title: When is the thread pool used?
body: <p>So I have an understanding of how Node.js works: it has a single listener thread that receives an event and then delegates it to a worker pool. The worker thread notifies the listener once it completes the work, and the listener then returns the response to the caller.</p>
  
  <p>My question is this: if I stand up an HTTP server in Node.js and call sleep on one of my routed path events (such as "/test/sleep"), the whole system comes to a halt. Even the single listener thread. But my understanding was that this code is happening on the worker pool.</p>
  
  <p>Now, by contrast, when I use Mongoose to talk to MongoDB, DB reads are an expensive I/O operation. Node seems to be able to delegate the work to a thread and receive the callback when it completes; the time taken to load from the DB does not seem to block the system.</p>
  
  <p>How does Node.js decide to use a thread pool thread vs the listener thread? Why can't I write event code that sleeps and only blocks a thread pool thread?</p>