In SERIALIZABLE it is totally possible to deadlock yourself with a simple invisible SELECT and a lone INSERT or UPDATE.A deadlock with an S-lock escalation means you need to check the isolation level.You won’t need SERIALIZABLE unless your code is broken.This is an easy fix for them and the deadlocks are gone, the world is safe again. They do not need the double read and also not the isolation level SERIALIZABLE. Session1 > SELECT * FROM sometable WHERE id = 10 FOR UPDATEl - lock escalation to X Session1 > SET TRANSACTION ISOLATION LEVEL SERIALIZABLE Session1 > START TRANSACTION READ WRITE Session1 > SELECT * FROM sometable WHERE id = 10 - implied S-lock granted on rec or gap If at that point in time another threads tries to run the exact same statement, which is what happens here, they already hold a second S-lock, preventing the first thread from completing their transaction (it is waiting until the second threads drops the S-lock, or it times out).Īnd then that second thread also tries to upgrade their S-lock into an X-lock, which it can’t do, because that first thread is trying to do the same thing, and we have the deadlock and a rollback. The database needs to acquire the X-lock, that is, it needs to upgrade the S-lock to an X-lock. Our transaction now did not get the required locks necessary for reading at the start of the transaction, because the later INSERT requires an X-lock, like any write statement would. We get an S-Lock, which is not good for writing. The LOCK IN SHARE MODE or equivalent FOR SHARE is not in the code, it is added implicitly by the isolation level SERIALIZABLE. Session1 > START TRANSACTION READ WRITE Session1 > SELECT * FROM sometable WHERE id = 10 FOR SHARE - S-lock granted on rec or gap So instead of a regular Read-Modify-Write The SERIALIZABLE isolation mode turns a normal SELECT statement into a Medusa’s freeze ray that shoots S-Locks all over the tables onto everything it looks at, preventing other threads from changing these things until we end our transaction and drop our locks (And that is why you should not use it, and why I personally believe that your code is broken if it needs it). It does answer the question of “Where to the S-Locks come from?”, though. It does not explain what it is good for (mostly: shooting yourself into the foot) and when you should use it (mostly: don’t). It then goes on to explain how SERIALIZABLE does nothing when there is no explicit transaction going on. This level is like REPEATABLE READ, but InnoDB implicitly converts all plain SELECT statements to SELECT. Previously, I made the decision to leave the fourth and most useless isolation level out of the picture: SET TRANSACTION ISOLATION LEVEL SERIALIZABLE. So when we spoke about transactions and isolation levels The isolation = Isolation.SERIALIZABLE is the culprit here. Or, if they understand what goes on with the database, to map this to the code. This is often a problem - Object Relational Mappers encapsulate the things that go on in the database so much that it is really hard for anybody - Developers, DBAs and everybody else - to understand what actually happens and make debugging quite painful. REQUIRES_NEW, timeout = MYSQL_TRANSACTION_TIMEOUT, rollbackFor = The last question can be actually answered by the developer, but because they are using Java, in true Java fashion it is almost - but not quite - useless to a database ( propagation = Propagation. what does the originating code look like?.how can a single INSERT transaction deadlock?.We get the exact same statement with the exact same value on the second thread, but with lock mode X locks gap.īoth transactions have an undo-log entry of the length 1 - one row, single insert and the insert has an S-lock. ` sometable ` trx id 9843342279 lock mode S locks gap before recĪnd that is weird because of the lock mode S locks gap in the last line. ![]() somedomain someuser update INSERT into sometable ( identifier_id, currency, balance ) VALUES ( 'd4e84cb1-4d56-4d67-9d16-1d548fd26b55', 'EUR', '0' ) *** ( 2 ) HOLDS THE LOCK ( S ): RECORD LOCKS space id 3523 page no 1106463 n bits 224 index PRIMARY of table ` somedb `. Mysql > pager less mysql > show engine innodb status \ G.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |