Load balancing (computing)

In computer networking, load balancing is a technique (usually performed by load balancers) to spread work between many computers, processes, hard disks or other resources in order to get optimal resource utilization and decrease computing time.

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.

Different virtual servers can be configured for different sets of physical services, such as TCP and UDP services in general. Protocol- or application-specific virtual servers that may be supported include HTTP, FTP, SSL, SSL BRIDGE, SSL TCP, NNTP, SIP, and DNS.

The load balancing methods (listed below) manage the selection of an appropriate physical server in a server farm. 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) (also known as Global Traffic Management) the load balancer distributes load to a geographically distributed set of server farms based on health, server load or proximity.