During a commit operation, all changes made during a transaction are made permanent and a rollback operation is not possible after that. Depending on the transaction isolation used, this may be the first time such changes are visible to other threads.

To support COMMIT, create a function that matches this definition:

        int  (*commit)(THD *thd, bool all);

The function name is then listed in the rollback (twelfth) entry of the handlerton.

The THD parameter is used to identify the transaction that needs to be committed, while the bool all parameter indicates if this is a full transaction commit or just the end of a statement that is part of the transaction.

Details of implementing a COMMIT operation will vary by storage engine. Examples can be found in ha_innodb.cc and ha_berkeley.cc.

If the server is in auto-commit mode, the storage engine should automatically commit all read-only statements such as SELECT.

In a storage engine, "auto-committing" works by counting locks. Increment the count for every call to external_lock(), decrement when external_lock() is called with an argument of F_UNLCK. When the count drops to zero, trigger a commit.