CMS Frontier Architecture. Online is the part of the system co-located with the CMS detector, and Offline starts at CERN and sends data throughout the world. Squid is the name of the standard http caching program, and Tomcat is the name of the standard software system that the Frontier server is built on. Applications on the worker node Farms make database requests that are converted to http by the Frontier client, then Squids either return the results immediately from their caches or forward the requests to the Frontier servers. The Frontier servers convert the requests from http into a Database request, and convert the answers into http to send back to the Squids and clients.