Push notification

I want to do push notification for my chat application. Any one help me to what to do?

There are two free good services to call from your server: OneSignal and Google Firebase.

Alternatively you can read specs on Apple and Google push notifications, and implement a client for each one. Then set up a queue backed by something like RabbitMQ which accepts Tasks like "send message X to phone Y". Worker for the queue would use both APIs to send message to iOS and Android devices.