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Given the popularity of Linux with many ISPs, it behooves the Linux system administrator to be aware of load balancing since most ISPs use load balancers to add scalability and fault tolerance to the web servers they provide to their customers. Load balancers are devices that distribute client requests from the Internet to a virtual cluster of servers (often called web farms). With a virtual cluster, more requests can be handled than a single server could process, and any server in a cluster can fail without interrupting service because the load balancer will simply bypass the disabled server, and the other servers in the cluster will continue to operate. A load balancer creates a virtual IP address. For example, if the address resolved by DNS for www.foo.com is 192.72.166.240, that address actually is the load balancer. Therefore, any traffic sent to www.foo.com actually is directed to the load balancer, which then directs requests to one of the servers in the web farm.

In a typical scenario, the load balancer is connected to two networks. One Ethernet port is given the IP address of the web site, and the other port is connected to the network where the actual servers are connected. In the same sense that multiple web servers can reside on a single physical server, load balancers can create many virtual clusters on the same group of servers, each with a different virtual IP address but directing requests to the same servers.

This enables an ISP to replicate content to as many servers as necessary in a web farm, so that one URL might be spread across ten servers, while another URL served by the same load balancer would only reside on three of the ten servers. Load balancers use different algorithms to distribute loads among the servers in a web farm. The earliest versions used a round-robin method, simply rotating through the list of servers, sending each successive request to the next server on the list. The problem that quickly became apparent was that different requests could produce vastly different loads on the server—running a CGI script used much more of the server's processing power than downloading a graphic, although the graphic may use a great deal more network bandwidth. Newer algorithms try to address this by sending the next request to the server that is responding the fastest or that has the least number of users connected to it. There are several types of load balancers: switches, software-only products and appliances. Load-balancing switches are physically the same as the usual 10/100/1000 Ethernet switch, but with load-balancing functionality added.