Re: problem with duplicate records

One can also use MATTOD and CVTCH to get a 16-character value that is guaranteed to be unique by the system - if the documentation is right. There are 16 values per character position, so the total possible is 16 to the 16th power (18,446,744,073,709,551,616) - however, not every value is used - I don't know the practical limit, but it still has to be considerable.

There is also the UUID - unique universal ID (universal unique?) - I think the function is GETUUID but not sure - look it up in the API finder. Another unique 16-character value, as I understand it.

These might be overkill - one can lock a data area, get the number, increment it and save the value, update and release the data area - all very fast.

Bruce Vining wrote:

I'm a little confused about the scenario in that the first note suggests an archive number while the second suggests a test for duplication of client calculated data, but if this is simply a sequential archive number then several possible solutions exist.
If volume is low, and expected to remain low, the external procedure could access a database record for update which contains the next available archive number. The procedure uses this number as is, adds 1 to the value and then updates the record with this new value for the next reader. A variant of this would be to use a *DTAARA.
If volume is high, the external procedure could utilize a shared memory location and use the Compare and Swap instruction CMPSWP. I discuss CMPSWP in the article found at http://www.mcpressonline.com/programming/cl/the-cl-corner-incrementing-a-numeric-value-across-jobs.html though this is a CL oriented article. You shouldn't have much difficulty mapping this to RPG though.
Other solutions also exist such as SQL ROWID, etc.
Bruce

Bruce Bruce Vining Services 507-206-4178

--- On Wed, 11/19/08, David FOXWELL <David.FOXWELL@xxxxxxxxx> wrote:

From: David FOXWELL <David.FOXWELL@xxxxxxxxx>
Subject: RE: problem with duplicate records
To: "RPG programming on the AS400 / iSeries" <rpg400-l@xxxxxxxxxxxx>
Date: Wednesday, November 19, 2008, 5:17 AM

Sorry, the contents of my file are misleading. The time isn't part of the
reference number.

ArchiveFile
Product Client Ref Time
Product1 Client1 RefNo1 10:41:57
Product2 Client2 RefNo1 10:41:57


Summing up, 2 jobs writing to the same file. One job is quicker than the other.
The first job calculates the data to write, tests to see it doesn't already
exist and writes the data. In RPG, how should we prevent another job from doing
the same and writing the same data before the first one?


-----Message d'origine-----
De : rpg400-l-bounces@xxxxxxxxxxxx [mailto:rpg400-l-bounces@xxxxxxxxxxxx] De la
part de David FOXWELL
Envoyé : mercredi 19 novembre 2008 09:33
À : RPG programming on the AS400 / iSeries
Objet : problem with duplicate records

Hi,

Two users have managed to create the same archive reference number while
creating two different clients.

Like this :

ArchiveFile
Product1 Client1 RefNo1 10:41:57(time)
Product2 Client2 RefNo1 10:41:57

The program would crash if the problem occurred with the same product. As it is
2 users in different services created the records at the same time.

The user enters the client details and hits the enter key.
The program writes the client to the client file then calls an external
procedure to find an unused archive number then write this number to the archive
file.

The client files look like this
Product1_clients
Client1 created at 10:41:53

Product2_clients
Client2 created at 10:41:56

I'm assuming that the creation of client1 took so long that the creation of
client2 caught up. The same time in the ArchiveFile is just a hazard.

I can think of ways to fix it, but I'd rather ask what are we doing wrong?

Thanks.
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