Introduction to Clustering

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At the end of this chapter you will be able to:

  • Cluster Introduction and Architecture
  • Networks and Clusters
  • Cluster Communication

Definition: Clustering

  • A cluster is a group of WebLogic Server instances, working in coordination.



Benefits of Clustering

  • There are two main benefits of clustering together WebLogic servers:
    • Scalability
    • High-Availability
  • Scalability allows you to add and remove servers without disrupting current services.
  • High-Availability ensures that when a server (in a cluster) fails, there are other servers to take over the work so the client is not affected.

Key Capabilities:

The key capabilities of a WebLogic cluster are:

  • Application Failover

When an object in an application is performing a task becomes unavailable, another object will take over and finish the job.

  • Site Failover

When all the services and applications in a single site fail they can switch to a separate site and continue processing.

  • Server Migration

When a server fails, pinned services can be migrated to another server in a cluster.

  • Load Balancing

The even distribution of tasks and communications across multiple servers.  

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Cluster Architecture:

  • Applications are generally broken into multiple tiers, each representing their distinct functionality:
    • Web tier
    • Presentation tier
    • Business or object tier
  • WebLogic provides clustering support for all three tiers.
  • Other services, such as JMS and JDBC, can take advantage of clusters but load-balancing and failover is a little different.

Deciding on Cluster Architecture

Good architecture is somewhat subjective but there are a few global considerations:

       1.  Performance

        2. Efficient replication

        3. Optimal load balancing

        4. Effective failover

        5. Reliable communication

  • There are two primary cluster architectures to choose from:
    • Basic Cluster architecture
    • Multi-tier architecture

Basic Cluster Architecture

Basic cluster architecture combines static HTTP, presentation logic, business logic, and objects into one cluster.

Basic cluster architecture  

Multi-Tier Cluster Architecture

The Web tier and the business logic with services can be separated into two clusters.

Multi tier architecture  

When to Use Multi-tier Architecture

  • The multi-tier cluster is recommended for Web Applications that require:
    • Load balancing for method calls to clustered EJBs
    • Flexibility for load balancing between servers that provide
  • HTTP content and servers that provide clustered objects
    • Higher availability (fewer single points of failure)
    • More flexible security

Basic Cluster Architecture Advantages and Disadvantages

  • The basic cluster architecture has these advantages:
    • Easy administration
    • Flexible load balancing
    • Robust security
  • The basic cluster architecture has these disadvantages:
    • Cannot load balance EJB method calls
    • Load balancing across the tiers may become unbalanced

Multi-tier Advantages and Disadvantages:

  • The multi-tier architecture has these advantages:
    • Improved load balancing
    • Load balancing of EJB methods
    • Higher availability
    • Improved security options
  • The multi-tier architecture has these disadvantages:
    • Can create a bottleneck when presentation tier makes frequent calls to the business logic
    • Increased licensing cost
    • Added firewall configuration complexity

Proxy Servers

  • Proxy servers are used to provide load balancing and failover for a cluster.
    • Are the client’s first level of interaction with the cluster
    • Give the cluster its single server appearance
  • A proxy server can be either software-based or hardware-based.
  • A software-based proxy server may be an internal WebLogic servlet or a 3rd party application.
  • A hardware-based proxy server is typically a physical load balancer.

Basic Cluster Proxy Architecture:

Similar to the basic cluster architecture, except for static content, is hosted on non-clustered HTTP servers.

Basic Cluster Proxy architecture  

Multi-Tier Cluster Proxy Architecture

Similar to the multi-tier cluster architecture, except for static content, is hosted on non-clustered HTTP servers.

Multi-tier cluster proxy architecture  


WLS HttpClusterServlet

WLS Http Cluster servlet  

WLS Plug-Ins…

  • WLS is compatible with major Web servers using the following plug-ins:
    • Sun Java System Web Server plug-in (formerly Netscape iPlanet or Sun One Web Server)
    • IIS plug-in (Microsoft IIS)
    • Apache plug-in
  • Plug-ins:
    • Delegate dynamic content requests to WLS
    • Round-robin across a cluster
    • Support routing based on URL path or on the MIME type of the requested file or both
    • Route HTTP requests to back-end WLS instances based on session cookie or URL rewriting
    • Avoid failed servers in the cluster

Proxy Plug-in Vs. Load Balancer

  • There are many advantages to using a physical load balancer instead of the proxy plug-in:
    • No need to configure client plug-ins
    • Eliminating the proxy layer reduces the number of connections
    • The availability of more sophisticated load balancing algorithms
  • There are a number of disadvantages as well:
    • Additional administration
    • Explicit configuration of “sticky” sessions for stateful web Applications

Architecture Recommendations

  • If possible, place static web content on separate web servers in the DMZ.
  • Use combined tier architecture if your presentation and control tier makes multiple invocations of the business tier.
  • Make sure that your architecture choice supports passing active and passive cookies between the cluster and client application.

Clusterin Networks

  • WebLogic Server clusters can be created in three different kinds of networks:
    • Local Area Networks
    • Metropolitan Area Networks
    • Wide Area Networks
  • When you are configuring your cluster, you will need to keep in mind the type of network you are using.

Local Area Networks

  • A local area network (LAN) serves a local set of computers.
    • They usually use high quality, high-speed communication links
    • Typical data transmission speeds are 100 megabits/second
  • Most clusters exist within a single LAN



Metropolitan Area Networks: 

  • A Metropolitan Area Network (MAN) is a network that usually spans a campus or a city.
  • You can have different clusters located reasonably close to each other within a MAN.


Wide Area Networks

  • A Wide Area Network (WAN) usually spans a wider geographical area and can be made up of smaller MANs and Local Area Networks (LAN).
  • You can have different clusters located in different regions within a WAN.

A cluster can be located in different LANs within a MAN or within a WAN


Cluster Communication

Server Communication in a Cluster

  • WebLogic Server instances in a cluster communicate with one another using two different techniques:
    • Multicast (UDP)
    • Sockets (peer-to-peer TCP)
  • IP multicast broadcasts one-to-many communications among clustered instances.
  • IP sockets are used for peer-to-peer communications between servers.

Detecting a Failure:  

  • WebLogic clusters detect the failure of a server instance in the following ways:
    • Through the use of IP sockets
    • Through the WebLogic server heartbeat
  • If a server in the cluster unexpectedly closes its socket, it will be marked as "failed" and its services will not be used.
  • Server instances use multicast to broadcast heartbeats every 10 seconds to other server instances in the cluster.
    • If three heartbeats are missed from a peer server, the server is marked as "failed" and its services will not be used

One-to-Many Communications

  • WebLogic Server uses one-to-many communication for:
  • Cluster-wide JNDI updates
  • Cluster “heartbeats”
  • Because all one-to-many communications occur over IP multicast, when designing a cluster, consider these factors:
    • If your cluster spans multiple subnets, your network must be configured to reliably transmit messages
    • A firewall can break IP multicast transmissions
    • The multicast address should not be shared with other applications
    • Multicast storms may occur

Peer-to-Peer Communications:

  • WebLogic Server uses peer-to-peer communications for:
  • Accessing non-clustered objects that reside on a remote server instance in the cluster
  • Replicating HTTP session states and stateful session EJB states between a primary and a secondary server
  • Accessing clustered objects that reside on a remote server instance (typically, in a multi-tier cluster architecture)

Multi-Tier Communications:

  • Multi-tier clusters will require more IP Sockets than a combined-tier cluster:
    • One socket for replicating session states
    • One socket for each WebLogic Server in the EJB cluster, for accessing remote objects
  • As an example, using a three-node cluster, the worst-case scenario would be five open sockets per server:
    • One primary and secondary replicated session
    • Each server simultaneously invokes a remote EJB method on each node in the cluster

Communication in a WAN:

  • In a WAN, the servers in your cluster may span multiple subnets.
  • In order for multicast messages to reliably transmit across the WAN your network must meet the following requirements:
    • Full support of IP multicast packet propagation
    • A network latency that allows for multicast messages to reach their destination in 200 to 300 milliseconds
    • A multicast time-to-live value high enough to ensure that routers do not discard multicast packets


In this chapter we discussed:

  • Cluster Introduction and Architecture
  • Networks and Clusters
  • Cluster Communication

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