5 ws 6 and as 4 are normally inactive or used elsewhere in order to save costs the relatively greater expandability of the web tier indicated by having three servers in reserve versus one for the application tier corresponds to the greater computing power and state maintenance normally required of the application servers and thus the greater expense involved in adding servers to the application tier application tier servers may but need not run the same software for example application server as 1 runs a database program that organizes information regarding flight schedules application server as 2 runs a flight scheduling application while application server as 3 runs a flight reservation application in times of high demand application server as 4 can provide additional capacity for the flight reservation function moreover each application server can run multiple applications some of which may run on only one application server some of which may run on all application servers and some of which may run on more than one but not all application servers however the application servers do not run the web serving applications that interface directly with client tier 19 web servers ws 1 ws 2 and ws 3 as well as the other web servers when active run respective copies of the same software each runs a web serving application that provides the main interface to browsers running on client computers 11 load balancer 21 distributes client requests in round robin fashion to the active web servers in addition each server can run one or more application programs other than web servers however the more complex application programs are run on the application tier servers the common functions of servers on web tier 17.