Answer by Pandaiolo for Designing a Firebase based scalable feed model

Firebase guys kinda replied on their blog : https://www.firebase.com/blog/2015-10-07-how-to-keep-your-data-consistent.html

The post is about "Data fanning" (spreading items across many nodes in one atomic write operation).

The technique greatly addresses the feed model of the original question

The post actually contains example code for implementing it :

• Function for creating the fannout object (actually a simple object with keys being API endpoints to be written)

  function fanoutPost({ uid, followersSnaphot, post }) {
          // Turn the hash of followers to an array of each id as the string
          var followers = Object.keys(followersSnaphot.val());
          var fanoutObj = {};
          // write to each follower's timeline
          followers.forEach((key) => fanoutObj['/timeline/' + key] = post);
          return fanoutObj;  
  }
  

• And the logic using this function :

  var followersRef = new Firebase('https://<YOUR-FIREBASE-APP>.firebaseio.com/followers');
  var followers = {};
  followersRef.on('value', (snap) => followers = snap.val());
  var btnAddPost = document.getElementById('btnAddPost');
  var txtPostTitle = document.getElementById('txtPostTitle');
  btnAddPost.addEventListener(() => {
        // make post
        var post = { title: txtPostTitle.value };
        // make fanout-object
        var fanoutObj = fanoutPost({ 
            uid: followersRef.getAuth().uid, 
            followers: followers, 
            post: post 
        });
        // Send the object to the Firebase db for fan-out
        rootRef.update(fanoutObj);
  });
  

Note: this is way more scalable than a loop writing each time in one follower feed. However, it could nevertheless be insufficient for millions of followers. In that case, it would be safer to trust a server operation making several writes. I think client-side can be used for up to a few hundreds followers, which is the average number of followers on social media. (This needs to be verified by testing though)