there is still the issue that each user would not know that the other user has changed another property; we need some kind of server push mechanism to inform us

there is still the issue that each user would not know that the other user has changed another property; we need some kind of server push mechanism to inform users that the underlying data has changed If the data occasionally changes, you can just respond to the update request with a 409 Conflict reporting the problem and allowing the user to fix it. If it's changing so fast that it requires real-time updates, the user can't probably keep up either, so you should re-think your mechanism.