We continue our quest to create an RPG program that "talks" to your PC application.

In a previous tip in this series, I described how to use ADO to call an RPG program from a VB application. Now, I'll detail the fundamental aspects of that RPG program. First, a quick recap: in our scenario, there's a VB application that requires data about a client's debt status. This data resides on a System i within the same network. The data cannot be retrieved via a simple query because a set of business rules is associated with it (the part about the debt limit for the client). The VB application is calling the RPG program via ADO, using a list of parameters (see the table in the previous article).

The objective of the RPG program is to get the debt data from the database and call an existing program to determine if the client can increase its debt. Let's go through an overview of the program's structure.

First of all, the files used by the program have to be defined with the USROPN keyword. This is required because the program is called without a library list and its first task will be to compose the proper one, based on the system type parameter.

       ...

      *------------------------------------------------------------------------*

      *   File Descriptions                                                    *

      *------------------------------------------------------------------------*

     FCLIENTF   IF   E           K Disk    Prefix(CLI_) UsrOpn

     FCLIINVF   IF   E           K Disk    Prefix(INV_) UsrOpn         

       ....

Then, we'll need to declare the variables required for the input/output parameter of the program and define the prototype for a CL program that sets the proper library list (explained later):

       ...

      *------------------------------------------------------------------------*

      *   Variables                                                            *

      *------------------------------------------------------------------------*

      * Parameter Fields

     D P_SysType       S              1A

     D P_ClientID      S                   Like(CLI_ID)

     D P_TotalDebt     S             21  2

     D P_Debt30        S                   Like(Inv_InvTot)

     D P_Debt60        S                   Like(Inv_InvTot)

     D P_Debt90Plus    S                   Like(Inv_InvTot)

     D P_DebtAllowed   S              1A

     D P_RetCode       S              1A                          

       ...

      * Prototype for the SETLIBL CL

     D SETLIBL         PR                  ExtPgm('SETLIBLCL')

The next detail is the program interface, i.e., the entry parameters:

                ...

      *------------------------------------------------------------------------*

      *   Parameters                                                           *

      *------------------------------------------------------------------------*

      * Entry parameters

     C     *Entry        PList

     C                   Parm                    P_SysType

     C                   Parm                    P_ClientID

     C                   Parm                    P_TotalDebt

     C                   Parm                    P_Debt30

     C                   Parm                    P_Debt60

     C                   Parm                    P_Debt90Plus

     C                   Parm                    P_DebtAllowed

     C                   Parm                    P_RetCode    

       ...

Now, for the program itself! As mentioned previously, the first task is to set up the proper library. This can be achieved using a CL program that contains CHGLIBL commands contained within IF statements. These IF statements check the value of the system type parameter and set the library list accordingly. Here's the code for the SETLIBL CL program:

             PGM        PARM(&P_SYSTYPE)

             DCL        VAR(&P_SYSTYPE) TYPE(*CHAR) LEN(1)

 /* Test Library list */

             IF         COND(&P_SYSTYPE = 'T') THEN(CHGLIBL LIBL(TSTFILES +

                          TSTOBJS TSTXXXXLIB))

 /* Production library list */

             IF         COND(&P_SYSTYPE = 'P') THEN(CHGLIBL LIBL(PRDFILES +

                          PRDOBJS PRDXXXXLIB))

             ENDPGM

Of course, you can make it more dynamic if you create two data areas with the library lists and use their values on the CHGLIBL command:

             PGM        PARM(&P_SYSTYPE)

             DCL        VAR(&P_SYSTYPE) TYPE(*CHAR) LEN(1)

             DCL        VAR(&PRDLIB) TYPE(*CHAR) LEN(255)

             DCL        VAR(&TSTLIB) TYPE(*CHAR) LEN(255)

/* Retrieve the library lists */

             RTVDTAARA  DTAARA(PRDLIBDA) RTNVAR(&PRDLIB)

             RTVDTAARA  DTAARA(TSTLIBDA) RTNVAR(&TSTLIB)

 /* Set Test Library list */

             IF         COND(&P_SYSTYPE = 'T') THEN(CHGLIBL LIBL(&TSTLIB))

 /* Set Production library list */

             IF         COND(&P_SYSTYPE = 'P') THEN(CHGLIBL LIBL(&PRDLIB))

             ENDPGM

So, back to our program. If setting the library list is the first task, the second is to open the files and clear any "trash" that the output variables may contain. I've created an initialization routine for that:

      //------------------------------------------------------------------------*

      //    Initialization                                                      *

      //------------------------------------------------------------------------*

      /FREE

       BegSr Init;

        // Set the library list

        CallP SetLibL (P_SysType);

        // Open all files

         Open CLIENTF;

         Open CLIINVF;

        // Other initialization code goes here...

        // Initialize output parms

         P_TotalDebt   = *Zeros;

         P_Debt30      = *Zeros;

         P_Debt60      = *Zeros;

         P_Debt90Plus  = *Zeros;

        P_DebtAllowed = *Blanks;

         P_RetCode     = '1';

       EndSr;             

Now, it's time to validate the input parameters. In this case, there's only one (the client ID), so a simple chain to the CLIENTF file will suffice. In a real-life situation, you will probably have a lot more to validate. Needless to say, an error during the validations should set the return code to failure ('0' in this case) and end the program.

Creating a validation routine is a good practice and, as you'll see later this article, makes the flow of the program logical and easy to understand.

Next, the program will retrieve the necessary data to fill the output parameters via business logic and a call to an existing program that returns the go/no go for additional debt. This will be done in the "process" routine.

Note: This program might be an old-style interactive program with loads and loads of business rules built in. Instead of rewriting it, you can reuse it with a few well-directed changes. First of all, add new parameters: the information that is now inputted on the screen can be transformed into entry parameters. I'm talking about the client ID (input), the "interactive mode Y/N" (also input), and the go/no go for additional debt.

Now, you need to change the display file definition, adding the USROPN keyword, and open the file as indicated by the "interactive mode Y/N" parameter. Finally, to get the response back to the calling program, you'll need to pass the value from the screen field (the go/no go for additional debt) to the last of the entry parameters. Be sure to control all the screen-related operations (WRITE, UPDATE, and so on to display file formats) with IF statements that check the "interactive mode Y/N" parameter.

The final step is to close all the files and end the program. This will be performed in the EndProg routine.

Here's an overview of the code:

       ...

      *------------------------------------------------------------------------*

      *   File Descriptions                                                    *

      *------------------------------------------------------------------------*

     FCLIENTF   IF   E           K Disk    Prefix(CLI_) UsrOpn

     FCLIINVF   IF   E           K Disk    Prefix(INV_) UsrOpn                  

       ....

      *------------------------------------------------------------------------*

      *   Variables                                                            *

      *------------------------------------------------------------------------*

      * Parameter Fields

     D P_SysType       S              1A

     D P_ClientID      S                   Like(CLI_ID)

     D P_TotalDebt     S             21  2

     D P_Debt30        S                   Like(Inv_InvTot)

     D P_Debt60        S                   Like(Inv_InvTot)

     D P_Debt90Plus    S                   Like(Inv_InvTot)

     D P_DebtAllowed   S              1A

     D P_RetCode       S              1A                          

       ...

      * Prototype for the SETLIBL CL

     D SETLIBL         PR                  ExtPgm('SETLIBLCL')

      /FREE

       //-----------------------------------------------------------------------*

       //   Main Code                                                           *

       //-----------------------------------------------------------------------*

       ExSr Init;

       ExSr Valid;

       ExSr Process;

       ExSr EndProg;

       //-----------------------------------------------------------------------*

       //    Initialization                                                     *

       //-----------------------------------------------------------------------*

       BegSr Init;

        // Set the library list

        CallP SetLibL (P_SysType);

        // Open all files

         Open CLIENTF;

         Open CLIINVF;

        // ...Other initialization code goes here...

        // Initialize output parms

         P_TotalDebt   = *Zeros;

         P_Debt30      = *Zeros;

         P_Debt60      = *Zeros;

         P_Debt90Plus  = *Zeros;

        P_DebtAllowed = *Blanks;

         P_RetCode     = '1';

       EndSr;             

       //-----------------------------------------------------------------------*

       //    Validate                                                           *

       //-----------------------------------------------------------------------*

       BegSr Valid;

        // Run validations on the input parms

        // ...VALIDATION CODE GOES HERE...

        // In case of error:

        If W_Error = *On;

          P_RetCode = '0';

          Return;

        EndIf;

       EndSr;    

       //-----------------------------------------------------------------------*

       //    Process Request                                                    *

       //-----------------------------------------------------------------------*

       BegSr Process;

        // ...RETRIEVE DATA WITH BUSINESS LOGIC...

        // ...FILL OUTPUT PARMS...

       EndSr;        

       //-----------------------------------------------------------------------*

       //    End program                                                        *

       //-----------------------------------------------------------------------*

       BegSr EndProg;

         Close CLIENTF;

         Close CLIINVF;

         *InLr = *On;

       EndSr;        

This is a rather crude interface program. You can enhance it in a number of ways. For instance, instead of a success/failure code, you might want to build in something more complex, such as an error code (001 meaning "client not found," 002 meaning "debt information not available," and so on). You might implement error-handling to prevent the program from "hanging" your VB application. Or whatever else is your cup of tea…

The sky is the limit! I've just presented the basic functionality required to get the interface working.

In the next tip, I'll continue to explore the application-to-application interfaces, this time using MQSeries.

Rafael Victória-Pereira has more than 20 years of IBM i experience as a programmer, analyst, and manager. Over that period, he has been an active voice in the IBM i community, encouraging and helping programmers transition to ILE and free-format RPG. Rafael has written more than 100 technical articles about topics ranging from interfaces (the topic for his first book, Flexible Input, Dazzling Output with IBM i) to modern RPG and SQL in his popular RPG Academy and SQL 101 series on mcpressonline.com and in his books Evolve Your RPG Coding and SQL for IBM i: A Database Modernization Guide. Rafael writes in an easy-to-read, practical style that is highly popular with his audience of IBM technology professionals.

Rafael is the Deputy IT Director - Infrastructures and Services at the Luis Simões Group in Portugal. His areas of expertise include programming in the IBM i native languages (RPG, CL, and DB2 SQL) and in "modern" programming languages, such as Java, C#, and Python, as well as project management and consultancy.

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MC Press books written by Rafael Victória-Pereira available now on the MC Press Bookstore.

		Evolve Your RPG Coding: Move from OPM to ILE...and Beyond Transition to modern RPG programming with this step-by-step guide through ILE and free-format RPG, SQL, and modernization techniques. List Price $79.95 Now On Sale
		Flexible Input, Dazzling Output with IBM i Uncover easier, more flexible ways to get data into your system, plus some methods for exporting and presenting the vital business data it contains. List Price $79.95 Now On Sale
		SQL for IBM i: A Database Modernization Guide Learn how to use SQL’s capabilities to modernize and enhance your IBM i database. List Price $79.95 Now On Sale