Re: Commit or Journal?

["Kahn, David" <KAHN@xxxxxxxxxxxxxxxxxx>]

David Gibbs (devdmg@silvon-server.uisinc.com) wrote:

>> 2 users access the same account number at the exact same time in the
>> same file and same interactive job and are presented with a full
>> screen where 20 comment lines can be typed in.
>
>If the file is opened for update, only 1 of those users will actually
>get the record locked.  As soon as the record is retrieved, update
>the record with a flag to indicate that it's being maintained.  The
>other user's program should check this flag and disallow maintenance
>if it's set.
>
>When the first user is done, re-read the record for update, clear
>the flag, and update the fields.

and Jorge Moreno (jmoreno@militarycars.com) wrote:

>I hope I can help you with this: If the account number does not exist in
>the file write and immediatly read for Update. If the account
>number exists read the record for update. The AS/400 Data Management will
>provide the record lock and you will be in charge in your
>program/application to update the record at your convenience and remove all
>phantom updates.

Both these solutions have drawbacks. Setting a flag in the record leaves
the possibility that the flag might never get reset if the job should
fail. The record lock is the simplest method but many people consider it
bad practice to hold a lock while waiting for a user response. The user
may have to take a phone call or decide go to lunch or even go home for
the day at that point. The same thing applies with the flag. An
alternative is not to hold a lock while the user is keying data, then to
re-read the record for update (locking it) and see if it's changed since
the previous read. If it hasn't changed, update it; if it has changed,
release the lock, show the new details to the user with a suitable
message.

How do you test whether the record has changed? Method 1 - store the
entire record and then compare the current value of each field that your
update would change with its stored value. Method 2 - have an update
time stamp on the record and simply compare that with it's previous
value.

Dave Kahn - TCO, Tengiz, Kazakstan
=========

e-mail:  kahn@tengizchevroil.com    (until September 30th)
         dkahn@cix.compulink.co.uk  (from  October 1st)

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