The STM model is where you essentially write programs in a mode where it looks like you put a gigantic lock around everything that matters. To be more or less serial, but all the complexity in your code are doing parallels in any way. If they do conflict every now and again, then that one of the guys gets reverted back to the start. And so the net effect is that it looks likeyou're running stuff serially for the programmer and you get correctness for free.