title: Getting ETags right
body: <p>I’ve been reading a book and I have a particular question about the ETag chapter. The author says that ETags might harm performance and that you must tune them finely or disable them completely.</p>
  
  <p>I already know what ETags are and understand the risks, but is it that hard to get ETags right?</p>
  
  <p>I’ve just made an application that sends an ETag whose value is the MD5 hash of the response body. This is a simple solution, easy to achieve in many languages.</p>
  
  <ul>
  <li><p>Is using MD5 hash of the response body as ETag wrong? If so, why?</p></li>
  <li><p>Why the author (who obviously outsmarts me by many orders of magnitude) does not propose such a simple solution?</p></li>
  </ul>
  
  <p>This last question is hard to answer unless you are the author :), so I’m trying to find the weak points of using an MD5 hash as an ETag.</p>