Ask HN: How do you implement a notification system in your web apps?

Not every web apps need a notification system.

But apps like CRMs, Social networks, Forums etc where a lot of things happen often needs an efficient way to notify their users and also the notifications need to be available in a notification feed inside the app (with unseen notification count).

Facebook has implemented a great notification system. With In-App real-time notifications, A great Notification Feed, and Native Desktop Push notifications too.

How do you do this usually for your web apps?