Transaction rollbacks

The following table describes the errors that indicate the rollback of a transaction.

A transaction rollback is not necessarily an error. Several situations can lead to transaction rollback, such as transaction timeout and participant leader switchover.

The following situations can trigger a leader switchover:

• O&M scenarios: switch replica leader, change primary zone, remove server, stop server
• Internal scheduling scenarios: migrate unit, bring replica offline, add replica in primary zone
• Internal exception scenarios: abnormal leader switchover

When RootService triggers a leader switchover, the system reserves a certain period for existing active transactions to complete and does not allow new transactions to start. After the period expires, all still-active transactions are migrated to the new leader. Then the leader switchover occurs, and the new leader continues to provide services.

In the following scenarios, transactions are killed during a leader switchover:

• The leader switchover is not triggered by RootService, for example, abnormal leader switchover
• Large transaction scenarios
• During migration of transactions to the new leader, the statement has not finished executing

When a transaction rollback error occurs, you can query the DBA_OB_SERVER_EVENT_HISTORY view to confirm whether a leader switchover has occurred. The following example shows a leader switchover process triggered by the switch replica leader command:

1. Query the log stream leader distribution before the leader switchover.

   obclient> 
   select TENANT_ID,LS_ID,SVR_IP,SVR_PORT,ROLE,PAXOS_MEMBER_LIST from GV$OB_LOG_STAT where tenant_id=1 and ls_id=1;
   +-----------+-------+-------------+----------+----------+---------------------------------------------------------------------------+
   | TENANT_ID | LS_ID | SVR_IP      | SVR_PORT | ROLE     | PAXOS_MEMBER_LIST                                                         |
   +-----------+-------+-------------+----------+----------+---------------------------------------------------------------------------+
   |         1 |     1 | xx.xx.xx.158|     2882 | FOLLOWER | xx.xx.xx.43:2882:1686125578254268,xx.xx.xx.106:2882:1,xx.xx.xx.158:2882:1 |
   |         1 |     1 | xx.xx.xx.43 |     2882 | FOLLOWER | xx.xx.xx.43:2882:1686125578254268,xx.xx.xx.106:2882:1,xx.xx.xx.158:2882:1 |
   |         1 |     1 | xx.xx.xx.106|     2882 | LEADER   | xx.xx.xx.43:2882:1686125578254268,xx.xx.xx.106:2882:1,xx.xx.xx.158:2882:1 |
   +-----------+-------+-------------+----------+----------+---------------------------------------------------------------------------+
   3 rows in set (0.01 sec)
   

2. Execute the leader switchover.

   obclient> 
   alter system switch replica leader ls=1 server='xx.xx.xx.158:2882' tenant='sys';
   Query OK, 0 rows affected (0.01 sec)
   

3. Query the log stream leader election events.

   obclient> 
   SELECT VALUE4 HAPPENED_TIME,svr_ip,VALUE1 TENANT_ID,VALUE2 LS_ID,event,name3,value3  FROM DBA_OB_SERVER_EVENT_HISTORY  WHERE module="ELECTION";
   +----------------------------+-------------+-----------+-------+---------------------------+-------+------------------------------------------------------------------------------------------------------------------+
   | HAPPENED_TIME              | svr_ip      | TENANT_ID | LS_ID | event                     | name3 | value3                                                                                                           |
   +----------------------------+-------------+-----------+-------+---------------------------+-------+------------------------------------------------------------------------------------------------------------------+
   | 2023-06-12 23:27:23.291335 | xx.xx.xx.106| 1         | 1     | prepare change leader     | INFO  | leader prepare to change leader : "xx.xx.xx.106:2882" -> "xx.xx.xx.158:2882", reason : MANUAL LEADER             |
   | 2023-06-12 23:27:23.294363 | xx.xx.xx.158| 1         | 1     | change leader to takeover | INFO  | new leader takeover : "xx.xx.xx.106:2882" -> "xx.xx.xx.158:2882"                                                 |
   | 2023-06-12 23:27:23.318691 | xx.xx.xx.43 | 1         | 1     | witness change leader     | INFO  | witness change leader : "xx.xx.xx.106:2882" -> "xx.xx.xx.158:2882"                                               |
   | 2023-06-12 23:27:23.292954 | xx.xx.xx.106| 1         | 1     | change leader to revoke   | INFO  | old leader revoke : "xx.xx.xx.106:2882" -> "xx.xx.xx.158:2882"                                                   |
   | 2023-06-12 23:27:23.541978 | xx.xx.xx.158| 1         | 1     | witness change leader     | INFO  | witness change leader : "xx.xx.xx.106:2882" -> "xx.xx.xx.158:2882"                                               |
   | 2023-06-12 23:27:23.542136 | xx.xx.xx.106| 1         | 1     | witness change leader     | INFO  | witness change leader : "xx.xx.xx.106:2882" -> "xx.xx.xx.158:2882"                                               |
   +----------------------------+-------------+-----------+-------+---------------------------+-------+------------------------------------------------------------------------------------------------------------------+
   6 rows in set (0.02 sec)