RE: Multi user programming?

[Buck Calabro <buck.calabro@xxxxxxxxxxxxxxxxx>]

Raja Rajendren wrote:

>Suppose I'm running a program which is 
>used to edit a record.  Now let us say
>I'm in edit mode, editing a record.  
>And when I try to run the same program in
>one more terminal and try to edit the 
>same record, which is already used by
>one more user, the system shows blank 
>screen with "X" at the bottom.  How to
>rectify this problem.  

In RPG, if your file is opened for Update (like UF on the F-specs), the
record is locked when you do a READ (or its variants READE, etc.) or CHAIN.
The record is unlocked when you move to another record, you use the UNLCK
opcode or update the record via UPDAT or EXCPT.

READ MASTER              - Locks record 1
READ MASTER              - Unlocks record 1, locks record 2
UPDAT MASTER     - Unlocks record 2

If you want to read without locking, specify N in column 43 (RPG/400) or use
the (N) opcode extender (RPG IV) for NO LOCK.  Be aware that if you do this
you cannot update the record you just read - in order to update you must
have a lock on it.

   READ MASTER                  n        - Does not lock record 3
   ...
3 CHAIN MASTER                           - Locks record 3
   UPDAT MASTER                          - Unlocks record 3

These rules are different when you operate under commitment control.

There was a great debate on whether it is good to lock the records or not,
and the outcome was "it depends on your application!"  If you don't lock the
records then you have the possibility of losing data:

JOB 1                                   JOB 2
1   CHAIN MASTER        n               1   CHAIN MASTER        n       
     EXFMTSCREEN                             EXFMTSCREEN
* This job changes the phone number     * This job changes the address
     UPDATMASTER                             UPDATMASTER

If JOB 1 UPDATes first, then JOB 2 will overwrite the phone number with the
one on JOB 2's screen.

If you do lock the records, then all other jobs (including batch) that need
to update record will fall over with a Record in use error.  It is fairly
easy to trap the "Record locked" error using the error indicator and the
file information data structure:

FMaster    UF   E           K Disk                      
F                                     INFDS(MasFeedback)
                                                        
D                SDS                                    
D  StsErrMsg             91    170                      
                                                        
DMasFeedback      DS                                    
D  StsMasStatus     *STATUS 
                                                        
C     MasKey        Chain     RMASTER                            8081

Indicator 80 is "no record found" and indicator 81 is "I/O error occurred."

StsErrMsg contains the error message you would have got had you not used
indicator 81.
StsMasStatus contains the file status - record lock is 01218.  Find these in
the RPG manual under Error Handling.
You can make the record lock error appear faster by doing an OVRDBF xxx
WAITRCD(1)

I hope this helps.

Buck Calabro
Aptis; Albany, NY
"We are what we repeatedly do.
 Excellence, then, is not an act, but a habit." --Aristotle


Billing Concepts Corp., a NASDAQ Listed Company, Symbol: BILL
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