Hi there @owais , welcome to the Community. Into the millions , you say?? Wow that’s a lot! Like: publish one message and you expect a million subscribers to each get a copy of the message, and in a Guaranteed/persistent fashion (in case they are offline).
Your testing is correct, a message in a queue can only be sent to one consumer, even if there are many consumers bound to the queue. If you need to do the publish-subscribe (one-to-many) pattern in Solace with Guaranteed delivery, then every consumer needs their own queue, and each queue is subscribed to a topic (e.g. banking/notification/> ) and whenever you publish on a topic that matches, each queue will get a copy. At least in Solace, we only write the message to the disk once, so large fanout doesn’t take up a lot of room on the broker. But if you are talking about millions of consumers, this is probably not easy, would require a networked mesh (DMR) of brokers, and very large ones.
Is this an IOT use case? Why such large scale? And what type of data? Depending on the use case, there may be other approaches to use. For example, if a consumer is offline for a while, do they want to receive all messages they missed while offline, or only the most recent one? How many different topics are we talking about here?