I’m looking for a pub/sub database that can handle high volume (hundreds of thousands to millions) of topics/channels where the volume or delivery latency of messages per topic/channel is of lower interest.

To better illustrate my use case with an example, image a live chat system where everyone can listen to everyone else’s message but only at random. Hence when I send a message, the server selects N random other users to send my message to. In this system, each user subscribes to their own topic/channel and the server publishes to their topic/channel at random.

I’ve hypothesized using Kafka, Redis pub/sub, or Google pub/sub, but none of these services mentions anything about high volume of topics/channels.