Apple’s Push Notification Service (APNS) is the iOS implementation of Push messaging to deliver asynchronous messages from a server to an iOS device.
To conserve device battery, Apple puts strict limits on what an application can do in the background and shuts down those apps which misbehave. If an iOS developer needs to notify a device user that action needs to be taken in app. In that case, the device can send an APNS message, which will display a push notification. For example, if a new chat message is available in a chat app, a server would send an APNS message to the iOS device notifying the user; the user can then unlock their phone and open the chat app.
To send APNS messages, an authorization key is required and an Apple developer account. Note that while APNS is the native push message implementation on iOS, many developers choose to use FCM, which is cross-platform and is implemented on both iOS and Android.
Sending a message from a client device is done using the PubNub API. PubNub then takes responsibility for delivering that message to the target device(s) using an APNS push message (if registered).
Both PubNub and APNS are highly scalable, and when used together, they can enhance your user’s mobile experience.