# Synchronous Callout Message Flow from IMS Applications

Implementing a callout is normally intrusive. If you want to use IMS application to send request messages using ICAL to the awaiting Substation ES process, you must first add code to your application logic.

Synchronous callouts do not use the IMS queues, so the normal rules for trigger or outbound processing do not apply. A sample COBOL program is supplied with the product installation.

The following figure shows the message flow for IMS synchronous callout:

The following is the process for a typical flow of a triggered message:

1. An IMS application (8, or 9) executes an ICAL call with a request and a response area.
2. After Substation ES IMS Interface (5) is notified of the message from XCF (6), it consumes the information from OTMA (7).
3. If the message is valid and recognized by Substation ES IMS Interface (5), the message is placed on a Substation ES inbound queue, and the trigger service is selected by ESB Interface (4).
4. ESB Interface (4) formats the message as specified on the trigger service, which is then sent to Substation ES Delivery Agent. If formatting or delivery fails, a NAK is sent back to IMS. Substation ES envelope information is also inserted as a property or field within the message.
5. ESB Interface (4) publishes the message to the ESB based on level of service and destination defined in the trigger service.
6. TIBCO messaging application publishes the reply message to the originating Substation ES instance with an active subscription for the reply message. The Substation ES envelope information must be returned unchanged with the reply.
7. Substation ES receives the reply message and transforms the contents of the message into a user-configured, formatted buffer area.
8. Substation ES sends the buffer, along with the control and execution information, to Substation ES IMS Interface (5).
9. Substation ES IMS Interface (5) constructs an OTMA message that contains a prefix and application information (LL and buffer) and sends the OTMA message to IMS on a designated Tpipe.
10. When IMS receives the OTMA message, it places the response in the application response area specified in the ICAL call.

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