ExtremeTech: Inside Microsoft’s New Digital Photo Project
"With the lofty goal of storing millions of digital images online, the underlying central database system must be attuned to handle an incredible volume of data while elegantly storing, searching, and displaying images by location.
For this monumental challenge, Microsoft employed a distributed approach: one central WWMX server (currently running Microsoft’s SQL Server database) stores just the metadata, image thumbnails, and pointers to where the full-sized images can be found. Those pictures are stored on PCs in a peer-to-peer (P2P) network. It’s similar to Kazaa or Napster, but with an atlas as the main interface."