Google Map : Update XML data based on the current viewable map area

Question

Please direct me to a right source.

I have a google map. I want to read data from XML file and display markers.

But I also want to update this XML file, based on the "current viewable map".

So if a user moves the map, then we update the XML file and display the appropriate markers on the map.

What would be the best way to do this?