![]() ![]() A wait timeout is analogous to a driver giving up and turning back because of delays. It's possible (even, likely) that the lock would have become available and have been acquired if the transaction had waited longer, but the timeout exists to avoid applications waiting on the database indefinitely. If T1 locks row 1 and then tries to lock row 2, and at the same time T2 locks row 2 and then tries to lock row 1, a deadlock will occur. In the case of InnoDB locks) elapses while a transaction awaits a lock, perhaps because a slow transaction is holding the lock and has not finished executing or perhaps because a number of transactions are queuing for the lock. ![]() ![]() A deadlock is analogous to a policeman solving gridlock (the situation at a road junction when no vehicle is able to move forward) by ordering a random participant to reverse.Ī wait timeout occurs when the configured timeout period (e.g. Thus the database picks a transaction to abort/rollback application code should detect this eventuality and handle accordingly, usually by attempting the transaction again. You can find innodblockwaittimeout which is the time limit with a default value 50 (seconds), and innodbtablelocks is the switch which the default value is. tr2 try replace record 1 and detect dead lock. tr1 try delete record 3 and wait tr2 release lock. avoid using the LOCK TABLES statement, because it does not offerĪny extra protection, but instead reduces concurrency.A deadlock occurs whenever a circular dependency arises among the locks that transactions must acquire in order to proceed: for example, imagine that transaction 1 holds lock A but needs to acquire lock B to proceed and transaction 2 holds lock B but needs to acquire lock A to proceed-the transactions are immediately deadlocked (no timeout required) and neither can proceed until one releases its locks. delete sql will delete record 1 and 3, and replace sql will update 3 and 1. I believe the developer's attitude is summed up by the following excerpt from the documetation: But your application code must catch the lock timeout. It's simpler to have a 1 second timeout (minimum) on the lock tables query and keep retrying to get the lock(s) until you succeed and then proceeding with your queries before unlocking the tables. Lock Timeouts - Helping your application to avoid long waits and improve response time to user actions. With transactions, the locks are grabbed at the last possible moment and if they can't be fetched and time-out, you then need to check for this failure and roll back before trying everything all over again. The nice thing about using LOCK TABLES, is that you can state the tables that you're queries are dependent upon before proceeding. ![]() I think there may still be cases where table locks are more suitable to the application than using transactions and are perhaps a lot easier to comprehend, even if they are worse performing. Others may regard this as a case of the XY Problem, where we are trying to fix a symptom (deadlocks) by changing the timeout period of locking tables when really we should resolve the root cause by switching over to transactions instead. I think that removing the variable was unhelpful. Unfortunately, the release notes don't specify an alternative to use, and I'm guessing that the general attitude is to switch over to using InnoDB transactions as James has stated in his answer. I think you are after the table_lock_timout variable which was introduced in MySQL 5.0.10 but subsequently removed in 5.5. ![]()
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