Home » Server Options » RAC & Failsafe » RAC/TAF does not support Transaction Failover - WHY
RAC/TAF does not support Transaction Failover - WHY [message #276727] |
Fri, 26 October 2007 02:14  |
vipassana
Messages: 2 Registered: October 2007
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Junior Member |
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I am trying to understand RAC and have some questions. Appreciate anyone who can help me understand
1) The TAF in a RAC setup does not support transaction failover (DMLs will be rolled back), but only Query failover is supported. Why is this? If it can have the query image and using the instance which is alive, why cant it retain the DML image also and provide failover? Can someone please explain?
When a SELECT failsover, the instance still dies. But the SELECT statement from the dying instance is somehow taken and re-executed on the alive instance and fetched. For the User, its as if nothing happened.
In case of say an UPDATE, why cant Oracle take the UPDATE statement and re-execute in the alive instance again, so that its much as in the case of SELECT statment for the User as explained above as if nothing happnened.
The point is why does RAC demand for the User to re-execute the Update statement? It can rollback the transaction and re-execute the update statement on its own since it has the intelligence of what the Update statement was much like the select.
Please let me know, this is really important for me to understand
2) When a Query failover happens, I believe the select statment is re-issued. In this case will the SCN be different from the original. If so, how is read consistency maintained.
Thanks
Vipassana
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Re: RAC/TAF does not support Transaction Failover - WHY [message #283804 is a reply to message #283775] |
Wed, 28 November 2007 03:20  |
trantuananh24hg
Messages: 744 Registered: January 2007 Location: Ha Noi, Viet Nam
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Senior Member |
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Thank you, Michel!
Because of the old employee who configured this RAC system, leaved this company, now I have to manage it. Recently, some customer complained that their connections failures.
Executed AWR, STATSPACK, I found that our application cause the DB hang.
Thank you for your instructor!
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