Discovery announcement messages are used to inform Subscribers about configuration changes in the Publisher. They are sent by the Publisher in the case of a configuration change. A Publisher can also be configured to send the discovery announcement messages periodically. A Message Oriented Middleware may be able to persist the latest announcement message for Subscribers.
Discovery probe messages are sent from Subscriber to Publisher and they are limited to Message Oriented Middleware and protocol mappings that support such a back channel. A discovery probe is typically answered with one or more discovery announcement messages.
Depending on the used Message Oriented Middleware and the protocol mapping, it may be possible and required for the Subscriber to request discovery announcement messages by sending discovery probe messages. One use case is a non reliable transport where the Subscriber did not receive the message or the Subscriber was not available at the time the Publisher sent the discovery announcement. Another use case is the collection of initial knowledge about a Publisher. Some discovery announcement messages may only be sent as result of a discovery probe message.