A redundant array of inexpensive servers (RAIS) is the use of multiple servers with load balancing to manage demand across servers and allow one server to take over for another in the case of failure.
While a sole powerful server may be able to keep up with demands, if it goes down the whole service or website is down. In much the same way as RAID uses multiple redundant duplicate disks of data in order to increase reliability in the case of hard disk failures, RAIS stripes and mirrors data across multiple servers using versioned memory.
This redundancy allows RAIS to protect against other critical hardware failures such as those associated with a CPU, motherboard or power supply. When one RAIS server fails, workloads are spread across remaining servers, taking over duty. In that situation, performance is reduced; however, more servers may be added to improve performance.