# Overview

Substation ES receives messages on a specified subject located in a recipe defined in the Transformer’s configuration file. A sample host-side application that performs the required function is initiated.

Sample code has been included to help you understand how each host-side application is initiated and what happens as a result of the function that is performed. The request or reply applications are written in C and run on the z/OS host.

The IVP Sender and IVP Generic Listener programs in the previous versions have been combined into a single program called IVP Sender Listener in Substation ES version 2.3. The modules are `SXJ3ISL1` for TIBCO Enterprise Message Service and `SXR3ISL1` for TIBCO Rendezvous.

Also included is an Error listener source `SXR3IELS`, which listens for errors published by instances of TIBCO Substation ES. For summary usage of sample IVPs resources, see Sample IVP Resources.

Note: Before executing IVPs, it is good practice to start the Generic Listener and the Generic Error Listener so that you can see the printed output, including errors, while executing.

For more information about Generic Listener, see Generic Sender or Listener Utility in the TIBCO Substation ES Operations and Administration manual. For more information about Generic Error Listener, see Generic Error Listener Utility in the same manual.

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