A load-sharing mirror or load-balancing mirror is a copy of a website or web service that not only acts as a backup for the main resource but actively participates in serving a portion of its traffic.
The load-sharing mirror uses an active running copy of a site that is stored on a separate server or servers that respond to a portion of requests for a service or web page. This sharing helps to distribute large workloads for heavy usage or demanding services as well as invisibly redirecting to more local regional servers across the Internet.
Mirror sites are often used as back up servers for disaster recovery. Even sites that use load-sharing mirrors often keep separate offline mirrors for protection in the event of catastrophes. Although load-sharing mirrors may be read-only, their intended purpose is more to maintain performance of a site or service and provide a failover mechanism rather than serve solely as backup.
The load between servers is often balanced by purpose-designed hardware but can be performed by additional software on a separate server as well. The servers may all be local to the load-sharing server or geographically distributed.
Those who run large web sites and services often use load sharing as a method of providing consistent, reliable performance and enhancing security by hiding the end server behind the original contact point.