2 Understanding WebLogic Server Clustering

This chapter is a brief introduction to clusters in WebLogic Server 10.3.6.

The chapter includes the following sections:

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.

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. 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.

To understand how the failover and load balancing mechanisms used in WebLogic Server relate to particular configuration options see Load Balancing in a Cluster, and Failover and Replication in a Cluster.

Detailed configuration recommendations are included throughout the instructions in Setting up WebLogic Clusters.

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.

The choice to cluster WebLogic Server instances is transparent to application developers and clients. However, understanding the technical infrastructure that enables clustering will help programmers and administrators maximize the scalability and availability of their applications.

What Are the Key Capabilities of a Cluster?

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 (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.


    For backward compatibility with previous versions, WebLogic Server allows you to use multicast for communications between clusters.

    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 pick up where the failed object stopped, and finish the job.

  • 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.


      For backward compatibility with previous versions, WebLogic Server also allows you to use multicast for communications between clusters.

A detailed discussion of how communications and replication techniques are employed by WebLogic Server is provided in Communications In a Cluster.

What Types of Objects Can Be Clustered?

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 JavaBeans (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

Different object types can have certain behaviors in common. When this is the case, the clustering support and implementation considerations for those similar object types may be same. In the sections that follow, explanations and instructions for the following types of objects are generally combined:

  • Servlets and JSPs

  • EJBs and RMI objects

The sections that follow briefly describe the clustering, failover, and load balancing support that WebLogic Server provides for different types of objects.

Servlets and JSPs

WebLogic Server provides clustering support for servlets and JSPs by replicating the HTTP session state of clients that access clustered servlets and JSPs. WebLogic Server can maintain HTTP session states in memory, a file system, or a database.

To enable automatic failover of servlets and JSPs, session state must persist in memory. For information about how failover works for servlets and JSPs, and for related requirements and programming considerations, see HTTP Session State Replication.

You can balance the servlet and JSP load across a cluster using a WebLogic Server proxy plug-in or external load balancing hardware. WebLogic Server proxy plug-ins perform round-robin load balancing. External load balancers typically support a variety of session load balancing mechanisms. For more information, see Load Balancing for Servlets and JSPs.

EJBs and RMI Objects

Load balancing and failover for EJBs and RMI objects is handled using replica-aware stubs, which can locate instances of the object throughout the cluster. Replica-aware stubs are created for EJBs and RMI objects as a result of the object compilation process. EJBs and RMI objects are deployed homogeneously—to all the server instances in the cluster.

Failover for EJBs and RMI objects is accomplished using the object's replica-aware stub. When a client makes a call through a replica-aware stub to a service that fails, the stub detects the failure and retries the call on another replica. To understand failover support for different types of objects, see Replication and Failover for EJBs and RMIs.

WebLogic Server clusters support multiple algorithms for load balancing clustered EJBs and RMI objects: round-robin, weight-based, random, round-robin-affinity, weight-based-affinity, and random-affinity. By default, a WebLogic Server cluster will use the round-robin method. You can configure a cluster to use one of the other methods using the Administration Console. The method you select is maintained within the replica-aware stub obtained for clustered objects. For details, see Load Balancing for EJBs and RMI Objects.

JMS and Clustering

The WebLogic Java Messaging Service (JMS) architecture implements clustering of multiple JMS servers by supporting cluster-wide, transparent access to destinations from any WebLogic Server server instance in the cluster. Although WebLogic Server supports distributing JMS destinations and connection factories throughout a cluster, the same JMS topic or queue is still managed separately by each WebLogic Server instance in the cluster.

Load balancing is supported for JMS. To enable load balancing, you must configure targets for JMS servers. For more information about load balancing and JMS components, see Load Balancing for JMS, For instructions on setting up clustered JMS, see Configure Migratable Targets for Pinned Services, and Deploying, Activating, and Migrating Migratable Services.

What Types of Objects Cannot Be Clustered?

The following APIs and internal services cannot be clustered in WebLogic Server:

  • File services including file shares

  • Time services

You can still use these services on individual WebLogic Server instances in a cluster. However, the services do not make use of load balancing or failover features.