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Title
What is a partial rollback and what causes it? -
Description
Shareplex replicates complete transactions that have DMLs followed by a COMMIT, or complete rollbacks that have equal forward DMLs followed by corresponding backward DMLs (that nullify the forward transactions) followed by a COMMIT, or something in between termed as partial rollbacks. At times there are issues encountered by Shareplex Post process when dealing with partial rollbacks so it is important to understand what a partial rollbacks is, in case the problems needed to be resolved by getting rid of a part of that partial rollback. This is for the purpose of knowing the implications of the workaround tried to get past the issues.
The purpose of this solution is to explain how the partial rollbacks may occur.
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Cause
General information.
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Resolution
In a database management system (DBMS), partial rollback is a mechanism for canceling only part of the operations executed in a transaction back to a savepoint. This is implemented by creating various milestones in the transactions termed as savepoint and then rolling back the actions up to designated savepoint(s). This way the transaction is not fully rolled back. The following PL/SQL block illustrates this:
INSERT INTO employee_sal VALUES (‘A’, 1000);
SAVEPOINT A_sal;
INSERT INTO employee_sal VALUES (‘B’, 2000);
SAVEPOINT B_sal;
ROLLBACK TO SAVEPOINT A_sal;
COMMIT;
The net effect of the above block is that the second INSERT is rolled back whereas the first INSERT is applied. This is termed as partial rollback. The partial rollback can also happen if there are any glitches in the transaction on source that cause a part of the transaction to be rolled back.
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Additional Information
Partial rollbacks are generally processed by Shareplex without any issues. At times the rowids of the forward transactions may not match the rowids of the backward transactions and under such circumstances the Post process may stop due to error with a string like “partial rollback rowid mismatch”. Please refer to KnowledgeBase and search with the appropriate string of the error message to know the exact workaround to be tried, as well as the consequences like resulting out of sync, if any.