Synchronous callout message flow

A synchronous callout message is placed on the tpipe hold queue and waits to be pulled by an external application or service, and the tpipe remains in a wait state until the response message from the external application returns before the next callout message can be delivered.

A typical message flow is as follows:

Figure 1. Synchronous callout message flow

1. A Java™ EE application in WebSphere® Application Server starts and WebSphere Application Server connects to IMS™ Connect through the IMS TM resource adapter. The IMS TM resource adapter, in turn, issues a RESUME TPIPE request to the tpipe and waits for the callout request from IMS Connect.
2. An initiating client, such as a terminal or an IMS Connect or OTMA client starts an IMS application.
3. The IMS application issues an IMS DL/I ICAL call and specifies the OTMA destination descriptor that contains the destination tpipe name where the callout request message is to be queued. When the request is placed on the tpipe hold queue, a correlation token is attached with the request.
4. If a callout request is not available at the time of the request, the IMS TM resource adapter is blocked and the Java application will wait for the next available callout message. In the case of an EJB application where the application itself handles the polling of IMS Connect for the callout messages, the IMS TM resource adapter waits until timeout occurs. As soon as the callout request is available in the tpipe, IMS Connect delivers the callout message to the IMS TM resource adapter.
5. The IMS TM resource adapter receives the callout request message and sends the callout request to the Java application. The application processes the callout request.
6. The Java application sends the response, with the correlation token attached, to the IMS TM resource adapter.
7. The IMS TM resource adapter forwards the response to IMS Connect.

   If the send-only-with-acknowledgment protocol is enabled, IMS TM Resource Adapter receives an acknowledgment from IMS when IMS receives the synchronous callout response message.

8. Based on the correlation token, the response is correlated back to the initiating request from IMS.