02 - Create the conflict

Ok, now we have set up the data for our test, lets do something that you wouldn't normally do. That is, we need to force a conflict between the source and target table by deliberately updating a record in the target table to a different value than in the source table.

1. On the target table on the target server, run the following SQL to update the value

sqlplus repoe/repoe@ttorcl_trg

The current value of the amount_received for prod_id 101 in the SALES table is NULL. Update this to 60 on the target.

UPDATE sales 
   SET amount_received = 60 
   WHERE sales_status = 'OVERDUE' 
   AND prod_id = 101;
COMMIT;

2. The AMOUNT_RECEIVED column in record PROD_ID = 101 in the target table now differs from the source.

Now lets update the same column in the same record on the source (with a different value) and see what happens when Replicate tries to apply that change on the target.

3. Update the AMOUNT_RECEIVED for PROD_ID = 101 on the source database. On source server run the following SQL to update the value

sqlplus repoe/repoe@ttorcl_src

UPDATE sales
   SET amount_received = 120 
   WHERE sales_status = 'OVERDUE' 
   AND prod_id = 101;
COMMIT;

4. This will create a conflict because the update statement will be replicated to the target database. The target value was already changed and so a conflict is created.

5. On the replication console the conflict will be shown.

APPLY IS running. Currently at plog 724 and SCN 14858536 and 1 apply conflicts so far and WAITING on manual resolve of apply conflict id 724010046998.

6. The type of conflict is shown against the sales table. The conflict is "Command affected 0 rows".

REPOE.SALES:                      80%  Mine:5/5             Unrecov:0/0         Applied:4/4         Conflicts:1/1       RETRY:Command affected 0 row(s).

7. Type list conflict in the command console to view the complete SQL statement causing the conflict

dbvrep> list conflict                                                                                                                                                           
Information for conflict 724010046998 (current conflict):
Table: REPOE.SALES at transaction 001e.008.00000060 at SCN 3144514
SQL text (with replaced bind values): update "REPOE"."SALES"
set AMOUNT_RECEIVED = 120
where (1=1)
and AMOUNT_RECEIVED IS NULL 
and PROD_ID = 101
Error: Command affected 0 row(s).
Handled as: RETRY
Conflict repeated 8 times.

8. As can be seen from the SQL statement in the above list conflict, the SQL statement contains the OLD value of "AMOUNT_RECEIVED IS NULL". This is the reason for the "Command affected 0 rows". The AMOUNT_RECEIVED was updated from NULL to 60 on the target database in step 1 and so the original value of "AMOUNT_RECEIVED IS NULL" is no longer valid. In order to detect such changes in the data, the replication has added the old value of the value to be update in the WHERE predicate. This is true for most logical replication technologies.

9. Decide which value should be kept on the target database. Do you want to keep the value of AMOUNT_RECEIVED = 60 or AMOUNT_RECEIVED = 120? In this case we are going to keep the value of AMOUNT_RECEIVED = 120 as this is the value on the source system.