, and finish the job.

Migration

WebLogic Server supports automatic and manual migration of a clustered server instance from one machine to another. A Managed Server that can be migrated is referred to as a migratable server. This feature is designed for environments with requirements for high availability. The server migration capability is useful for:

• Ensuring uninterrupted availability of singleton services—services that must run on only a single server instance at any given time, such as JMS and the JTA transaction recovery system, when the hosting server instance fails. A Managed Server configured for automatic migration will be automatically migrated to another machine in the event of failure.
• Easing the process of relocating a Managed Server, and all the services it hosts, as part of a planned system administration process. An administrator can initiate the migration of a Managed Server from the Administration Console or command line.

The server migration process relocates a Managed Server in its entirety—including IP addresses and hosted applications—to one of a predefined set of available host machines.

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 unicast, IP sockets, and JNDI.