Publish/subscribe messaging allows you to publish one message to many subscribers: A producing application publishes a message on a given subject area, or topic. Consumer applications that have subscribed to the topic each receive a copy of the message. A topic space is a hierarchy of publish/subscribe topics, with a publication point defined automatically on each messaging engine in an associated service integration bus.
A topic space has a set of default publish/subscribe permissions for all topics in the hierarchy. An administrator can configure individual publish/subscribe topics with specific permissions and mediations, and also inherit such configurations from higher publish/subscribe topics in the topic space hierarchy, and from the topic space itself.
A producing application can attach to any messaging engine on the bus, and messages are stored in the message store for the messaging engine to which the producer is attached. As a result, a topic space can have messages stored in a number of message stores at various (producing) messaging engines in the bus. Messages for a publish/subscribe topic are published to a publication point and automatically forwarded to all other publication points for which there are subscribers on that topic.
The default messaging provider supports durable subscriptions to publish/subscribe topics. These enable a subscriber to receive a copy of all messages published to a topic, even messages published during periods of time when the subscriber is not connected to the server. For a given JMS connection factory or activation specification, all publish/subscribe messages to be delivered to durable subscriptions are stored on the publication point of the messaging engine named by the durable subscription home property. Therefore if that messaging engine is unavailable, subscribers cannot retrieve messages. These undelivered messages are preserved and sent to the durable subscriptions after the messaging engine restarts.