MySQL trigger + SELECT FOR UPDATE not locking - mysql

Question

I’ve got a table of bookings with start and end times, and no two bookings can overlap.
I need to check that a new booking won’t overlap with any existing bookings. However we’ve got very high load so there’s a race condition: two overlapping bookings can be both successfully inserted because the first booking was inserted after the second booking checked for overlaps.
I’m trying to solve this by taking a lock on a related resource using a BEFORE INSERT database trigger.
DELIMITER //
CREATE TRIGGER booking_resource_double_booking_guard BEFORE INSERT ON booking_resource
FOR EACH ROW BEGIN
DECLARE overlapping_booking_resource_id INT DEFAULT NULL;
DECLARE msg VARCHAR(255);
-- Take an exclusive lock on the resource in question for the duration of the current
-- transaction. This will prevent double bookings.
DECLARE ignored INT DEFAULT NULL;
SELECT resource_id INTO ignored
FROM resource
WHERE resource_id = NEW.resource_id
FOR UPDATE;
-- Now we have the lock, check for optimistic locking conflicts:
SELECT booking_resource_id INTO overlapping_booking_resource_id
FROM booking_resource other
WHERE other.booking_from < NEW.booking_to
AND other.booking_to > NEW.booking_from
AND other.resource_id = NEW.resource_id
LIMIT 1;
IF overlapping_booking_resource_id IS NOT NULL THEN
SET msg = CONCAT('The inserted times overlap with booking_resource_id: ', overlapping_booking_resource_id);
SIGNAL SQLSTATE '45000' SET MESSAGE_TEXT = msg;
END IF;
END
//
If I put this trigger in the database and insert two bookings asynchronously from the command line, this trigger successfully blocks the overlapping booking. I’ve tried this with putting a SLEEP before the last IF statement in the trigger, to make sure that the lock has really been taken out.
However, I have a load testing environment in Jenkins which runs a lot of bookings concurrently using jMeter. When I put this trigger there and run the load tests, no overlapping bookings are caught, i.e. double bookings are made.
Some checks I’ve done:
I’ve logged out the SQL queries that the load test script generates when creating a booking, and it is the same as the SQL I use in the command line.
The trigger is definitely being triggered in the load test environment, and it is definitely not catching any overlapping bookings. I ascertained this by inserting the “overlapping_booking_resource_id” variable from the trigger into another table. All the values were null.
The trigger works in the load test environment when inserting bookings from the command line, i.e. it prevents the overlapping booking from being inserted.
If I make the constraint for what a “double booking” is slightly too strict, i.e. adjacent bookings count as double bookings, then I do see things being caught by the trigger – that is, the apache log records several errors with the message ‘The inserted times overlap with booking_resource_id:’
I’m wondering if maybe the lock is only taken out until the end of the trigger, and there is still a race condition between the end of the trigger and actually inserting into the table. However this doesn’t explain why none of the overlapping bookings are ever caught.
I’m really stuck now. Does anyone have any ideas as to what I have done wrong?

Answer 1

A less elegant but more robust method would be to use a table made for locking records accross the system.
DELIMITER //
CREATE TRIGGER booking_resource_double_booking_guard BEFORE INSERT ON booking_resource
FOR EACH ROW BEGIN
DECLARE overlapping_booking_resource_id INT DEFAULT NULL;
DECLARE msg VARCHAR(255);
-- Take an exclusive lock on the resource in question for the duration of the current
-- transaction. This will prevent double bookings.
---CHANGED HERE
REPEAT
BEGIN
DECLARE CONTINUE HANDLER FOR SQLWARNING
BEGIN
SET locked = FALSE;
END;
locked=TRUE;
INSERT INTO lockresource values(NEW.resource_id);
END;
UNTIL LOCKED END REPEAT;
---TIL HERE
-- Now we have the lock, check for optimistic locking conflicts:
SELECT booking_resource_id INTO overlapping_booking_resource_id
FROM booking_resource other
WHERE other.booking_from < NEW.booking_to
AND other.booking_to > NEW.booking_from
AND other.resource_id = NEW.resource_id
LIMIT 1;
IF overlapping_booking_resource_id IS NOT NULL THEN
SET msg = CONCAT('The inserted times overlap with booking_resource_id: ', overlapping_booking_resource_id);
SIGNAL SQLSTATE '45000' SET MESSAGE_TEXT = msg;
END IF;
END
//
---ADDED FROM HERE
DELIMITER //
CREATE TRIGGER booking_resource_double_booking_guard_after AFTER INSERT ON booking_resource
FOR EACH ROW BEGIN
DECLARE CONTINUE HANDLER FOR SQLWARNING BEGIN END;
delete from lockresource where lockid=NEW.resource_id;
END
//
Anyway, that'd be the idea and would certainly prevent any loss of lock until completion of your validation.