1. A method of operating a publish/subscribe message broker comprising: receiving a message on a topic from a publisher; identifying the message as a start of a sequence of messages; transmitting the message to one or more existing subscribers to the topic; and transmitting, in response to a new subscriber registering to the topic and a further message associated with the sequence being received on the topic from a publisher, the further message to the one or more existing subscribers to the topic, but not to the registered new subscriber to the topic.