A web application, when in active use, is a large decentralized system. Thousands (and hopefully more) of participants, each with their own machine running a web browser, are generating events to -- and requesting resources from -- the web server portion of the web application.
It can become centralized if the web server controls who can access the web application. This doesn't have to happen; it is a decision (whether consciously or unconsciously made) by its designers.
