I was looking for an answer for two days now and finally made the decision to just ask about the following question in here:
I was working with Googles Firebase / Firestore as backend for my Flutter application and everything worked so far.
Now I came to the decision to write my very own API with "the help" of Node JS and Express. I am already able to store user data inside my MongoDB database using http requests. I wrote different service files in Dart that send requests and so on. Now with Firestore I was able to setup some streams that listen to the documents based on the users ID. For example I listened to direct messages and group messages associated to that user and immediately update the UI (chat) when there was a new message.
I was wondering if there is a possibility to achieve the same thing with my self written API. I read some things about "Change Streams" and so on, but couldn't really figure out how to implement such a Stream with Node JS and listen to it on the Dart Frontend as soon as the user uses the application. Same thing e.g. whenever a user I refer to in some way, uploads a new post.