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The consequence is that the SQL issued at the apply APPLY database is not the same issued against mine MINE – instead, each SQL updates/deletes/inserts exactly one row, applying just one change.
Definition of a conflict
A conflict occurs when there is a potential for the data between the source and target databases to be out of sync. This is also referred to as data divergence.
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- SQL affects zero rows. This causes data divergence and so a conflict is raised. This could also mean that the target data has changed independently of the source data.
- SQL affects more than one row. This causes data divergence and so a conflict is raised.
- Oracle error. This can range from usual primary key or foreign key violation (another type of data divergence) to purely technical reasons (cannot extend datafile).
- Lock timeout. If the apply APPLY waits for a row lock more than WATCHDOG_TIMEOUT seconds, a conflict is also reported.
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How is a conflict detected?
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| PROD_ID | AMOUNT_RECEIVED |
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| 101 | 250 |
When the following SQL is issued on the source database:
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| width | 95 |
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| bgColor | CCC |
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update SALES set AMOUNT_RECEIVED = 120 where PROD_ID = 101;
The replicated SQL that is applied on the target database is as follows :
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| width | 95 |
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(note the WHERE clause contains the old value of AMOUNT_RECEIVED):
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update SALESset AMOUNT_RECEIVED = 120 where (1=1) and AMOUNT_RECEIVED =250 250 and PROD_ID = 101; |
If in the meantime, another transaction on the target database had changed AMOUTAMOUNT_RECEIVED from 250 to another value (say 199), then the above update would affect zero rows and the conflict would occur. This is because the WHERE clause (AMOUNT_RECEIVED = 250) would not be satisfied.
This ensures that the user is notified and no previous data will be lost.