Asynchronous transactions in MySQL InnoDB? - mysql

Question

I want to implement parallel processing of multiple DB transactions which lock only a few rows for short periods of time. For Example we have this query executed every time an user opens the page:
START TRANSACTION;
SELECT * FROM table_1 WHERE worktime < UNIX_TIMESTAMP() FOR UPDATE;
...WORK...
...UPDATE...
COMMIT;
In a multiuser environment, this kind of row locking would lead to Deadlocks every time the select statement would be executed. Currently I would solve the problem using a second table to store the locked IDs:
START TRANSACTION;
LOCK TABLE table_1 WRITE, table_locks WRITE;
SELECT id FROM table_1 WHERE worktime < UNIX_TIMESTAMP() AND id NOT IN table_locks;
...insert locked Ids into Table "table_locks"...
...this prevents other calls to read from this table...
UNLOCK TABLES;
COMMIT;
...Perform calculations and Updates...
DELETE FROM table_locks WHERE id = ...
The problem of this method is, that if something goes wrong after "locking" a row by storing its ID in the table_locks table, this Row would never be updated anymore. Of course I can set a timeout to release such locks automatically after some time, but this doesen't seem properly done to me. But is there something possible like:
SELECT * FROM table_1 WHERE worktime < UNIX_TIMESTAMP() AND NOT LOCKED BY OTHER TRANSACTION FOR UPDATE
?

Answer 1

You could mark rows to be done by your session:
UPDATE table_1
SET marked_by_connection_id = CONNECTION_ID(),
marked_time = NOW()
WHERE worktime < UNIX_TIMESTAMP() AND marked_by_connection_id IS NULL;
Then you can feel free to work on any row that has your connection id, knowing that another session will not try to claim them:
SELECT * FROM table_1 WHERE marked_by_connection_id = CONNECTION_ID();
. . .
No locking or non-autocommit transaction is needed.
At the end of your session, unmark any rows you had marked:
UPDATE table_1 SET marked_by_connection_id = NULL
WHERE marked_by_connection_id = CONNECTION_ID();
Or alternatively your app could unmark individual rows as it processes them.
But perhaps your session dies before it can unmark those rows. So some rows were marked, but never processed. Run a cron job that clears such abandoned marked rows, allowing them to get re-processed by another worker, although a bit late.
UPDATE table_1 SET marked_by_connection_id = NULL
WHERE marked_time < NOW() - INTERVAL 30 MINUTE;