CSC 411/511: DBMS Design Dr. Nan WangCSC411_L12_JDBC_MySQL 1 Transations CSC411_L12_JDBC_MySQLDr. Nan Wang 2 Transaction Databases are all about sharing data, so it is common for multiple users to be accessing and even changing the same data at the same time. The simultaneous execution of operations is called concurrency. Sometimes concurrency can get us into trouble if our changes require multiple SQL statements. Lets look at an example. 2 CSC411_L12_JDBC_MySQLDr. Nan Wang 33 CSC411_L12_JDBC_MySQLDr. Nan Wang 44 CSC411_L12_JDBC_MySQLDr. Nan Wang 55 CSC411_L12_JDBC_MySQLDr. Nan Wang 66 CSC411_L12_JDBC_MySQLDr. Nan Wang 77 CSC411_L12_JDBC_MySQLDr. Nan Wang 88 CSC411_L12_JDBC_MySQLDr. Nan Wang 99 CSC411_L12_JDBC_MySQLDr. Nan Wang 10 CSC411_L12_JDBC_MySQLDr. Nan Wang 11 CSC411_L12_JDBC_MySQLDr. Nan Wang 12 Example We always want to make sure that we have all of the ingredients for every item before we submit a customers order. 12 CSC411_L12_JDBC_MySQLDr. Nan Wang 13 CSC411_L12_JDBC_MySQLDr. Nan Wang 14 Now consider the following scenario: Two customers, Bob and Mary, arrive at about the same time and each go to a different register. Bob orders a Garden Salad. Bobs register runs Query 11.1, which indicates that there are enough ingredients to make his order. While Bob decides if he wants to order a drink, Mary also orders a Garden Salad. Marys register runs Query 11.1, which again indicates that there are enough ingredients to make her order. Mary immediately pays for her order, and her register executes Update Bob nally decides against a drink, and his register executes Update Of course, we only have enough ingredients to make one Garden Salad. This is a classic problem in database systems; it is called the isolation or serializability problem. In general, if two or more users access the same data and one or more of the statements changes the data, we have a conict. If the users perform multiple steps, conicts can cause incorrect results to occur. 14 CSC411_L12_JDBC_MySQLDr. Nan Wang 15 Transaction To deal with this problem, databases allow the grouping of a sequence of SQL statements into an indivisible unit of work called a transaction. A transaction ends with either a commit or a rollback: Commit: commit permanently stores all of the changes performed by the transaction. Rollback: rollback removes all of the updates performed by the transaction, no matter how many rows have been changed. A rollback can be executed either by the DBMS to prevent incorrect actions or explicitly by the user. 15 CSC411_L12_JDBC_MySQLDr. Nan Wang 16 ACID The DBMS provides the following guarantees for a transaction, called the ACID properties: Atomicity: Either all (commit) or none (rollback) of the changes within a transaction are made permanent. Consistency: If a transaction executes on a consistent database, then when it terminates the database will still be consistent. Isolation: A transaction can execute on a database as though it is the only transaction running. Durability: Changes made by any committed transaction are permanent, even surviving system crashes and hardware failures. 16 CSC411_L12_JDBC_MySQLDr. Nan Wang 17 Solution for Bobs problem For Bob and Mary, Query 11.1 and the Update 11.2 are not two independent operations on the database but are actually part of a single order. The logical unit of work is both the query and the update. We use transactions to let Bob and Mary execute Query 11.1 and Update 11.2 as one unit of work. This avoids any conicting updates to the database (either Bob, Mary, or neither gets the Garden Salad, not both), and we know that if a transaction commits, all of its changes are permanent. 17 CSC411_L12_JDBC_MySQLDr. Nan Wang 18 Ending a Transaction A transaction ends with either a commit or rollback. A commit is explicitly executed by the user using the COMMIT statement: COMMIT [WORK] COMMIT attempts to commit all of the changes made since the beginning of the transaction. If a problem is detected, COMMIT signals an error, and the transaction is rolled back. Once a commit successfully completes, the changes are permanent. 18 CSC411_L12_JDBC_MySQLDr. Nan Wang 19 Heres a situation where a fear of commitment can be healthy. A rollback can be executed either by the DBMS (to prevent incorrect actions) or explicitly by the user using the ROLLBACK statement: ROLLBACK [WORK][TO SAVEPOINT ] ROLLBACK undoes all of the changes made since the beginning of the current transaction. If you have an active transaction and you kill your query processor without executing a COMMIT, the DBMS should roll back all of your changes. 19 CSC411_L12_JDBC_MySQLDr. Nan Wang 20 Starting a Transaction START TRANSACTION [ ] BEGIN TRANSACTION [ ] While a transaction is running, all of the data accessed by the transaction are protected. For example, start a transaction and execute Query Now, in a separate session try to execute Update What usually happens is that Update 11.2 does not complete.The database has detected a possible conict between Query 11.1 and Update Whichever transaction tries to access the data second is blocked by the atabase. No operations can be submitted by that transaction until the block ends, and that wont happen until the rst transaction terminates. 20 CSC411_L12_JDBC_MySQLDr. Nan Wang 21 Auto-Commit Most DBMSs include an auto-commit mode where a commit is automatically attempted after every SQL statement. With auto-commit, all transactions consist of only a single SQL statement. auto-commit is the default mode. Changing to the manual-commit mode where a COMMIT statement must be executed to commit is DBMS-specic. Some DBMSs will temporarily suspend the auto- commit mode if the user enters a START TRANSACTION. 21 CSC411_L12_JDBC_MySQLDr. Nan Wang 22 Savepoints SQL allows you to create named placeholders, called savepoints, in the sequence of statements in a transaction. You can rollback to a savepoint instead of to the beginning of the transaction. Only the changes made after the savepoint are undone. To set a savepoint, use the SAVEPOINT command: SAVEPOINT If we create a savepoint named sp, we can rollback to that savepoint with the following: ROLLBACK TO SAVEPOINT sp Executing ROLLBACK without designating a savepoint or executing a COMMIT deletes all savepoints back to the start of the transaction. A rollback to a particular savepoint deletes all intervening savepoints. 22 CSC411_L12_JDBC_MySQLDr. Nan Wang 23 Suppose Bob is ordering food for his entire soccer team, one player at a time, and the last player wants to change his order. ROLLBACK will undo the order for all players; however, if we created a savepoint before each players order, we could easily rollback just the order of the last player. 23 CSC411_L12_JDBC_MySQLDr. Nan Wang 24 CSC411_L12_JDBC_MySQLDr. Nan Wang 25 Transaction Characteristics SET TRANSACTION [, ] READ ONLY: only statements that do not change the data are allowed. READ WRITE: both statements that access and manipulate data are allowed. SERIALIZABLE: prevents all possible conicts. REPEATABLE READ: allows a problem known as phantom read. A phantom read happens when a transaction reads a set of rows (such as an entire table). A second transaction then inserts a row into the table. If the rst transaction repeats the read, the results will now be different. READ COMMITTED READ UNCOMMITTED 25 CSC411_L12_JDBC_MySQLDr. Nan Wang 26 Locking Issues Deadlock and lock escalation When a deadlock occurs in any computer system, progress cannot occur. For example, if two transactions execute Query 11.1 at the same time, the database will prevent any other transaction from changing the values read. Now suppose the rst transaction tries to execute Update The DBMS will block the transaction, preventing it from doing any work. If the second transaction also performs Update 11.2, then it will also be blocked. Neither transaction can make progress. They are deadlocked. DBMS will detect this deadlock and rollback one of the transactions, allowing the other to make progress. 26 CSC411_L12_JDBC_MySQLDr. Nan Wang 27 Lock Escalation Lock escalation occurs as a result of a transaction updating a large amount of data in a single table. Usually, a DBMS will only lock as much of a database as needed for the transaction to perform its operations. However, if a transaction updates a large portion of a table, the DBMS may lock the entire table. This will help the performance of the update transaction, but it may cause serious performance problems for concurrent transactions, because they will not be able to read any of the table. The solutions to the lock escalation problem are application specic and include dividing the update transaction into independent transactions, running the transactions under a reduced isolation level, or accepting the reduced performance. 27 CSC411_L12_JDBC_MySQLDr. Nan Wang 28 Summary A transaction is a logical unit of work. By combining multiple SQL statements into a single transaction we can execute many complex statements as though they are a single statement. This allows us to correctly update different tables at one time. Transactions can either commit, which means that all of the operations are saved to the database, or rollback, which means all of the operations are removed from the database. 28 CSC411_L12_JDBC_MySQLDr. Nan Wang 29 Exercise 29 CSC 411/511: DBMS Design Dr. Nan WangCSC411_L12_JDBC_MySQL 30 Questions?