For those who are not aware of Kindle's whispersync, here is how it works (from amazon.com):

"...Whispersync synchronizes the bookmarks and furthest page read among devices registered to the same account. Whispersync is on by default to ensure a seamless reading experience for a book read across multiple Kindles."

Can anyone give some details on how the Whispersync feature is implemented in Kindle and in the Backend of Amazon?

I am guessing this implementation involves a very simple hashmap for each user account. Each hashmap maps Books with satellite information about the book. Satellite information contains bookmarks, furthest page read, device on which it was read, etc..

Thanks!