Caveat here is that you're duplicating data in a multi-master scenario. If you have a single write master and multiple read-only instances then consistency is easy to maintain (and suffering when you write from a distance) - obviously you're giving up strict read consistency, but that's enormously easier to model and understand.
edit: I guess this is written from a very web-tech point of view, where the assumption is often made that reads will dominate writes. Obviously that's not always the case, and the tradeoffs get a lot more complicated!
edit: I guess this is written from a very web-tech point of view, where the assumption is often made that reads will dominate writes. Obviously that's not always the case, and the tradeoffs get a lot more complicated!