title: User session in facebook messenger bot
body: <p>In normal web application we maintain session by sending UserID/sessionID/tokenID via cookie or as querystring parameter or in HTTP request in header.</p>
  
  <p>How can we have a session when creating a chatbot using facebook messenger? This will be helpful in getting the context of the conversation.</p>
  
  <p>From the documentation there is <code>metadata</code> field in the message object which can be set. </p>
  
  <blockquote>
    <p>Metadata is a custom string that will be re-delivered to webhook listeners</p>
  </blockquote>
  
  <p>Can this be used for maintaining the session or is there a better option?</p>