A publish/subscribe topology consists of the brokers, the collectives, and the connections between them, that support publish/subscribe applications in the broker domain.
A publish/subscribe application can consist of a network of brokers connected together. The brokers can all be on the same physical system, or they can be distributed over several physical systems. By connecting brokers together, publications can be received by a client on any broker in the network.
Publications are sent only to brokers that have subscribers that have expressed an interest in the topics being published. This helps to optimize network traffic.
The following diagram shows a network of six collectives.