title: How to send device to device messages using Firebase Cloud Messaging?
body: <p>After searching the docs I could not find any info on how to send device to device messages using FCM without the use of an external server.</p>
  
  <p>For example, if I was creating a chat application I would need to send push notifications to users about unread messages since they won't be online all the time and I can't have a persistent service in the background that would always be connected to the real time database because that would be too resource heavy.</p>
  
  <p>So how would I send a push notification to a user "A" when a certain user "B" sends him/her a chat message? Do I need an external server for this or can it be done with just Firebase servers?</p>