What functions do CICS Clients provide?

CICS® Clients provide a standard set of functions for client/server computing. This section gives an overview of the most important functions; it is not meant to be exhaustive.

The External Call Interface

The External Call Interface (ECI) is an application programming interface (API) that allows a non-CICS program running on a Client to call a CICS program located on a CICS server. This enables the Client to make use of existing server routines that could be used, for example, to make enquiries on a database.

The Client program can make the following types of call to a CICS server:

Dynamic routing of ECI calls

CICS Transaction Server for z/OS® and CICS Transaction Server for OS/390® Release 3 allow you to route ECI calls dynamically. In these products, if a program defined as DYNAMIC is the subject of an ECI program-link call from a CICS Client, the dynamic routing program is invoked, and can select a remote region on which the server program is to execute.

This means that CICS Clients can benefit from the workload balancing capabilities of CICS TS for OS/390 Release 3 and later.

For definitive information about how to route program-link requests dynamically, see your Intercommunication Guide.

The External Presentation Interface

The External Presentation Interface (EPI) is an API that allows a non-CICS Client program to appear to a CICS server as one or more standard 3270 terminals. This enables the Client to access, for example, CICS on System/390® transactions written for 3270 terminals, without needing to change the System/390 code.

The Client program can start CICS transactions and send and receive standard 3270 datastreams to and from the transactions. It can present the 3270 data to the user by emulating a 3270 terminal, or by means of a graphical user interface such as Windows (Windows Clients).

The EPI consists of a set of calls that can be made from a Client program.

Depending on the Client version and platform, the Client program can be written in any of a number of languages, including:

The EPI calls are provided in a library that is linked to the application. Among the functions available are calls to:

The External Security interface

The External Security Interface (ESI) is an API that allows a non-CICS Client program to verify and change the passwords used by Clients to connect to a CICS server.

Terminal emulation

CICS Clients can run 3270 terminal emulators. A Client terminal emulator transmits or receives standard CICS transaction routing flows to or from a CICS server. This allows a user to interact with the server, and run transactions, as if the Client were a locally-attached 3270 terminal.

It is possible to run multiple terminal emulators on a single Client. The emulators can be connected to the same CICS server, or to different servers. In the former case, each instance of the emulator requires a unique name and represents a separate terminal to the server.

Users can customize the colors and keyboard mapping of their emulators.

Related concepts
What is a CICS Client?
What protocols are supported?
Benefits of Client support
Related reference
Further information about Clients
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