I have a innoDB table which records online users. It gets updated on every page refresh by a user to keep track of which pages they are on and their last access date to the site. I then have a cron that runs every 15 minutes to DELETE old records.
I got a 'Deadlock found when trying to get lock; try restarting transaction' for about 5 minutes last night and it appears to be when running INSERTs into this table. Can someone suggest how to avoid this error?
=== EDIT ===
Here are the queries that are running:
First Visit to site:
INSERT INTO onlineusers SET
ip = 123.456.789.123,
datetime = now(),
userid = 321,
page = '/thispage',
area = 'thisarea',
type = 3
On each page refresh:
UPDATE onlineusers SET
ips = 123.456.789.123,
datetime = now(),
userid = 321,
page = '/thispage',
area = 'thisarea',
type = 3
WHERE id = 888
Cron every 15 minutes:
DELETE FROM onlineusers WHERE datetime <= now() - INTERVAL 900 SECOND
It then does some counts to log some stats (ie: members online, visitors online).
One easy trick that can help with most deadlocks is sorting the operations in a specific order.
You get a deadlock when two transactions are trying to lock two locks at opposite orders, ie:
If both run at the same time, connection 1 will lock key(1), connection 2 will lock key(2) and each connection will wait for the other to release the key -> deadlock.
Now, if you changed your queries such that the connections would lock the keys at the same order, ie:
it will be impossible to get a deadlock.
So this is what I suggest:
Make sure you have no other queries that lock access more than one key at a time except for the delete statement. if you do (and I suspect you do), order their WHERE in (k1,k2,..kn) in ascending order.
Fix your delete statement to work in ascending order:
Change
DELETE FROM onlineusers
WHERE datetime <= now() - INTERVAL 900 SECOND
To
DELETE FROM onlineusers
WHERE id IN (
SELECT id FROM onlineusers
WHERE datetime <= now() - INTERVAL 900 SECOND
ORDER BY id
) u;
Another thing to keep in mind is that mysql documentation suggest that in case of a deadlock the client should retry automatically. you can add this logic to your client code. (Say, 3 retries on this particular error before giving up).