XA Transaction Optimization

TenDB Cluster has the ability to quickly process suspended transactions when dealing with distributed transactions, which relies on the support of TenDB internals. This article introduces the new features of TenDB in regards to XA transactions.

Suspended Transactions Detect

In order to recover suspended transactions (transactions which fail to COMMIT or ROLLBACK and is in PREPARED state), quick detection of them is of greate importance.

Normally, we define a transaction which continuously in the PREPARED state for a certain time, like 30 seconds, to be a transaction that requires active intervention. The specific time can be determined by the application.

TenDB provides a statement xa recover with time,which can detect PREPARED state transactions and their duration in the PREPARED state. Examples are as follows:

mysql> xa recover;
| formatID | gtrid_length | bqual_length | data |
|        1 |            3 |            0 | 123  |
1 row in set (0.00 sec)

mysql> xa recover with time;
| formatID | gtrid_length | bqual_length | data | prepare_time        |
|        1 |            3 |            0 | 123  | 2020-03-09 12:03:32 |
1 row in set (0.00 sec)

By using xa recover with time,we are able to quickly detect those PREPARED transactions and handle them if needed.

Commit Logs

Whether a transaction needs COMMIT or ROLLBACK depends on whether other sibling nodes of transactions are commited.

In order to quickly detect whether other transactions are committed, TenDB provides a one-phase commit instruction so that it can write logs at the same time as the transaction commit: xa commit $xid one phase with logs

This instruction make the transaction's xid to be writtent to the system table mysql.xa_commit_logs. Both committing transactions and writting xid into xa_commit_log is atomic. If the transaction can be commited, then xid must have been recorded into xa_commit_log. If xid is not recorded in xa_commit_log, the transaction is rolled back. Examples are as follows:

mysql> xa start '123';
Query OK, 0 rows affected (0.00 sec)

mysql> insert into t1 values(1,'a');
Query OK, 1 row affected (0.00 sec)

mysql> xa end '123';
Query OK, 0 rows affected (0.00 sec)

mysql> xa commit '123' one phase with logs;
Query OK, 0 rows affected (0.00 sec)

mysql> select * from mysql.xa_commit_log;
| id | xid | commit_time         |
|  1 | 123 | 2020-03-09 15:04:23 |
1 row in set (0.00 sec)

The transaction xid '123' has been recorded into the mysql.xa_commit_log. </br>

Since the submitted xid will be written tomysql.xa_commit_log, in order to avoid the excessive write amount of mysql.xa_commit_log, TenDB write it in a circular fashion. The mysql.xa_commit_log table is defined as follows:

mysql> show create table mysql.xa_commit_log\G
*************************** 1. row ***************************
       Table: xa_commit_log
Create Table: CREATE TABLE `xa_commit_log` (
  `id` bigint(20) NOT NULL,
  `xid` char(128) NOT NULL,
  PRIMARY KEY (`id`),
  KEY `kx` (`xid`)
1 row in set (0.00 sec)

The system variable max_xa_commit_logs controls the maximum number of records inxa_commit_log. The default value is 1000000. If this value is exceeded, the overwrite will start from id 1.

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