Release 10.1A: OpenEdge Development:
Messaging and ESB

> ## Consuming messages

> A Message Consumer consumes messages either asynchronously or synchronously. The message consumption type is set for a session. Messages are processed by the Message Consumer when the 4GL is in a `WAIT–FOR` state or other IO-blocking state. While the application is in such a state, all other UI and non-UI events are handled normally. `WAIT–FOR` can be called explicitly by the 4GL code. It can also be called through the waitForMessages procedure on the Session object, which works the same for GUI, character, batch, AppServer, and WebSpeed applications. Once a message is consumed, the content of the message is inaccessible.

> The following sections describe:

> - Creating a Message Consumer object

> - Creating a message handler process

> - Setting reply properties

> - Receiving messages from a queue

> - Subscribing to a topic

> - Terminating the Message Consumer object

> - Processing messages

> - Controlling flow of messages

> - Reusing messages

> - Message-reception issues

> - Reply mechanisms

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