10 Transaction Management and Concurrency Control MIS 304 Winter 2005 10 2 Review from last week The Internet is largely dependent on database technology Database Middleware links HTML and HTTP based systems to traditional Relational Database. The HTML and HTTP architectures can make it more difficult to implement user friendly interfaces. 10 3 A leftover from last time. Demonstration of Altova XML Spy feature to created Relational tables from an XML document. 10 4 Goals for this class Understand how transactions are used in the databases and their applications. Understand the technology of the database transactions. Understand concurrency and locking technology and how they effect databases. 10 5 TRANSACTION 10 6 Sources of Transaction What are the sources of transactions? What generates the data? 10 7 Consistent State 10 8 Table: Stock X = 40 PartNo=12345 Consistent state Table: Stock X = 30 PartNo=12345 Consistent state UPDATE Stock SET X = X 10 WHERE PartNo = Transaction(s) Example of a Transaction 10 9 What is a Transaction? Any action that reads from and/or writes to a database may consist of Simple SELECT statement to generate a list of table contents A series of related UPDATE statements to change the values of attributes in various tables A series of INSERT statements to add rows to one or more tables A combination of SELECT, UPDATE, and INSERT statements 10 What is a Transaction? ( continued ) A logical unit of work that must be either entirely completed or aborted Successful transaction changes the database from one consistent state to another One in which all data integrity constraints are satisfied Most real-world database transactions are formed by two or more database requests The equivalent of a single SQL statement in an application program or transaction 10 11 The Relational Schema for the Ch09_SaleCo Database 10 12 Double Entry Bookkeeping The bane of the database programmer. WHY? 10 13 Accounting Transactions Represent a large component of all Transactions. Double Entry Bookkeeping means that there are at least TWO components per transaction. 10 14 Evaluating Transaction Results An accountant wishes to register the credit sale of 100 units of product X to customer Y in the amount of $500.00: Reducing product Xs Quantity on hand by 100. Adding $ to customer Ys accounts receivable. UPDATE PRODUCT SET PROD_QOH = PROD_QOH WHERE PROD_CODE = X; UPDATE ACCREC SET AR_BALANCE = AR_BALANCE WHERE AR_NUM = Y; If the above two transactions are not completely executed, the transaction yields an inconsistent database. 10 15 Evaluating Transaction Results Not all transactions update the database SQL code represents a transaction because database was accessed Improper or incomplete transactions can have a devastating effect on database integrity Some DBMSs provide means by which user can define enforceable constraints based on business rules Other integrity rules are enforced automatically by the DBMS when table structures are properly defined, thereby letting the DBMS validate some transactions 10 16 Tracing the Transaction in the Ch09_SaleCo Database Figure 9.2 10 17 Transaction Properties Atomicity Requires that all operations (SQL requests) of a transaction be completed Durability Indicates permanence of databases consistent state 10 18 Transaction Properties ( continued ) Serializability Ensures that the concurrent execution of several transactions yields consistent results Isolation Data used during execution of a transaction cannot be used by second transaction until first one is completed 10 19 Transaction Management with SQL ANSI has defined standards that govern SQL database transactions Transaction support is provided by two SQL statements: COMMIT and ROLLBACK ANSI standards require that, when a transaction sequence is initiated by a user or an application program, it must continue through all succeeding SQL statements until one of four events occurs 10 20 The Transaction Log Stores A record for the beginning of transaction For each transaction component (SQL statement) Type of operation being performed (update, delete, insert) Names of objects affected by the transaction (the name of the table) Before and after values for updated fields Pointers to previous and next transaction log entries for the same transaction The ending (COMMIT) of the transaction 10 21 A Transaction Log 10 22 Concurrency Control Coordination of simultaneous transaction execution in a multiprocessing database system Objective is to ensure transaction serializability in a multiuser database environment 10 23 Concurrency Control Important simultaneous execution of transactions over a shared database can create several data integrity and consistency problems lost updates uncommitted data inconsistent retrievals 10 24 Normal Execution of Two Transactions 10 25 Lost Updates 10 26 Correct Execution of Two Transactions 10 27 An Uncommitted Data Problem 10 28 Retrieval During Update 10 29 Transaction Results: Data Entry Correction 10 30 Inconsistent Retrievals 10 31 The Scheduler Special DBMS program: establishes order of operations within which concurrent transactions are executed Interleaves the execution of database operations to ensure serializability and isolation of transactions 10 32 The Scheduler ( continued ) Bases its actions on concurrency control algorithms Ensures computers central processing unit (CPU) is used efficiently Facilitates data isolation to ensure that two transactions do not update the same data element at the same time 10 33 Read/Write Conflict Scenarios: Conflicting Database Operations Matrix 10 34 Concurrency Control with Locking Methods Lock Guarantees exclusive use of a data item to a current transaction Required to prevent another transaction from reading inconsistent data Lock manager Responsible for assigning and policing the locks used by the transactions 10 35 Lock Granularity Indicates the level of lock use Locking can take place at the following levels: Database Table Page Row Field (attribute) 10 36 Lock Granularity ( continued ) Database-level lock Entire database is locked Table-level lock Entire table is locked Page-level lock Entire diskpage is locked 10 37 Lock Granularity ( continued ) Row-level lock Allows concurrent transactions to access different rows of the same table, even if the rows are located on the same page Field-level lock Allows concurrent transactions to access the same row, as long as they require the use of different fields (attributes) within that row 10 38 A Database-Level Locking Sequence 10 39 An Example of a Table-Level Lock 10 40 Example of a Page-Level Lock 10 41 An Example of a Row-Level Lock 10 42 Lock Types Binary lock Has only two states: locked (1) or unlocked (0) Exclusive lock Access is specifically reserved for the transaction that locked the object Must be used when the potential for conflict exists Shared lock Concurrent transactions are granted Read access on the basis of a common lock 10 43 An Example of a Binary Lock 10 44 Two-Phase Locking to Ensure Serializability Defines how transactions acquire and relinquish locks Guarantees serializability, but it does not prevent deadlocks Growing phase, in which a transaction acquires all the required locks without unlocking any data Shrinking phase, in which a transaction releases all locks and cannot obtain any new lock 10 45 Two-Phase Locking to Ensure Serializability ( continued ) Governed by the following rules: Two transactions cannot have conflicting locks No unlock operation can precede a lock operation in the same transaction No data are affected until all locks are obtained that is, until the transaction is in its locked point 10 46 Two-Phase Locking Protocol 10 47 Deadlocks Condition that occurs when two transactions wait for each other to unlock data Possible only if one of the transactions wants to obtain an exclusive lock on a data item No deadlock condition can exist among shared locks Control through Prevention Detection Avoidance 10 48 How a Deadlock Condition Is Created 10 49 Concurrency Control with Time Stamping Methods Assigns a global unique time stamp to each transaction Produces an explicit order in which transactions are submitted to the DBMS Uniqueness Ensures that no equal time stamp values can exist Monotonicity Ensures that time stamp values always increase 10 50 Wait/Die and Wound/Wait Schemes Wait/die Older transaction waits and the younger is rolled back and rescheduled Wound/wait Older transaction rolls back the younger transaction and reschedules it 10 51 Wait/Die and Wound/Wait Concurrency Control Schemes 10 52 Concurrency Control with Optimistic Methods Optimistic approach Based on the assumption that the majority of database operations do not conflict Does not require locking or time stamping techniques Transaction is executed without restrictions until it is committed Phases are read, validation, and write 10 53 Database Recovery Management Database recovery Restores database from a given state, usually inconsistent, to a previously consistent state Based on the atomic transaction property All portions of the transaction must be treated as a single logical unit of work, in which all operations must be applied and completed to produce a consistent database If transaction operation cannot be completed, transaction must be aborted, and any changes to the database must be rolled back (undone) 10 54 Transaction Recovery Makes use of deferred-write and write-through Deferred write Transaction operations do not immediately update the physical database Only the transaction log is updated Database is physically updated only after the transaction reaches its commit point using the transaction log information 10 55 Transaction Recovery ( continued ) Write-through Database is immediately updated by transaction operations during the transactions execution, even before the transaction reaches its commit point 10 56 A Transaction Log for Transaction Recovery Examples 10 57 Summary Transaction Sequence of database operations that access the database Represents real-world events Must be a logical unit of work No portion of the transaction can exist by itself Takes a database from one consistent state to another One in which all data integrity constraints are satisfied 10 58 Summary ( continued ) SQL provides support for transactions through the use of two statements: COMMIT and ROLLBACK Concurrency control coordinates the simultaneous execution of transactions Scheduler is responsible for establishing order in which concurrent transaction operations are executed 10 59 Summary ( continued ) Lock guarantees unique access to a data item by a transaction Database recovery restores the database from a given state to a previous consistent state