To communicate with our CICS business logic application on the OS/390 server, the Visual Basic component uses the CICS External Call Interface (ECI) API. This is explained later in Understanding the CICS External Call Interface (ECI). The ECI enables a non-CICS client application to call a CICS program synchronously or asynchronously, as a subroutine. The client program communicates with the server CICS program, using a data area called a COMMAREA. Check back with Saving Data: Using a Scratchpad Facility in Chapter 5 if you have forgotten what a COMMAREA means in CICS.
In making use of the CICS ECI, we have chosen to implement two different access methods, so that you can compare and contrast the approaches:
The first of these uses the native CICS API. This requires the Visual Basic application programmer to have a reasonable understanding of what CICS requires in terms of the COMMAREA field definitions used by the CICS applications, as well as the ability to set up various buffers that are needed in the sending and receiving of the data. The bulk of the code for this method is contained in the Visual Basic module comsubs.bas.
The second method makes use of the VisualAge Interspace product. This product enables the Visual Basic application programmer to make use of CICS applications easily, without the need to understand exactly what CICS is doing. Within Interspace, the programmer can create the module that is required to communicate ...