What's an event loop anyways?

Event loops are a paradigm for processing events different than your typical single-threaded or multi-threaded application. Your request gets broken down into async "events" that are executed in a loop to improve performance and minimize synchronization across threads. It is famously used by Node.js as the backbone of their event processing and also by several other technologies like Redis and Nginx . In this article I'll explain the reason for why this paradigm was created and what it tries to optimize. By the end you'll come out a little wiser, and know more than just "don't block the event loop" :). Motiviation - why event loops? To understand why we need event loops we will explore a simple but key example. Take this straightforward HTTP request code, which sends a request and then tries to read the response from the socket: def send_http_request_GET ( domain : str , request : str ) -> HttpResponse : socket_fd = get_socket_for_domain ( domain ) write_res = os . write ( socket_fd , request ) data = os . read ( socket_fd , 1024 ) return HttpResponse ( data ) We do two things in this call - write data out and read data in. Both of these actions will end up triggering syscalls through the kernel that write and fetch data. In terms of time spent on the CPU, this is relatively inexpensive; sending out packets takes very little time, and eventually reading the response will also take very little CPU time. The key time lost is from waiting on the server to respond to us. os.read will block this thread until the response is available, meaning that the thread cannot be used for any other processing during this time. If our service is single-threaded, this means that we can't make any requests in parallel and are stuck waiting on any previous requests to finish. But of course, most services are not single-threaded, so this isn't a huge problem? Let's continue with the example code, imagining that instead we are processing these requests with multiple threads pulling from a