Eventual consistency: dual software models, software with gaps
Maintaining consistency within parallel, distributed, high-performance, and stream oriented programming is all about accepting one thing: when it comes to consistency, that is dealing with failures, don't promise too much too soon! And that is the whole notion of eventual consistency: you sometimes need to wait for things to get resolved. "How much you wait" depends typically on how much money you have invested in infrastructure, yet it also depends on how fine your system captures failures. Let's start with the eventual consistency. In traditional transactional models, consistency, the knowledge that your system is "doing well", is assured by approaching each operation "transactionaly": if an operation fails, make sure nothing is messed up and that consistency is maintained. Yet in the "new" world of massive parallelism, need for high-performance, and the like of streaming models, it is often simply impossible to maintain consistency...