The publisher maintains the state of its own data by updating it whenever any changes are received so that as a new client subscribes it can be sent the latest state of the data as a whole. As such changes are received they are also published as deltas to all currently subscribed clients.

Receiving messages from a remote service

Remote service can also provided a data feed into a publisher. The remote service can publish to topics and the updates are applied to the topics and passed onto subscribed clients.