A service program on IBM i (AS/400, IBM i) is one of the core building blocks of ILE (Integrated Language Environment).
Think of it as IBM i’s version of a shared library (.dll on Windows, .so on Unix/Linux, .jar on Java).
🧠 Definition
A service program is:
-
A collection of procedures (functions/subroutines) and possibly data
-
Compiled once and stored in a system object (
*SRVPGM) -
Designed to be reused by multiple programs or other service programs
-
Lives inside an activation group when called, and can stay resident in memory for fast reuse
✅ Why Use Service Programs?
| Benefit | Explanation |
|---|---|
| Code reuse | Write procedures once, use in many programs |
| Easier maintenance | Fix logic in one place instead of 50 programs |
| Smaller programs | Main programs don’t need to duplicate code |
| Better performance | Service program can stay loaded in memory across calls |
| Encapsulation | Hide implementation details, expose only needed APIs |
🧩 Example Structure
A typical service program flow:
Module (SRVPGMMOD) --> Service Program (*SRVPGM) --> Called by RPG/CL/COBOL programs
🛠 Steps to Create a Service Program
1. Write RPGLE Module (Procedures)
MATHMOD.RPGLE
ctl-opt dftactgrp(*no) actgrp(*caller);
dcl-proc AddNums export;
dcl-pi *n int(10);
a int(10);
b int(10);
end-pi;
return a + b;
end-proc;
dcl-proc SubNums export;
dcl-pi *n int(10);
a int(10);
b int(10);
end-pi;
return a - b;
end-proc;
Compile as a module:
CRTRPGMOD MODULE(MYLIB/MATHMOD) SRCFILE(MYLIB/QRPGLESRC)
2. Create a Service Program
CRTSRVPGM SRVPGM(MYLIB/MATHSRV) MODULE(MYLIB/MATHMOD) EXPORT(*ALL)
This packages the procedures into a reusable service program.
3. Call from a Program
MAINPGM.RPGLE
ctl-opt dftactgrp(*no) actgrp(*caller);
dcl-pr AddNums int(10) extproc(*dclcase);
a int(10);
b int(10);
end-pr;
dcl-pr SubNums int(10) extproc(*dclcase);
a int(10);
b int(10);
end-pr;
dsply ('5 + 3 = ' + %char(AddNums(5: 3)));
dsply ('10 - 7 = ' + %char(SubNums(10: 7)));
Compile program and bind it to the service program:
CRTBNDRPG PGM(MYLIB/MAINPGM) SRCFILE(MYLIB/QRPGLESRC) BNDSRVPGM((MYLIB/MATHSRV))
📌 Notes About Service Programs
-
Activation Group: Usually compiled with
ACTGRP(*CALLER)so they share the caller’s AG. -
Exported Procedures: Only procedures declared with
EXPORTare callable externally. -
Signature: Service programs have a signature (like a version ID). Changing the signature without rebinding can cause binding errors.
🔍 Service Program vs Program
| Feature | Program (*PGM) |
Service Program (*SRVPGM) |
|---|---|---|
| Entry point | Yes (main procedure) | No (only procedures) |
| Standalone runnable | Yes | No (must be bound or called) |
| Reusable code | Limited | High |
| Used for APIs | Sometimes | Always |
👉 In short:
A program = something you call to run.
A service program = a library of procedures you bind into other programs.