Transactions Dr. Charles Severance www.php-intro.com

Transactions Transaction processing is designed to maintain a system’s integrity (typically a database or some modern filesystems) in a known, consistent state, by ensuring that interdependent operations on the system are either all completed successfully or all canceled successfully. Text http://en.wikipedia.org/wiki/Transaction_processing

Transactions and MySQL CREATE TABLE webauto_rps ( rps_guid VARCHAR(64) NOT NULL, ... ) ENGINE = InnoDB DEFAULT CHARSET=utf8;

Bank Balance: 200 ATM ATM

Bank Balance: 200 ATM ATM Balance: 200

Bank Balance: 200 ATM ATM Balance: 200 Balance: 200

Bank Balance: 200 ATM ATM Balance: 200 Balance: 200

Bank Balance: 100 ATM ATM Balance: 100 Balance: 200 $100

Bank Balance: 100 ATM ATM Balance: 100 Balance: 100 $100 $100

Bank Balance: 100 ATM ATM Balance: 100 Balance: 100 $100 $100 Awesome!

Transactions Allows a sequence of steps to work as a unit or not work at all Handles the fact that some things take time Locks data on read so that it cannot be read by another transaction until the current transaction completes

Transactions in MySQL Tables where transactions will be used require the use of the InnoDB engine. BEGIN - Starts a transaction SELECT .... FOR UPDATE – Reads and locks row (or rows) COMMIT - Completes a transaction ROLLBACK - Gives up on a transaction

CREATE TABLE accounts ( number INT, balance FLOAT, PRIMARY KEY(number) ) ENGINE InnoDB; INSERT INTO accounts(number, balance) VALUES(12345, 1025); INSERT INTO accounts(number, balance) VALUES(67890, 140); mysql> select * from accounts; +--------+---------+ | number | balance | | 12345 | 1025 | | 67890 | 140 | 2 rows in set (0.02 sec) mysql>

BEGIN; SELECT balance FROM accounts WHERE number=12345 FOR UPDATE; UPDATE accounts SET balance=balance+25 WHERE number=12345; COMMIT; mysql> select * from accounts; +--------+---------+ | number | balance | | 12345 | 1050 | | 67890 | 140 | 2 rows in set (0.00 sec) mysql>

mysql> BEGIN; mysql> UPDATE accounts SET balance=balance-250 WHERE number=12345; mysql> UPDATE accounts SET balance=balance+250 WHERE number=67890; mysql> SELECT * FROM accounts; +--------+---------+ | number | balance | | 12345 | 800 | | 67890 | 390 | mysql> ROLLBACK; Query OK, 0 rows affected (0.34 sec) | 12345 | 1050 | | 67890 | 140 |

Transaction started but not committed. SELECT waits

Transaction started but not committed. SELECT timeout

Transaction started but not committed. UPDATE waits

Transaction committed. UPDATE completes instantly

Balances properly reflect both reductions

Transaction Deadlock If process A Locks X, and process B locks Y, and then process A waits for Y and process B waits for Y - they wait forever. This is called a “deadlock”. Deadlocks are avoided by making sure all transactions take locks in the same order - this way one will “win” at the beginning.

Start two transactions and lock two accounts

Attempt to lock an account but need to wait....

Attempt to lock an already locked account - deadlock

First transaction aborted so second select immediately gets lock

tsugi/samples/rps/play.php $stmt = $pdo->prepare("SELECT rps_guid, play1, play2, displayname FROM {$p}rps LEFT JOIN {$p}user ON {$p}rps.user1_id = {$p}user.user_id WHERE play2 IS NULL ORDER BY started_at ASC LIMIT 1 FOR UPDATE"); $stmt1 = $pdo->prepare("UPDATE {$p}rps SET user2_id = :U2ID, play2 = :PLAY WHERE rps_guid = :GUID"); $pdo->beginTransaction(); $stmt->execute(); $row = $stmt->fetch(PDO::FETCH_ASSOC); if ( $row == FALSE ) { $pdo->rollBack(); } else { $stmt1->execute(array(":U2ID" => $_SESSION['id'], ":PLAY" => $play, ":GUID" => $row['rps_guid'])); $pdo->commit(); }

Transaction Trade-offs Transactions are essential in situations where data is read, updated, and written in a way that timing is critical. But transactions incur a cost because they lock areas of the database and pause other operations waiting for the transaction to finish. There is both a resource and performance cost if transactions are used excessively.

Summary M ySQL transactions Locks and deadlocks Transaction trade-offs

Acknowledgements / Contributions Continue new Contributors and Translators here These slides are Copyright 2010- Charles R. Severance (www.dr-chuck.com) as part of www.wa4e.com and made available under a Creative Commons Attribution 4.0 License. Please maintain this last slide in all copies of the document to comply with the attribution requirements of the license. If you make a change, feel free to add your name and organization to the list of contributors on this page as you republish the materials. Initial Development: Charles Severance, University of Michigan School of Information Insert new Contributors and Translators here including names and dates