title: How do I 'hook' into the requested page with the Express Framework?
body: <p>I have this example code from the express tutorial pages.</p>
  
  <pre><code>app.use(express.static('/path/to/html/files'));
  </code></pre>
  
  <p>However, in my application, some of the requested pages should only be generated on demand.<br>
  So, I want the filename the user needs, pass it to a program running in the background (I do not want to spawn this process repeatedly as it takes some time to initialize) and then wait for it to signal that my file is available.  </p>
  
  <p>What is the recommended way to do this ?</p>