Introduction to WebLogic Server Clustering

 

Contents

  1. What Is a WebLogic Server Cluster?
  2. How Does a Cluster Relate to a Domain?
  3. What Are the Benefits of Clustering?
  4. What Are the Key Capabilities of a Cluster?
  5. What Types of Objects Can Be Clustered?
  6. What Types of Objects Cannot Be Clustered?
  7. What Are the New Clustering Features in WebLogic Server 8.1?

 

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.

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. For example, if you create a JDBC connection pool in one domain, you cannot use it with a server instance or cluster in another domain. (Instead, create a similar connection pool in the second domain.)

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, adhere to certain guidelines for cluster configuration.

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.

  • Failover

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

  • 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.

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 Java Beans , servlets, and Java Server Pages . 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 objects
  • Java Messaging Service destinations
  • Java Database Connectivity connections

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 filesystem, 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.

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.

 

JDBC Connections

WebLogic Server allows you to cluster JDBC objects, including data sources, connection pools and multipools, to improve the availability of cluster-hosted applications. Each JDBC object you configure for your cluster must exist on each managed server in the cluster - when you configure the JDBC objects, target them to the cluster.

  • Data Sources - In a cluster, external clients must obtain connections through a JDBC data source on the JNDI tree. The data source uses the WebLogic Server RMI driver to acquire a connection. The cluster-aware nature of WebLogic data sources in external client applications allows a client to request another connection if the server instance hosting the previous connection fails. Although not strictly required, BEA recommends that server-side clients also obtain connections via a data source on the JNDI tree.

  • Connection Pools - Connection pools are a collection of ready-to-use database connections. When a connection pool starts up, it creates a specified number of identical physical database connections. By establishing connections at start-up, the connection pool eliminates the overhead of creating a database connection for each application. BEA recommends that both client and server-side applications obtain connections from a connection pool through a data source on the JNDI tree. When finished with a connection, applications return the connection to the connection pool.

  • Multipools - Multipools are multiplexers for basic connection pools. To the application they appear exactly as basic pools, but multipools allow you to establish a pool of connection pools, in which the connection attributes vary from connection pool to connection pool. All of the connections in a given connection pool are identical, but the connections in each connection pool in a multipool should vary in some significant way such that an expected failure of one pool will not invalidate another pool in the multipool. Usually these pools will be to different instances of the same database.

    Multipools are only useful if there are multiple distinct database instances that can equally handle an application connection, and the application system takes care of synchronizing the databases when application work is distributed among the databases. In rare cases it may be valuable to have the pools to the same database instance, but as different users. This would be useful if the DBA disabled one user, leaving the other user viable.

    By default, a clustered multipool provides high availability (DBMS failover). A multipool can be optionally configured to also provide load balancing.

For more information about JDBC, see "Creating and Deploying JDBC Components - Connection Pools, MultiPools, and Data Sources" in the Administration Console Online Help.

 

Getting Connections with Clustered JDBC

To ensure that any JDBC request can be handled equivalently by any cluster member, each managed server in the cluster must have similarly named/defined pools and, if applicable, multipools. Data sources, if intended for use in external clients, should be targeted to the cluster so they are cluster-aware and their connections can be to any cluster members.

  • External Clients Connections - External clients that require a database connection perform a JNDI lookup and obtain a replica-aware stub for the data source. The stub for the data source contains a list of the server instances that host the data source - which should be all of the Managed Servers in the cluster. Replica-aware stubs contain load balancing logic for distributing the load among host server instances.

  • Server-Side Client Connections - For server-side use, application code can use a server-side pool driver directly instead of a JNDI lookup and a data source, but if a data source is used, it will be a local object. A server-side data source will not go to another cluster member for its JDBC connections. The data source obtains a connection from the pool it references. The connection is pinned to the local server instance for the duration of the database transaction, and as long as the application code retains it (until the connection is closed).

 

Failover and Load Balancing for JDBC Connections

Clustering your JDBC objects does not enable failover of connections but it can ease the process of reconnection when a connection fails. In replicated database environments, multipools may be clustered to support database failover, and optionally, load balancing of connections. See the following topics for more information:

 

JMS and Clustering

The WebLogic Java Messaging Service 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, 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.

Automatic failover is not supported by WebLogic JMS in this release.

 

What Types of Objects Cannot Be Clustered?

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

  • File services
  • Time services
  • WebLogic Events (deprecated in WebLogic Server 6.0)
  • Workspaces (deprecated in WebLogic Server 6.0)

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.

 

What Are the New Clustering Features in WebLogic Server 8.1?

The new features in WebLogic Server 8.1 that pertain to clustering are describe in the following sections.

 

Server Affinity for Client Connections to RMI Objects

WebLogic Server 8.1 provides new server affinity features that reduce the number of sockets used for RMI communications. For details, see Round-Robin Affinity, Weight-Based Affinity, and Random-Affinity.

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