Introduction to Clustering

 

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.

 

Multi-Tier Cluster Architecture

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

 

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.

 

Multi-Tier Cluster Proxy Architecture

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

 

 

WLS HttpClusterServlet

 

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

Summary:

In this chapter we discussed:

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

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