I'm building text communicator using serverpod and flutter.
My app have group chats and direct chats between users. Let's focus on group chats only.
When my app is open I want to receive all the messages that should reach the current user.

In this discussion I want to focus on 2 serverpod features: streams and server events in the way that my app will be scalable and will preserve performance with group chats that have a lot of users in it.

Scenario:
User A
User A is a member of 100 different groups (including "Group 1").

User B sends a message to Group 1
Group 1 in the moment of sending a message have 100 members
User A should receive a message when app is active.

How should communication be designed? I will put here a couple of ideas:

1. Streams - observe all groups separately:
   Sending a message will send a single message on {group id} server event topic.
   User A after opening an app will call 100 time observeGroup. Observe group is a streaming method (new streams).
   Observe group endpoint will only listen to server events on topic equal to group id. Received server event messages are returned to client with stream.

So 100 opened streaming methods listening to 100 topics on server events. Sending chat message as one server event.

2. Streams - observe messages to user id:
   Sending a message will query database for all user ids that are in the group (100). Then will trigger 100 times post message method with every userId as a topic.
   User A after opening an app will call 1 time observeMessages. Observe messages is a streaming method. Observe messages endpoint will only listen to server events on topic equal to user id. Received server event messages are returned to client with stream (group id is in the returned message).

So 1 opened streaming method listening to 1 topic. Sending chat message as 100 server events.

3. Mix
   Sending a message will send a single message on {group id} server event topic.
   User A after opening an app will call 1 time observeMessages. Observe messages is a streaming method. Observe messages endpoint invokes listen on 100 server events topics - one for every group that user is in). Received server event messages are returned to client with stream (group id is in the returned message).

So 1 opened streaming method listening to 100 topics. Sending chat message as 1 server events.

{This could be improved with regex or templates when listening to server events, but as far as I know this is not possible in serverpod.}

I'm glad for every advice and help.