Types of Clusters

August 14,  2003
Don Burleson
 

• Failover Clusters
• Scalable High Performance Clusters
• Application Clusters
• Network Load balancing clusters
• Other types of clusters

Let us examine the similarities and differences of each class of clusters.

Failover Clusters

If one of the nodes in a cluster becomes unavailable, due to either planned downtime for maintenance or unplanned downtime due to failure, another node takes over to provide the service to the end-user—a process known as failover. When failover occurs, users who are accessing the cluster service continue to access the service, and are unaware that it is being provided from a different server (node). This architecture emphasizes the availability of the database or application service, rather than performance or load balancing.

High availability clusters allow multiple servers, in conjunction with shared disk storage units, to quickly recover from failures. Hardware or software failures, affecting either a critical component or an entire system, trigger a failover from one system in the cluster to another. Application processing and access to disk-based data is typically restored within minutes, although recovery times vary depending upon the specific characteristics of the application and the system configuration.

Scalable High Performance Clusters

This class of cluster provides scalability, high-performance, load balancing, and high availability through the use of parallel middleware. These are also sometimes called parallel clusters or high performance computing clusters (HPCC). They provide a single system image, or in other words, the application can be executed on any of the servers within the cluster. They are used to execute compute-intensive and data-intensive problems by running the job on multiple nodes at the same time.

These clusters utilize parallel-aware software that breaks the task down into smaller chunks, which are dispatched across a network of interconnected systems. The network processes the chunks concurrently and communicates with the nodes using message-passing libraries to coordinate and synchronize the results. Cluster-aware applications take advantage of the network architecture in three ways:

• Parallel Computing, Multi-Threading, and Modularization
• Load Balancing
• Automatic Failure

Parallel database clusters like Oracle Real Application Clusters, IBM Sysplex Database Cluster, and Beowulf Computers fit into this group of clusters. Beowulf clusters are of particular interest to the scientific and educational communities. Beowulf clusters are built by using commodity systems – PCs, workstations, or servers running software such as Linux. They feature a parallel computing system consisting of multiple interconnected nodes and are able to execute parallel jobs.

Through message passing, nodes synchronize data and share results. Oracle RAC uses message passing to process requests and update data on each node, resulting in fast data access and a consistent database.

Application Server Clusters

Nodes running application server clusters are connected to provide availability and scalability. In this type of cluster, each node runs an instance of the application server. They run the application server independently from each other’s instance. There is no concept of failover. When clustered in this way, the state of certain objects is maintained synchronously. Web server clients can connect to either of the application server instances. Application servers such as WebLogic Server, Websphere, and Oracle Application server are examples of this category of clusters.

As an example, the WebLogic Server cluster consists of multiple copies of the WebLogic program running simultaneously and working together to provide increased scalability and reliability. The cluster appears to the web server clients as a single WebLogic Server instance. In WebLogic deployment, Servlets, JSPs, EJB, Remote Method Invocation (RMI) objects, Java Messaging Service (JMS) destinations, and Java Database Connectivity (JDBC) connections are usually clustered.

Clustering in J2EE, as in the Oracle Applications server, is implemented across a number of tiers, namely the client tier, Web tier, EJB tier, and EIS tier. Every tier has load balancing and failover.

Other Types of Clusters

The term cluster is loosely used to specify a condition when two networking components or two storage units are joined to provide backup and failover facility. Two or more clusters can also be located at remote locations and be linked to form a global cluster. Some of the other widely known clusters include:

• NetApp Filer Clusters
• Network Load Balancing Clusters
• Global Clusters

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