Simon Willison’s Weblog

Question: how do you upgrade servers when you need to pass new information between them? It's a fool's game to try to upgrade both servers at the same time. So you need a communication protocol that is not only backward compatible (a new server can speak the old protocol) but also forward compatible (an old server can speak the new protocol). Protocol Buffers provide that because new additions to the protocol can be ignored by the old server.

— Matt Cutts