title: blocking code in non-blocking http server
body: <p>For example, I have a http server, which is node.js, so is non-blocking.</p>
  
  <p>For each request will do a DB operation. </p>
  
  <p>what I cannot understand is that what is the difference between a blocking DB operation and a non-blocking DB operation?</p>
  
  <p>Since the web server is non-blocking, so a blocking DB operation inside the request makes no difference?</p>