Re: No exception with concurrent updates

Dave,

On Jul 14 2004, Dave Cramer wrote:

> Vladimir,
> 
> This simply won't work, you will need to either synchronize transaction,
> or use some other locking mechanism. As I said, postgres supports
> concurrent transactions, they will both succeed simultaneously. 
> 
I thought that this is taken care of when you set TRANSACTION_SERIALIZABLE on 
the Connection object.

> Your options are select the row for update, which will lock it from
> other transactions updating it.
> 
> synchronize transaction so that only one gets in there at a time.
> 
> use pessimistic locking which involves a timestamp and you have to check
> to make sure that the row has not changed before you update it, although
> without synchronization this won't work either.
> 
> Dave
> 
> 
This programme was a part of the comparison between PostgreSQL and InterBase 
(known now under name Firebird for open source developement) regarding their 
concurrent updates handling, which as I know doesn't use 2 Phase Commit either. 
The term for MVCC in IB is multi-generational database architecture. By the way 
there exist the NO WAIT parameter in IB, which can be specified when starting a 
transaction and does not require a transaction to wait until the end of 
concurrent transaction. Using this mode the first version of my programme 
(without threads) did not halt - I received the exception and the second 
transaction's update did not commit.

Thanks for the prompt responses,

Vladimir

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Vladimir Stankovic                    Tel: +44 20 7040 0273
Research Student                      Fax: +44 20 7040 8585
Centre for Software Reliability       Email: V.Stankovic@city.ac.uk
City University
Northampton Square, London EC1V 0HB
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