Load Balancing Servers A load balancer can be used to increase the capacity of a server farm beyond that of a single server. It can also allow the service to continue even in the face of server down time due to server failure or server maintenance. A load balancer consists of a virtual server (also referred to as vserver or VIP) which, in turn, consists of an IP address and port. This virtual server is bound to a number of physical services running on the physical servers in a server farm. These physical services contain the physical server's IP address and port. A client sends a request to the virtual server, which in turn selects a physical server in the server farm and directs this request to the selected physical server. Load balancers are sometimes referred to as "directors"; while originally a marketing name chosen by various companies, it also reflects the load balancer's role in managing connections between clients and servers. 
Load Balancing clusters are often used for - Webservers suffering high load(huge number of visitors)Key databases (Oracle, Sybase, DB2).
- Mail Servers with huge traffic.
- LDAP servers ( Directory services).
Persistence can be configured on a virtual server; once a server is selected, subsequent requests from the client are directed to the same server. Persistence is sometimes necessary in applications where client state is maintained on the server, but the use of persistence can cause problems in failure and other situations. Further, persistence mechanisms can vary across load balancer implementations. A more common method of managing persistence is to store state information in a shared database, which can be accessed by all real servers, and to link this information to a client with a small token such as a cookie, which is sent in every client request. Load balancers also perform server monitoring of services in a web server farm. In case of failure of a service, the load balancer continues to perform load balancing across the remaining services that are UP. In case of failure of all the servers bound to a virtual server, requests may be sent to a backup virtual server (if configured) or optionally redirected to a configured URL. For example, a page on a local or remote server which provides information on the site maintenance or outage. Among the server types that may be load balanced are: - Server farms
- Caches
- Firewalls
- Intrusion detection systems
- SSL offload or compression appliances
- Content Inspection servers (such as anti-virus, anti-spam)
In Global Server Load Balancing (GSLB) the load balancer distributes load to a geographically distributed set of server farms based on health, server load or proximity.
Some Server load balancing methods - Least connections
- Round robin
- Least response time
- Least bandwidth
- Least packets
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