Storage Engine (Table Types) - Deadlocks

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Whenever multiple transactions obtain locks, there is the danger of encountering a deadlock condition. Deadlocks occur when two transactions attempt to obtain conflicting locks in a different order.

For example, consider these two transactions running against the StockPrice table:

Transaction #1:

BEGIN;
UPDATE StockPrice SET close = 45.50 WHERE stock_id = 4 and date = '2002-05-01';
UPDATE StockPrice SET close = 19.80 WHERE stock_id = 3 and date = '2002-05-02';
COMMIT;

Transaction #2:

BEGIN;
UPDATE StockPrice SET high = 20.12 WHERE stock_id = 3 and date = '2002-05-02';
UPDATE StockPrice SET high = 47.20 WHERE stock_id = 4 and date = '2002-05-01';
COMMIT;

If you’re unlucky, each transaction will execute its first query and update a row of data, locking it in the process. Each transaction will then attempt to update its second row only to find that it is already locked. Left unchecked, the two transactions will wait for each other to complete—forever.

To combat this problem, database systems implement various forms of deadlock detection and timeouts. The more sophisticated systems, such as InnoDB, will notice circular dependencies like the previous example and return an error. Others will give up after the query exceeds a timeout while waiting for a lock. InnoDB’s default timeout is 50 seconds. In either case, applications that use transactions need to be able to handle deadlocks and possibly retry transactions.

Transaction Logging

Some of the overhead involved with transactions can be mitigated through the use of a transaction log. Rather than directly updating the tables on disk each time a change occurs, the system can update the in-memory copy of the data (which is very fast) and write a record of the change to a transaction log on disk. Then, at some later time, a process (or thread) can actually apply the changes that the transaction log recorded. The serial disk I/O required to append events to the log is much faster than the random seeks required to update data in various places on disk.

As long as events are written to the transaction log before a transaction is considered committed, having the changes in a log will not affect the durability of the system. If the database server crashes before all changes have been applied from the transaction log, the database will continue applying changes from the transaction log when it is restarted and before it accepts new connections.

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