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Re: GUID's and uniqueness

From: Daniel Morgan <>
Date: Wed, 15 Sep 2004 07:36:51 -0700
Message-ID: <1095259080.39387@yasure>

Louis Frolio wrote:
> Daniel Morgan <> wrote in message news:<1095218401.235027_at_yasure>...

>>Louis Frolio wrote:
>>>Greetings All, I have read many upon many articles here regarding GUID
>>>data types and uniqueness.  There have been many opinions regarding
>>>the effectiveness of GUID's and when they should/should not be used. 
>>>However, every article strongly implies, if it does not state it
>>>outright, that GUID's are always unique.  My question is this, what
>>>happens if you have a database that uses GUID's and the NIC is changed
>>>out on the box?  From what I understand the MAC address of the NIC is
>>>used as part of the algorithm to generate a GUID.  If you change out
>>>the NIC after generating 1 billion GUID's do you run the chance of
>>>generating a duplicate GUID?
>>>I look forward to your insightfulness on this issue.
>>>Regards, Louis.
>>GUIDs are not unique except in very specific controlled situations.
>>They are highly likely to be unique ... but there are not guarantees.
>>And I have personally seen problems where they weren't.

> Daniele, would be so kind as to elaborate a bit with regards to the
> situation where duplicate GUID's appeared? I ask because I am
> designing an application that requires that it be replication
> friendly, and not just one way replication. I need the app to be able
> to do two way replciation to multiple nodes. So, any details you can
> provide regarding your experience with GUID's would be hugely helpful
> to me.
> Regards, Louis.

We had a problem with migrating data from one machine to another. The initial deployment was on a 2CPU Windows box. When this proved wholly inadequate production was moved to a 4CPU Sun box. Thereafter there were two or three incidents of primary key violations that were identified as being caused by duplicating a GUID.

It is quie a simple thing to move a production app from one hardware platform to a different one. Quite another thing if that migration requires replacing the primary keys and cascading to their foreign key relationships.

Daniel A. Morgan
University of Washington
(replace 'x' with 'u' to respond)
Received on Wed Sep 15 2004 - 09:36:51 CDT

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