Function Modules and Function Groups — Deep Dive

What Are Function Modules?

Function modules are SAP's reusable code units — stored in the system dictionary, callable from any program, and discoverable via SE37. Think of them as globally available functions with a formal interface.

You've already called function modules (REUSE_ALV_GRID_DISPLAY, CONVERSION_EXIT_ALPHA_OUTPUT). Now let's build our own.

Creating a Function Module in SE37

Step 1: Create a Function Group

Function modules live in function groups — containers that share global data. In SE37, go to Function Group → Create:

• Function Group: Z_HR_UTILS
• Short Text: "HR Utility Functions"
• Package: $TMP (for learning)

Step 2: Create the Function Module

In SE37, enter the name Z_CALCULATE_BONUS and click Create:

• Function Group: Z_HR_UTILS
• Short Text: "Calculate employee bonus based on salary and rating"

Step 3: Define the Interface

On the Import tab:

Parameter Name    Type         Reference Type    Optional    Description
─────────────     ──────       ──────────────    ─────────   ───────────
IV_SALARY         TYPE         P DECIMALS 2      -           Annual salary
IV_RATING         TYPE         C LENGTH 1        -           Performance rating (A/B/C)
IV_YEARS          TYPE         I                 X           Years of service

On the Export tab:

Parameter Name    Type         Reference Type    Description
─────────────     ──────       ──────────────    ───────────
EV_BONUS          TYPE         P DECIMALS 2      Calculated bonus amount
EV_BONUS_PCT      TYPE         P DECIMALS 1      Bonus percentage applied

On the Exceptions tab:

Exception Name           Description
──────────────           ───────────
INVALID_RATING           Rating must be A, B, or C
SALARY_NOT_POSITIVE      Salary must be greater than zero

Step 4: Write the Code

FUNCTION z_calculate_bonus.
*"----------------------------------------------------------------------
*"  IMPORTING
*"    VALUE(IV_SALARY) TYPE P DECIMALS 2
*"    VALUE(IV_RATING) TYPE C LENGTH 1
*"    VALUE(IV_YEARS) TYPE I OPTIONAL
*"  EXPORTING
*"    VALUE(EV_BONUS) TYPE P DECIMALS 2
*"    VALUE(EV_BONUS_PCT) TYPE P DECIMALS 1
*"  EXCEPTIONS
*"    INVALID_RATING
*"    SALARY_NOT_POSITIVE
*"----------------------------------------------------------------------

  " Validate inputs
  IF iv_salary <= 0.
    RAISE salary_not_positive.
  ENDIF.

  " Determine bonus percentage based on rating
  CASE iv_rating.
    WHEN 'A'.
      ev_bonus_pct = 20.
    WHEN 'B'.
      ev_bonus_pct = 12.
    WHEN 'C'.
      ev_bonus_pct = 5.
    WHEN OTHERS.
      RAISE invalid_rating.
  ENDCASE.

  " Add loyalty bonus: 0.5% per year of service
  IF iv_years IS SUPPLIED AND iv_years > 0.
    ev_bonus_pct = ev_bonus_pct + ( iv_years * '0.5' ).
  ENDIF.

  " Calculate bonus amount
  ev_bonus = iv_salary * ev_bonus_pct / 100.

ENDFUNCTION.

Step 5: Activate and Test

Activate (Ctrl+F3), then test (F8). SE37 provides a test screen where you enter parameter values and see results — invaluable for debugging.

Calling Your Function Module

REPORT z_call_fm.

DATA: lv_bonus     TYPE p DECIMALS 2,
      lv_bonus_pct TYPE p DECIMALS 1.

CALL FUNCTION 'Z_CALCULATE_BONUS'
  EXPORTING
    iv_salary = CONV #( 120000 )
    iv_rating = 'A'
    iv_years  = 5
  IMPORTING
    ev_bonus     = lv_bonus
    ev_bonus_pct = lv_bonus_pct
  EXCEPTIONS
    invalid_rating      = 1
    salary_not_positive = 2
    OTHERS              = 3.

CASE sy-subrc.
  WHEN 0.
    WRITE: / |Bonus: { lv_bonus } ({ lv_bonus_pct }%)|.
  WHEN 1.
    WRITE: / 'Error: Invalid rating'.
  WHEN 2.
    WRITE: / 'Error: Salary must be positive'.
  WHEN OTHERS.
    WRITE: / 'Unknown error'.
ENDCASE.

Bonus: 27,000.00 (22.5%)

RFC-Enabled Function Modules

To call a function module from another SAP system (or from an external program), mark it as RFC-enabled in the Attributes tab:

Processing Type: Remote-Enabled Module

RFC-enabled function modules can be called across SAP systems, from Java programs, from Python (using PyRFC), and from web services. This is how SAP systems communicate with each other.

* Call a function module on a remote SAP system
CALL FUNCTION 'Z_CALCULATE_BONUS'
  DESTINATION 'RFC_PRODUCTION'    " RFC destination configured in SM59
  EXPORTING
    iv_salary = CONV #( 120000 )
    iv_rating = 'A'
  IMPORTING
    ev_bonus = lv_bonus
  EXCEPTIONS
    communication_failure = 1  MESSAGE lv_error_msg
    system_failure        = 2  MESSAGE lv_error_msg
    invalid_rating        = 3
    salary_not_positive   = 4
    OTHERS                = 5.

The DESTINATION parameter routes the call to a remote system. RFC destinations are configured in transaction SM59 by the Basis team.

Function Group Global Data

Function modules in the same function group share global data:

* In the TOP include of function group Z_HR_UTILS:
DATA: gv_company_code TYPE bukrs,
      gt_config       TYPE TABLE OF zhrc_config.

All function modules in Z_HR_UTILS can access gv_company_code and gt_config. This is useful for caching — load configuration once, share across all function calls.

Finding Standard Function Modules

SE37 has a search function. Common ways to find the right function module:

• Search by name: Use wildcards like *CUSTOMER*CREATE*
• Search by function group: Browse function groups related to your module
• Google it: "SAP function module to create sales order" → results usually point you to the right FM
• SE37 documentation: Most SAP function modules have inline documentation

Key Takeaways

• Function modules are reusable, callable from any program, and testable in SE37.
• Interface has four categories: IMPORTING (in), EXPORTING (out), CHANGING (in+out), EXCEPTIONS (errors).
• Always handle exceptions — at minimum, handle OTHERS as a catch-all.
• RFC-enabled function modules can be called from remote systems and external programs.
• Function groups share global data across their function modules — useful for caching.
• Test function modules directly in SE37 (F8) before calling them from your program.

Next Lesson

SAP comes with thousands of standard function modules. The most important ones are BAPIs. In Lesson 24: BAPIs — SAP's Standard APIs, we'll learn how to use these pre-built APIs to create, read, update, and delete SAP business objects.