I am reading this writing: https://medium.com/@narengowda/system-design-dropbox-or-google-drive-8fd5da0ce55b. In the Synchronization Service part, it writes that:
The Response Queues that correspond to individual subscribed clients are responsible for delivering the update messages to each client. Since a message will be deleted from the queue once received by a client, we need to create separate Response Queues for each client to be able to share an update message which should be sent to multiple subscribed clients.
The context is that
we need a response queue to send the file updates from one client to other clients. I am confused by this statement. If Dropbox has 100 million clients, we need to create 100 million queues, based on the statement. It is unimaginable to me. For example, a Kafka cluster can support up to 5K topics (https://stackoverflow.com/questions/32950503/can-i-have-100s-of-thousands-of-topics-in-a-kafka-cluster#:~:text=The%20rule%20of%20thumb%20is,5K%20topics%20should%20be%20fine.). We need 20K Kafka clusters in this case. Which queuing system can do 100 million "topics"?