# Overview

Substation ES CICS Interface manages the communications and coordinates the data exchange between Substation ES and the CICS transaction processing system.

TIBCO messaging applications can subscribe to data and request acknowledgments or request sets of results from CICS transaction processing applications. In addition, Substation ES can publish data on behalf of CICS applications to subscribing TIBCO messaging applications.

When received by Substation ES, Rendezvous and Enterprise Message Service messages are transformed into formatted data buffers. The data buffers are then sent to CICS Interface, which combines them with a request header, depending on the method of invocation (MOI). Different MOIs are used by CICS Interface from which requests are interpreted. A reply from the CICS program can be returned to an awaiting user subscription as a Rendezvous or Enterprise Message Service message.

CICS Interface, which operates as a subtask within Substation ES region, communicates with CICS regions with IBM External CICS Interface (EXCI) protocol. CICS Interface employs a multithreaded design that can accommodate multiple sessions or pipes that communicate with each CICS region. Up to 30 CICS Interface instances can operate and coexist concurrently within the Substation ES region. The only limit is that each interface is restricted to communicating with only one CICS region at a time.

You can specify the number of CICS interfaces used by Substation ES through System Initialization Parameters, which provide a high degree of scalability for handling the data processing requirements of a given site.

The following are some of the tasks performed by the CICS Interface:

- Enables multiple CICS interfaces to be allocated for each Substation ES, enabling communication with many CICS regions.
- Provides flexible configuration options that are specified during the startup of Substation ES.
- Requires no special APIs to perform CICS-initiated outbound events (triggered events).
- Supports reliable and guaranteed message delivery services.
- Supports simultaneous connection of up to four Substation ES interface instances to a single CICS region.
- Support alternate routing, enabling each recipe service to define both primary and secondary back-end systems in which services can be executed. If the primary system is unavailable, Substation ES automatically tries to schedule execution on the alternate, as specified.

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