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Load balancing is a technique used to spread work between many processes, computers, disks or other resources. It's one form of Optimisation.
Web server load balancing
One major issue for large internet sites is how to handle the load of the large number of visitors they get. It's routinely encountered as a scalability problem as a site grows. The ways to do this vary. One example of a site using the approach is the Wikimedia Foundation and its projects. In June 2004 the load was balanced using a combination of:
- Round robin DNS distributed page requests evenly to one of three Squid cache servers.
- Squid cache servers used response time measurements to distribute page requests between seven web servers. In addition, the Squid servers cached pages and delivered about 75% of all pages without ever asking a web server for help.
- The PHP scripts which run the web servers distribute load to one of several database servers depending on the type of request, with updates going to a master database server and some database queries going to one or more slave database servers.
Alternative methods include use of layer 4 routers.