In the first section of this chapter, I claimed the superiority of the evented execution model over the threaded execution model, in the context of web servers. That being said, to be fair, the threaded model has an advantage over the evented model: it is simpler to program with. Indeed, in such a case, the framework is responsible for creating the threads and the JVM is responsible for scheduling the threads, so that you don't even have to think about this at all, yet your code is concurrently executed.

On the other hand, with the evented model, concurrency control is explicit and you should care about it. Indeed, the fact that the same execution thread is used to run several concurrent actions has an important implication on your code: it should not block the thread. Indeed, while the code of an action is executed, no other action code can be concurrently executed on the same thread.

What does blocking mean? It means holding a thread for too long a duration. It...