We use a generator to flag a process as locked or not. I will try and
explain,

- The value of the generator starts of as zero.
- The first user to execute the SQL SELECT GEN_ID(EXPORTLOCK_SEQ, 1) FROM
RDB$DATABASE and gets a value of one is deemed to have the lock.
- While user 1 has the lock other users run the same SQL and if they get a
value bigger than one they do not have the lock and then run the following
SQL to undo their effort to get the lock i.e. SELECT GEN_ID(EXPORTLOCK_SEQ,
-1) FROM RDB$DATABASE.
- As soon as user 1 runs his SELECT GEN_ID(EXPORTLOCK_SEQ, -1) FROM
RDB$DATABASE and all the other users have undone their locks the value of
the generator should be back at zero.
- The next user to run SELECT GEN_ID(EXPORTLOCK_SEQ, 1) FROM RDB$DATABASE
will get a value of 1 and he is deemed to have the lock again.

And so the cycle continues. The client has about 150 users trying to get the
lock at a point in time. Somewhere along the line at the client's site (we
have not been able to figure out why) this gets broken and the lock stays
on. In our test environment we have not been able to break the logic, the
code is pretty well protected with try finally's and works.

The client does run our software using Citrix terminals. Not sure if this is
a possible cause although we have already had them rebuild the Citrix
environment and yet this still happens.

Are there any flaws in the logic above?


Peter Ypenburg
AGM: Product Development
Barloworld Optimus
Tel +2711 286 1587 Fax +2711 286 1630 Cell +2782 789 9167
www.barloworldoptimus.com

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-----Original Message-----
From: firebird-support@yahoogroups.com
[mailto:firebird-support@yahoogroups.com] On Behalf Of Mitchell Peek
Sent: Thursday, May 12, 2005 9:40 PM
To: firebird-support@yahoogroups.com
Subject: Re: [firebird-support] Getting a unique value from a generator

helen or someone will reply later and tell me how wrong I am ;), but....

First, it isn't really possible for 2 users to execute the statement at
the *exact* same time, one has to come before the other... but anyway....

Generators operate outside of user transactions. Test it. A rollback
leaves the generator as it was whether you rollback or commit.
Therefore, it isn't possible for the same value to be returned if you
assume they are the only two statements going on. However, if there
were a third user such that the execution occurred in this manner....

user 1

SELECT GEN_ID(EXPORTLOCK_SEQ, -1)
FROM RDB$DATABASE;

user 2

SELECT GEN_ID(EXPORTLOCK_SEQ, 1)
FROM RDB$DATABASE

user 3

SELECT GEN_ID(EXPORTLOCK_SEQ, -1)
FROM RDB$DATABASE


then users 1 and 3 will get the same value.

Is this a possibility?






Peter Ypenburg wrote:

>Hi There,
>
>Is it possible for two users who run the following SQL at the exact same
>time (each in their own transaction) to get the same value back from a
>generator. The SQL is as follows:
>
>SELECT GEN_ID(EXPORTLOCK_SEQ, -1)
>FROM RDB$DATABASE
>
>Thanks.
>
>Peter Ypenburg
>AGM: Product Development
>Barloworld Optimus
>Tel +2711 286 1587 Fax +2711 286 1630 Cell +2782 789 9167
>www.barloworldoptimus.com
>
>Disclaimer
>
>The information transmitted is the property of Barloworld Optimus Pty (Ltd)
>and is intended only for the person or entity to which it is addressed and
>may contain confidential and/or privileged material. Statements and

opinions

>expressed in this e-mail may not represent those of the company. Any

review,

>retransmission, dissemination and other use of, or taking of any action in
>reliance upon, this information by persons or entities other than the
>intended recipient is prohibited. If you received this in error, please
>contact the sender immediately and delete the material from any computer.
>
>
>
>++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
>
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>on the main (top) menu. Try Knowledgebase and FAQ links !
>
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>
>++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
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++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

Visit http://firebird.sourceforge.net and click the Resources item
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Also search the knowledgebases at http://www.ibphoenix.com

++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

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