This chapter introduces WebLogic Server clusters, groups of WebLogic Server server instances running simultaneously and working together to provide increased scalability and reliability.
This chapter includes the following sections:
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 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. A domain also contains the application components deployed in the domain, and the resources and services required by those application components and the server instances in the domain. Examples of the resources and services used by applications and server instances include machine definitions, optional network channels, connectors, and startup classes.
You can use a variety of criteria for organizing WebLogic Server instances into domains. For instance, you might choose to allocate resources to multiple domains based on logical divisions of the hosted application, geographical considerations, or the number or complexity of the resources under management. For additional information about domains see Understanding Domain Configuration for Oracle WebLogic Server.
In each domain, one WebLogic Server instance acts as the Administration Server—the server instance which configures, manages, and monitors all other server instances and resources in the domain. Each Administration Server manages one domain only. If a domain contains multiple clusters, each cluster in the domain has the same Administration Server. All server instances in a cluster must reside in the same domain; you cannot "split" a cluster over multiple domains. Similarly, you cannot share a configured resource or subsystem between domains.
Clustered WebLogic Server instances behave similarly to non-clustered instances, except that they provide failover and load balancing. The process and tools used to configure clustered WebLogic Server instances are the same as those used to configure non-clustered instances. However, to achieve the load balancing and failover benefits that clustering enables, you must adhere to certain guidelines for cluster configuration.
Coherence clusters consist of multiple managed Coherence server instances that work together to distribute data in-memory to increase application scalability, availability, and performance. A client interacts with the data in a local cache and the distribution and backup of the data is automatically performed across cluster members.
Coherence clusters are different than WebLogic Server clusters. They use different clustering protocols and are configured separately. A WebLogic Server domain can contain a single Coherence cluster. Multiple WebLogic Server clusters can be associated with a Coherence cluster.
For details on configuring and managing Coherence clusters, see Administering Clusters for Oracle WebLogic Server.
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.
This section defines, 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 doing a particular "job"—some set of processing tasks—becomes unavailable for any reason, a copy of the failed object finishes 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.
Load Balancing
Load balancing is the even distribution of jobs and associated communications across the computing and networking resources in your environment.
A clustered application or application component is one that is available on multiple WebLogic Server instances in a cluster. If an object is clustered, failover and load balancing for that object is available. Deploy objects homogeneously—to every server instance in your cluster—to simplify cluster administration, maintenance, and troubleshooting.
Web applications can consist of different types of objects, including Enterprise Java Beans (EJBs), servlets, and Java Server Pages (JSPs). Each object type has a unique set of behaviors related to control, invocation, and how it functions within an application. For this reason, the methods that WebLogic Server uses to support clustering—and hence to provide load balancing and failover—can vary for different types of objects. The following types of objects can be clustered in a WebLogic Server deployment:
Servlets
JSPs
EJBs
Remote Method Invocation (RMI) objects
Java Messaging Service (JMS) destinations
Coherence cluster and managed Coherence servers
Timer services
The following APIs and internal services cannot be clustered in WebLogic Server:
File services including file shares
A dynamic cluster is any cluster that contains one or more dynamic servers. Dynamic clusters are based on a single shared server template. You use the server template to specify the configuration of the servers in your dynamic cluster so that each server does not need to be manually configured when expanding the cluster. Dynamic clusters allow you to easily scale up the number of server instances in your domain. When configuring your dynamic cluster, you can specify the number of server instances you anticipate needing at peak load. WebLogic Server will create the specified number of server instances and apply the calculated attribute values to each one. When you need additional server capacity, you can then start a server instance without having to first manually configure it and add it to the cluster. For more information on dynamic clusters, see "Creating Dynamic Clusters" in Administering Clusters for Oracle WebLogic Server.