Take an iOS app like Instagram. Instagram is fundementally a real-time application that updates its UI whenever a user interacts with you. For example, if someone likes your post and you are using the app, the UI is updated to trigger dopamine release and inform you that something has hapened to one of your posts. Similarly, when someone sends you a direct message on instagram, while using the app, you see the message spring down from the top, all in real-time.
In terms of implementing such real time features it is obvious that a naive HTTPS polling approach is far too inefficient. Thus this leaves two strategies:
1.) APNS Push Notifications:
When a user likes a post, sends a direct message, comments (etc.), send an HTTP POST to a backend server that will then update the database and send a silent Apple push-notification to the device of the recipient. The recipient, which is using the app, will receive the pushed payload and will send an HTTP GET to the backend server to fetch the needed data (ie. the contents of the direct message sent). The UI is updated in quasi "real-time".
Whenever any user opens the iOS app, connect the user to the server via a websocket. This means, that all users currently using the app are connected to the server via their own websocket. When a user likes a post, sends a direct message, comments (etc.), the app sends a message to the server through the socket indicating the action. The server, before updating the database, finds the socket associated with the recipient and forwards the message through the socket to the recipient. Upon reception of the message, the UI is updated in real-time
Which of these approaches is scalable and better suited for a production environment?
Source: Ios Questions