unity - How do I know the data has arrived at server without requesting every frame of the game?

I have a game where I have to get data from the server but the problem is I don't know when the data will become available on server. So I decided to use co-routine and hit server at after specific time or on every frame. But certainly this is not the right, scale able and efficient approach.

Now my question is that how do I know that data has arrived at server so that I can use this data to run my game. Or how should I direct the back-end team to design the server in an efficient way that it responds efficiently.

Our back-end team currently uses REST Web services with JSON with Python. And they want to push data to my unity player without my frequent requests.