You can guarantee consistency; but what you must then lose is the ability to make progress in all situations ('liveness'). A trivial commit protocol that is co

You can guarantee consistency; but what you must then lose is the ability to make progress in all situations ('liveness'). A trivial commit protocol that is consistent but not live simply sends no messages. All updates that succeed are consistent, but no updates succeed. An eventually consistent protocol is often correct for stronger consistency guarantees, but instead sacrifices consistency in the case of network problems rather than liveness. (sidebar: this is really what the CAP theorem is driving at. You must choose between consistency and liveness if the network can lose messages).