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| unique MsgId |
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| mgrabinski |
 Posted: Thu Oct 24, 2002 9:45 pm Post subject: unique MsgId |
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Master
Joined: 16 Oct 2001
Posts: 246
Location: Katowice, Poland
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Hi all,
I know that MsgId is unique within a queue manager. I know also that part of it contains part of the manager name. But still, it is possible to have two messages with same MsgId in a complex network of managers. Am I right?
_________________
Marcin Grabinski <>< |
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| bower5932 |
| Posted: Fri Oct 25, 2002 6:57 am Post subject: |
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Jedi Knight
Joined: 27 Aug 2001
Posts: 3023
Location: Dallas, TX, USA
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There is a footnote at the bottom of the MQMD MsgId page in the Application Programming Reference:
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3. A MsgId generated by the queue manager consists of a 4-byte product identifier 'AMQ ' or 'CSQ ' in either ASCII or EBCDIC, where ' ' represents a blank), followed by a product-specific implementation of a unique string. In MQSeries this contains the first 12 characters of the queue-manager name, and a value derived from the system clock. All queue managers that can intercommunicate must therefore have names that differ in the first 12 characters, in order to ensure that message identifiers are
unique. The ability to generate a unique string also depends upon the system clock not being changed backward. To eliminate the possibility of a message identifier generated by the queue manager duplicating one generated by the application, the application should avoid generating identifiers with initial characters in the range A through I in ASCII or EBCDIC (X'41' through X'49' and X'C1' through X'C9'). However, the application is not prevented from generating identifiers with initial characters in these ranges.
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So, I guess the answer is: Yes, in theory, you could get a duplicate message id. However, it is unlikely. |
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| Tibor |
| Posted: Sat Oct 26, 2002 1:22 pm Post subject: |
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Grand Master
Joined: 20 May 2001
Posts: 1033
Location: Hungary
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| bower5932 wrote: |
| So, I guess the answer is: Yes, in theory, you could get a duplicate message id. However, it is unlikely. |
I had seen same message id in a local queue, on two sequential messages, generated by qmgr. I was very surprised and looked it twice, because I learned of the unique ids. I don't know since then there was either an MQ bug or a timestamp slip...
Other case: when an application can set message id.
Tibor |
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