Push Messaging VS In-app Messaging

http://info.localytics.com/blog/push-messaging-vs-in-app-messaging-whats-the-difference-for-app-marketing

https://www.kahuna.com/blog/in-app-messaging-vs-push-notifications-best-practices/

https://medium.com/@Imaginovation/app-development-decisions-push-notifications-vs-in-app-messaging-5d7ac1e56233

Pubsub/In-app Messaging

https://pusher.com/

https://www.pubnub.com/

https://firebase.google.com/products/in-app-messaging/

https://www.ably.io/compare/ably-vs-pubnub

https://aws-amplify.github.io/docs/ios/pubsub

https://framework.realtime.co/messaging/