Firebase child_added strict ordering

Question

Per this question Why do my Firebase 'child_added' events show up out-of-order? it seems that Firebase makes no garauntees about all clients seeing additions to a list in the same order. Is there some way to force Firebase to use strict ordering and only see local events once they have been confirmed by the server?

Answer 1

The items could appear out of order because it's a real-time distributed system. If multiple records arrive to the server, their keys or priorities could cause them to be inserted before a record that has already been added. Multiply this by 100 users all working in data that is at a different state on their local machine, and it makes sense that ordering is fluid as things move or shift in the list.

However, you can obtain a "snapshot" of the data using the value event, and iterate the items in order using forEach to guarantee the order.

var fb = new Firebase(URL);

// obtain a snapshot of data
fb.once('value', function(snap) {

   // iterate the values in order
   snap.forEach(function(childSnap) {
      console.log(childSnap.name(), childSnap.getPriority(), childSnap.val());
   });

});