Most EAI systems treat a message as an event with an attached payload. In Message Management systems the payload is the focus, the event and the history are attached. Additionally, Oracle AQ allows the creation and tracking of messages that are directed to multiple consumers. Furthermore, users may be notified of the arrival of a message through event channels. This allows users to monitor one or more queues while working on other tasks, e.g., making a data warehouse analysis.
Messages have to be moved from providers to consumers.
A consumer can reside on the same node as the provider using the same EAI system or on another node. If the provider and the consumer reside on different nodes, messages must be propagated between the nodes. Any expression permitted in SQL is allowed in a subscription. This applies also to data supported by the Oracle8 extensibility feature. Foe example, one could subscribe to specials of Italian or Chinese restaurants within 6 miles of my house or in the catalogue of a department store to new dresses of a certain type in one of my favorite colors or pattern Subscriptions and queries act complementary, i.e., if a query is asked a the end of a time period or a subscription is provided at the beginning of the same time period, the result is the same, assuming all messages are still available.