Re: GUID's and uniqueness

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

Louis Frolio wrote:
> Daniel Morgan <damorgan_at_x.washington.edu> 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
damorgan_at_x.washington.edu
(replace 'x' with 'u' to respond)

Received on Wed Sep 15 2004 - 09:36:51 CDT