MySQL Internals Manual  /  Transaction Handling in the Server  /  Transaction Life Cycle

6.4 Transaction Life Cycle

When a new connection is established, thd->transaction members are initialized to an empty state. If a statement uses any tables, all affected engines are registered in the statement engine list. In non-autocommit mode, the same engines are registered in the normal transaction list. At the end of the statement, the server issues a commit or a rollback for all engines in the statement list. At this point the transaction flags of an engine, if any, are propagated from the statement list to the list of the normal transaction. When commit/rollback is finished, the statement list is cleared. It will be filled in again by the next statement, and emptied again at the next statement's end.

The normal transaction is committed in a similar way (by going over all engines in thd->transaction.all list) but at different times:

  • When the user issues an SQL COMMIT statement

  • Implicitly, when the server begins handling a DDL statement or SET AUTOCOMMIT={0|1} statement

The normal transaction can be rolled back as well:

  • When the user isues an SQL ROLLBACK statement

  • When one of the storage engines requests a rollback by setting thd->transaction_rollback_request

For example, the latter condition may occur when the transaction in the engine was chosen as a victim of the internal deadlock resolution algorithm and rolled back internally. In such situations there is little the server can do and the only option is to roll back transactions in all other participating engines, and send an error to the user.

From the use cases above, it follows that a normal transaction is never committed when there is an outstanding statement transaction. In most cases there is no conflict, because commits of a normal transaction are issued by a stand-alone administrative or DDL statement, and therefore no outstanding statement transaction of the previous statement can exist. Besides, all statements that operate via a normal transaction are prohibited in stored functions and triggers, therefore no conflicting situation can occur in a sub-statement either. The remaining rare cases, when the server explicitly must commit a statement transaction prior to committing a normal transaction, are error-handling cases (see for example SQLCOM_LOCK_TABLES).

When committing a statement or a normal transaction, the server either uses the two-phase commit protocol, or issues a commit in each engine independently. The server uses the two-phase commit protocol only if:

  • All participating engines support two-phase commit (by providing a handlerton::prepare PSEA API call), and

  • Transactions in at least two engines modify data (that is, are not read-only)

Note that the two-phase commit is used for statement transactions, even though statement transactions are not durable anyway. This ensures logical consistency of data in a multiple- engine transaction. For example, imagine that some day MySQL supports unique constraint checks deferred until the end of the statement. In such a case, a commit in one of the engines could yield ER_DUP_KEY, and MySQL should be able to gracefully abort the statement transactions of other participants.

After the normal transaction has been committed, the thd->transaction.all list is cleared.

When a connection is closed, the current normal transaction, if any, is rolled back.

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