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Message Posted: Wed, 19 Jan 2011 @ 23:38:59 GMT


     
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Subj:   Re: What is 2-phase commit?
 
From:   Baker, Andy

Comitting updates to data sets (tables) that are managed by two different database system instances (either same dbms or different) within the same transaction.

That is, a unit of work (transaction) may apply updates to tables in two or more databases systems (not databases in the Teradtaa sense but different teradata systems or one Teradata system and one Oracle system etc) and all or none have to committed or be rolled back.

Typically the transaction mgt system is actually or logically separate from the databasemanagement system(s) - the transaction mgt system manages transactions (msg queues etc) and the database management systems manage databases. The two phase memans the transaction system (the co- ordinator) has to ask each particapant (the database systems) to prepare for commit and then to actually commit - the two phases - after phase 1 - each can still rollback if soemmthing bad happens - - after phase 2 - it is all committed. If something happens in the tiny interval between phase 1 and Phase 2 the transaction is called 'In Doubt' - rare (as the time to do phase 2 is miniscule) and not necessarily anything wrong and these can be followed up and resolved by an administrator.

It originated I think with IMS DC (now TM) or CICS as the co-ordinator and IMS/DB and DB2 as the database particpants.


Regards

Andy Baker



     
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