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TechTip: Changing Triggers on a Busy System

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When your shop runs 24/7 and you have very little scheduled downtime, changing trigger programs can be very hard to do. If you implement triggers in the normal fashion, when a change needs to be done, you are forced to either wait for scheduled downtime or kick everyone off the system. This is because, in order to compile the trigger program, you must have an exclusive lock on the object that the trigger program is attached to.

The technique I have come up with gets around that problem by using two programs for the trigger, instead of one. This works because you can recompile a called program at any time, and the program that is calling it does not care. The two files to create are a driver program that is specified on the Add Physical File Trigger (ADDPFTRG) command and a workhorse program that the driver calls.

The following example is based upon the trigger example from IBM's Information Center.

I have included only the source for the database file that has the trigger attached to it, which is ATMTRANS. Also, my trigger examples do not show any processing, just how to code the two programs.

The file layout for ATMTRANS is shown in Figure 1:

  A          R ATMTRANR
  A            ATMID          5A         COLHDG('ATM' 'MACHINE' 'ID')
  A            ACCTID         5A         COLHDG('ACCT' 'NUM.')
  A            TCODE          1A         COLHDG('TRANS' 'CODE')
  A            AMOUNT         5S 0       COLHDG('AMT. DEP.' 'WITHDRAWN')
  A          K ATMID

Figure 1: ATMTRANS is the source for the database file that has the trigger attached to it.


The driver program TDRVATM is shown in Figure 2:

  Hoption(*nodebugio)
   * This is the DRIVER program for a database trigger.  This is the program
   * added by the ADDPFTRG command.  This program will never need to be modified
   * unless the database file is modified.
   *
   * The purpose is only to receive the old record and new record, and call
   * the WORKHORSE program to do the actual processing.
  Dparm1            ds
  DPARM2            DS
   * The parameters come into the program
  C     *ENTRY        PLIST
  C                   PARM                    PARM1
  C                   PARM                    PARM2
   * Call the WORKHORSE program to do the actual work
  C                   CALL      'TWRKATM'
  C                   PARM                    PARM1
  C                   PARM                    PARM2
   * OK, done
  C                   EVAL      *INLR = *ON

Figure 2: This is the driver program for a database trigger.


The workhorse program TWRKATM is shown in Figure 3:

  Hoption(*nodebugio)
 * This is the WORKHORSE program for a database trigger. This is the program
 * that will actually be doing the work. The reason they are separated is that
 *  a called program can be modified and recompiled at any time.
Dparm1            ds
 * Phys. File Name
D FNAME                   1     10
 * Phys. File Library
D LNAME                  11     20
 * Member Name
D MNAME                  21     30
 * Trigger Event
D TEVEN                  31     31
 * Trigger Time
D TTIME                  32     32
 * Commit Lock Level
D CMTLCK                 33     33
 * Reserved
D RSVFILL1               34     36
 * CCSID
D CCSID                  37     40B 0
 * Reserved
D RSVFILL2               41     48
 * Offset to Orig Rec
D ORECOFF                49     52B 0
 * Length of Orig Rec
D ORECLEN                53     56B 0
 * Offset to Orig Null
D ORECNULOFF             57     60B 0
 * Length of Orig Null
D ORECNULLEN             61     64B 0
 *Offset to New Rec
D NRECOFF                65     68B 0
 * Length of New Rec
D NRECLEN                69     72B 0
 * Offset to New Null
D NRECNULOFF             73     76B 0
 * Length of New Null
D NRECNULLEN             77     80B 0
 * Reserved
D RSVFILL3               81     96
 * Old Record Image
D OLDREC                 97    112
 * Old Record Null Map
D OLDRECNULL            113    116
 * New Record Image
D NEWREC                117    132
 * New Record Null Map
D NEWRECNULL            133    136
 *
DPARM2            DS
D LENGTH                  1      4B 0
 * Set up a Data Structure to put the Old and New records into.  Use the
 * DDS of the file, since that's the easiest.  Also, use PREFIX to make the
 * Old and New record fields unique.
DOLDRECDS       E DS                  EXTNAME(ATMTRANS)
D                                     PREFIX(O_)
DNEWRECDS       E DS                  EXTNAME(ATMTRANS)
D                                     PREFIX(N_)
 * The parameters come into the program
C     *ENTRY        PLIST
C                   PARM                    PARM1
C                   PARM                    PARM2
 * Move the new record into the data structure
C                   MOVE      OLDREC        OLDRECDS
C                   MOVE      NEWREC        NEWRECDS
C                   SELECT
C                   WHEN      TEVEN = '1'
C                   EXSR      @INSERT
C                   WHEN      TEVEN = '2'
C                   EXSR      @DELETE
C                   WHEN      TEVEN = '3'
C                   EXSR      @UPDATE
C                   ENDSL
C                   EVAL      *INLR = *ON
 * Routine to be used when a record is inserted into the file
C     @INSERT       BEGSR
 *
 *
C                   ENDSR
 * Routine to be used when a record is deleted from the file
C     @DELETE       BEGSR
 *
 *
C                   ENDSR
 * Routine to be used when a record is updated in the file
C     @UPDATE       BEGSR
 *
 *
C                   ENDSR

Figure 3: This is the workhorse program for a database trigger.


With this technique, whenever you need to change or even disable the trigger events, you only have to modify the workhorse program and re-compile. The only time the driver program would need to be modified is when the layout for the database file changes--which, if you were using the traditional trigger approach, would have to be done anyway.

Note: This example uses hard-coded buffers; for a soft-coded example,
please go to www.ibm.com/redbooks. The Redbook SG256403 contains an example on pages 345-350 for RPG.

--Tim Grove
Robert Weed Plywood Corp.
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