Commit Processing SQL Net Oracle Tips by Burleson Consulting

The Data Warehouse Development Life Cycle

Managing Two-Phase Commits (2PCs) With SQL*Net

When a distributed update (or delete) has finished processing, SQL*Net will coordinate COMMIT processing, which means that the entire transaction will roll back if any portion of the transaction fails. The first phase of this process is a prepare phase to each node, followed by the COMMIT, and then terminated by a forget phase.

If a distributed update is in the process of issuing the 2PC and a network connection breaks, Oracle will place an entry in the Remote DBA_2PC_PENDING table. The recovery background process (RECO) will then roll back or commit the good node to match the state of the disconnected node to ensure consistency. You can activate RECO via the ALTER SYSTEM ENABLE DISTRIBUTED RECOVERY command.

The Remote DBA_2PC_PENDING table contains an ADVISE column that directs the database to either commit or roll back the pending item. You can use the ALTER SESSION ADVISE syntax to direct the 2PC mechanism. For example, to force the completion of an INSERT, you could enter the following:

ALTER SESSION ADVISE COMMIT;
INSERT INTO PAYROLL@LONDON . . . ;

When a 2PC transaction fails, you can query the Remote DBA_2PC_PENDING table to check the STATE column. You can enter SQL*Remote DBA and use the Recover In-Doubt Transaction dialog box to force either a roll back or a commit of the pending transaction. If you do this, the row will disappear from Remote DBA_P2C_PENDING after the transaction has been resolved. If you force the transaction the wrong way (for example, roll back when other nodes committed), RECO will detect the problem, set the MIXED column to yes, and the row will remain in the Remote DBA_2PC_PENDING table.

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This is an excerpt from "High Performance Data Warehousing", copyright 1997.

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