The focus of the AppFabric Web Farm guide is to assist you in properly installing and configuring Windows Server AppFabric on an existing IIS Web farm. Within a small Windows Server AppFabric installation or within a development environment, having a single AppFabric server is sufficient. However, in a larger production environment, a Web farm using multiple AppFabric servers is recommended to handle increased user load. Web farms not only increase performance by reducing the load on each server, but also increase availability. If one server in the Web farm is disabled (for example, taken offline for maintenance), other servers in the Web farm take over, so users are never aware of the offline server. Servers within a Web farm share a pool of resources and minimize chances for performance bottlenecks due to resource contention.
http://download.microsoft.com/download/3/1/C/31CED722-2E5F-48D6-96B1-E73AAFD9873F/AppFabricWebFarm.docx
Maheshkumar S Tiwari edited Revision 1. Comment: Added Tag