I would like to save those data in server and retrieve the setup(hotspot, fov...etc) when they come back, so I think to save as XML file is a good idea to store those data, am I looking at right direction?
depends... if you want all users to have the same hotspots, then you do it on the server
if you want every user to have his own positions, then you do it locally/client-side
if you want every user to have his own positions and do it on the server, you need cookies anyway
i think rewriting the xml is too complicated...
you'd have to handle requests from different users with session ids etc
to just save all hotspots positions in an array and save this as a cookie is cleaner and easier to code i think.
just save the cookie on each change and onstart you check the cookie and setup the hotspots