MySQL deadlock
MySQL deadlock
http://dev.mysql.com/doc/refman/5.6/en/glossary.html#glos_deadlock
Deadlock
A situation where different transactions are unable to proceed, because each holds a lock that the other needs. Because both transactions are waiting for a resource to become available, neither will ever release the locks it holds.
A deadlock can occur when the transactions lock rows in multiple tables (through statements such as UPDATE or SELECT ... FOR UPDATE), but in the opposite order. A deadlock can also occur when such statements lock ranges of index records and gaps, with each transaction acquiring some locks but not others due to a timing issue.
to reduce the possibility of deadlocks, use transactions rather than LOCK TABLE statements;
keep transactions that insert or update data small enough that they do not stay open for long periods of time;
when different transactions update multiple tables or large ranges of rows, use the same order of operations (such as SELECT ... FOR UPDATE) in each transaction;
create indexes on the columns used in SELECT ... FOR UPDATE and UPDATE ... WHERE statements.
The possibility of deadlocks is not affected by the isolation level, because the isolation level changes the behavior of read operations, while deadlocks occur because of write operations.
If a deadlock does occur, InnoDB detects the condition and rolls back one of the transactions (the victim). Thus, even if your application logic is perfectly correct, you must still handle the case where a transaction must be retried. To see the last deadlock in an InnoDB user transaction, use the command SHOW ENGINE INNODB STATUS. If frequent deadlocks highlight a problem with transaction structure or application error handling, run with the innodb_print_all_deadlocks setting enabled to print information about all deadlocks to the mysqld error log.
For background information on how deadlocks are automatically detected and handled, see Section 14.2.2.8, “Deadlock Detection and Rollback”. For tips on avoiding and recovering from deadlock conditions, see Section 14.2.2.9, “How to Cope with Deadlocks”.
Deadlock Detection and Rollback
InnoDB automatically detects transaction deadlocks and rolls back a transaction or transactions to break the deadlock.
InnoDB tries to pick small transactions to roll back, where the size of a transaction is determined by the number of rows inserted, updated, or deleted.
InnoDB is aware of table locks if innodb_table_locks = 1 (the default) and autocommit = 0, and the MySQL layer above it knows about row-level locks. Otherwise, InnoDB cannot detect deadlocks where a table lock set by a MySQL LOCK TABLES statement or a lock set by a storage engine other than InnoDB is involved. Resolve these situations by setting the value of the innodb_lock_wait_timeout system variable.
When InnoDB performs a complete rollback of a transaction, all locks set by the transaction are released. However, if just a single SQL statement is rolled back as a result of an error, some of the locks set by the statement may be preserved（保护保持）. This happens because InnoDB stores row locks in a format such that it cannot know afterward which lock was set by which statement.
If a SELECT calls a stored function in a transaction, and a statement within the function fails, that statement rolls back. Furthermore, if ROLLBACK is executed after that, the entire transaction rolls back.
How to Cope with Deadlocks
Deadlocks are a classic problem in transactional databases, but they are not dangerous unless they are so frequent that you cannot run certain transactions at all. Normally, you must write your applications so that they are always prepared to re-issue a transaction if it gets rolled back because of a deadlock.
InnoDB uses automatic row-level locking. You can get deadlocks even in the case of transactions that just insert or delete a single row. That is because these operations are not really “atomic”; they automatically set locks on the (possibly several) index records of the row inserted or deleted.
You can cope with deadlocks and reduce the likelihood of their occurrence with the following techniques:
At any time, issue the SHOW ENGINE INNODB STATUS command to determine the cause of the most recent deadlock. That can help you to tune your application to avoid deadlocks.
If frequent deadlock warnings cause concern, collect more extensive debugging information by enabling the innodb_print_all_deadlocks configuration option. Information about each deadlock, not just the latest one, is recorded in the MySQL error log. Disable this option when you are finished debugging.
Always be prepared to re-issue a transaction if it fails due to deadlock. Deadlocks are not dangerous. Just try again.
Keep transactions small and short in duration to make them less prone（有…倾向的） to collision（碰撞; 冲突; ）.
Commit transactions immediately after making a set of related changes to make them less prone to collision. In particular, do not leave an interactive mysql session open for a long time with an uncommitted transaction.
If you use locking reads (SELECT ... FOR UPDATE or SELECT ... LOCK IN SHARE MODE), try using a lower isolation level such as READ COMMITTED.
When modifying multiple tables within a transaction, or different sets of rows in the same table, do those operations in a consistent order each time. Then transactions form well-defined queues and do not deadlock. For example, organize database operations into functions within your application, or call stored routines, rather than coding multiple similar sequences of INSERT, UPDATE, and DELETE statements in different places.
Add well-chosen indexes to your tables. Then your queries need to scan fewer index records and consequently set fewer locks. Use EXPLAIN SELECT to determine which indexes the MySQL server regards as the most appropriate for your queries.
Use less locking. If you can afford to permit a SELECT to return data from an old snapshot, do not add the clause FOR UPDATE or LOCK IN SHARE MODE to it. Using the READ COMMITTED isolation level is good here, because each consistent read within the same transaction reads from its own fresh snapshot.
If nothing else helps, serialize your transactions with table-level locks. The correct way to use LOCK TABLES with transactional tables, such as InnoDB tables, is to begin a transaction with SET autocommit = 0 (not START TRANSACTION) followed by LOCK TABLES, and to not call UNLOCK TABLES until you commit the transaction explicitly. For example, if you need to write to table t1 and read from table t2, you can do this:
SET autocommit=0; LOCK TABLES t1 WRITE, t2 READ, ...; ... do something with tables t1 and t2 here ... COMMIT; UNLOCK TABLES;Table-level locks prevent concurrent updates to the table, avoiding deadlocks at the expense of less responsiveness for a busy system.
Another way to serialize transactions is to create an auxiliary（辅助的） “semaphore” table that contains just a single row. Have each transaction update that row before accessing other tables. In that way, all transactions happen in a serial fashion. Note that the InnoDB instant deadlock detection algorithm also works in this case, because the serializing lock is a row-level lock. With MySQL table-level locks, the timeout method must be used to resolve deadlocks.
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