feat: Massive real-time flag synchronization for clients

[t-kikuc]

Describe the feature you'd like

Enable synchronization of flags in real-time for a massive number of clients, more than 3M concurrent clients.

We need to consider some ways, such as WebSocket.
This feature might cost so much.

Background

Currently, the real-time flag synchronization for clients is supported via Firebase Cloud Messaging (FCM).
https://docs.bucketeer.io/integration/pushes

However, the synchronization pace is not so fast for massive clients due to the FCM quota.

Alternative solutions

No response

[Ubisoft-potato]

We need to consider some ways, such as WebSocket.

Like you said, using WebSocket could cost too much resource, maybe we can use HTTP SSE since the user only need to receive server's update, it's single way communication.