The totalSales() API, even though expensive in terms of resources, was not affecting the ability of the server to accept concurrent requests. What we learned about the event loop in
Chapter 1, Welcome to the Node.js Platform, should provide an explanation for this behavior: invoking an asynchronous operation causes the stack to unwind back to the event loop, leaving it free to handle other requests.
However, what happens when we run a long, synchronous task that never gives back the control to the event loop? This kind of task is also known as CPU-bound, because its main characteristic is that it is heavy on CPU utilization rather than being heavy on I/O operations.
Let's work immediately on an example to see how these types of task behave in Node.js.