Event-based Concurrency (Advanced)

Thus far, we’ve written about concurrency as if the only way to build concurrent applications is to use threads. Like many things in life, this is not completely true. Specifically, a different style of concurrent programming is often used in both GUI-based applications [O96] as well as some types of internet servers [PDZ99]. This style, known as event-based concurrency, has become popular in some modern sys-tems, including server-side frameworks such as node.js [N13], but its roots are found in C/UNIX systems that we’ll discuss below. The problem that event-based concurrency addresses is two-fold. The first is that managing concurrency correctly in multi-threaded applications can be challenging; as we’ve discussed, missing locks, deadlock, and other nasty problems can arise. The second is that in a multi-threaded application, the developer has little or no con-trol over what is scheduled at a given moment in time; rather, the programmer simply creates threads and then hopes that the under-lying OS schedules them in a reasonable manner across available CPUs. Given the difficulty of building a general-purpose scheduler that works well in all cases for all workloads, sometimes the OS will schedule work in a manner that is less than optimal. The crux: THE CRUX: HOW TO BUILD CONCURRENT SERVERS WITHOUT THREADS How can we build a concurrent server without using threads, and thus retain control over concurrency as well as avoid some of the problems that seem to plague multi-threaded applications