title: Curiosity: Why does Firebase not guarantee order of delivery for push messages?
body: <p>I've been working with FB Messaging for a little while now and I came across something in the docs that made me curious as to what the reasoning is behind this behaviour:</p>
  
  <blockquote>
    <p>There is no guarantee of the order in which messages get sent.</p>
  </blockquote>
  
  <p>It's mentioned a couple of times throughout the documentation but this is one of the places: <a href="https://firebase.google.com/docs/reference/admin/node/admin.messaging.MessagingOptions" rel="noreferrer">MessagingOptions</a> (under collapseKey)</p>
  
  <p>I've been wondering why Firebase doesn't just send the messages in the order they are received on their servers?</p>
  
  <p>Is this just Google's (Firebase) Cloud Messaging platform works? Does it have to do with performance and efficiency? </p>