Apple recently open-sourced FoundationDB, two years after acquiring the company and depriving the world of some promising technology. FoundationDB is a distributed key-value store featuring ACID transactions, painless scaling and easy fault tolerance, all tested with ridiculous thoroughness. In short, a seriously impressive feat of engineering. Before the acquisition its creators often touted the concept of “layers”, meaning stateless programs which add new features to the otherwise simple database. A SQL layer could make FoundationDB behave like a relational database while an AMQP layer could make it function as a message broker.