Re: Question on File locking

[Booth Martin <booth@xxxxxxxxxxxx>]

One solution would be to write the records, without a sequence number, 
to a data queue and write a small program that has the sole function of 
creating the correct sequence number and writing the record to the 
physical file.


On 8/11/2014 3:58 PM, Vinay Gavankar wrote:
> Hi,
>
> I have a logical file with an Id and Sequence number as a Unique key. The
> first record for an Id is written with Seq No. as 99999 and subsequent
> records are to be written with (current lowest sequence - 1).
>
> There is only one program that is writing to this file, but is running
> concurrently.
>
> What would be a fool-proof way of writing to the file without having to
> trap for error on write and looping.
>
> For example, when getting the lowest sequence, I can keep that record
> locked, and then write a record with a lower sequence and then unlock the
> file.
>
> But if another instance of the program hits the same Id at the same time
> when the record is locked, it will wait for those microseconds for the lock
> to be released, but once the lock is released, wouldn't it get that record,
> instead of the the one that was written with a lower sequence number? In
> which case, it would end up with duplicate sequence number when it tries to
> write a record.
>
> Is my understanding of what would happen wrong?
>
> Would it make any difference if I were to use Setll & read instead of a
> Chain to get to the lowest sequence number?
>
> TIA for any suggestions.
>
> Vinay