What is a Cluster
cluster is a group of servers running a Web application simultaneously, appearing to the world as if it were a single server. To balance server load, the system distributes requests to different nodes within the server cluster, with the goal of optimizing system performance. This results in higher availability and scalability -- necessities in an enterprise, Web-based application.
High availability can be defined as redundancy. If one server cannot handle a request, can other servers in the cluster handle it? In a highly available system, if a single Web server fails, then another server takes over, as transparently as possible, to process the request.
Scalability is an application's ability to support a growing number of users. If it takes an application 10 milliseconds(ms) to respond to one request, how long does it take to respond to 10,000 concurrent requests? Infinite scalability would allow it to respond to those in 10 ms; in the real world, it's somwhere between 10 ms and a logjam. Scalability is a measure of a range of factors, including the number of simultaneous users a cluster can support and the time it takes to process a request.
Two Methods of load balancing :
1) DNS round robin
2) Hardware load balancers.
The DNS server generally contains a single IP address mapped to a particular site name. In our fictional example, our site www.loadbalancedsite.com maps to the IP address 203.24.23.3
To balance server loads using DNS, the DNS server maintains several different IP addresses for a site name. The multiple IP addresses represent the machines in the cluster, all of which map to the same single logical site name. Using our example, www.loadbalancedsite.com could be hosted on three machines in a cluster with the following IP addresses:
203.34.23.3
203.34.23.4
203.34.23.5
In this case, the DNS server contains the following mappings:
www.loadbalancedsite.com 203.34.23.3
www.loadbalancedsite.com 203.34.23.4
www.loadbalancedsite.com 203.34.23.5
Advantages :
Inexpensive and easy to set up. The system administrator only needs to make a few changes in the DNS server to support round robin, and many of the newer DNS servers already include support. It doesn't require any code change to the Web application; in fact, Web applications aren't aware of the load-balancing scheme in front of it.
Simplicity. It does not require any networking experts to set up or debug the system in case a problem arises.
Disadvantages :
1) No support for server affinity. Server affinity is a load-balancing system's ability to manage a user's requests, either to a specific server or any server, depending on whether session information is maintained on the server or at an underlying, database level.
2) No support for high availability
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