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Transcript
10 Transaction Management and Concurrency Control MIS 304 Winter 2005 10 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. 2 10 A leftover from last time. • Demonstration of Altova XML Spy feature to created Relational tables from an XML document. 3 10 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. 4 10 TRANSACTION 5 10 Sources of Transaction What are the sources of transactions? What generates the data? 6 10 Consistent State 7 Example of a Transaction Table: Stock X = 40 PartNo=12345 UPDATE Stock SET X = X – 10 WHERE PartNo = 12345 Table: Stock X = 30 PartNo=12345 10 Consistent state Transaction(s) Consistent state 8 10 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 9 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 10 The Relational Schema for the Ch09_SaleCo Database 11 10 Double Entry Bookkeeping • The bane of the database programmer. WHY? 12 10 Accounting Transactions • Represent a large component of “all” Transactions. • “Double Entry” Bookkeeping means that there are at least TWO components per transaction. 13 10 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 X’s Quantity on hand by 100. – Adding $500.00 to customer Y’s accounts receivable. UPDATE PRODUCT SET PROD_QOH = PROD_QOH - 100 WHERE PROD_CODE = ‘X’; UPDATE ACCREC SET AR_BALANCE = AR_BALANCE + 500 WHERE AR_NUM = ‘Y’; • If the above two transactions are not completely executed, the transaction yields an inconsistent database. 14 10 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 15 10 Tracing the Transaction in the Ch09_SaleCo Database Figure 9.2 16 10 Transaction Properties • Atomicity – Requires that all operations (SQL requests) of a transaction be completed • Durability – Indicates permanence of database’s consistent state 17 10 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 18 10 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 19 10 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 20 10 A Transaction Log 21 10 Concurrency Control • Coordination of simultaneous transaction execution in a multiprocessing database system • Objective is to ensure transaction serializability in a multiuser database environment 22 10 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 23 10 Normal Execution of Two Transactions 24 10 Lost Updates 25 10 Correct Execution of Two Transactions 26 10 An Uncommitted Data Problem 27 10 Retrieval During Update 28 10 Transaction Results: Data Entry Correction 29 10 Inconsistent Retrievals 30 10 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 31 10 The Scheduler (continued) • Bases its actions on concurrency control algorithms • Ensures computer’s 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 32 10 Read/Write Conflict Scenarios: Conflicting Database Operations Matrix 33 10 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 34 10 Lock Granularity • Indicates the level of lock use • Locking can take place at the following levels: – Database – Table – Page – Row – Field (attribute) 35 10 Lock Granularity (continued) • Database-level lock – Entire database is locked • Table-level lock – Entire table is locked • Page-level lock – Entire diskpage is locked 36 10 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 37 10 A Database-Level Locking Sequence 38 10 An Example of a Table-Level Lock 39 10 Example of a Page-Level Lock 40 10 An Example of a Row-Level Lock 41 10 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 42 10 An Example of a Binary Lock 43 10 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 44 10 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 45 10 Two-Phase Locking Protocol 46 10 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 47 10 How a Deadlock Condition Is Created 48 10 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 49 10 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 50 10 Wait/Die and Wound/Wait Concurrency Control Schemes 51 10 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 52 10 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) 53 10 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 54 10 Transaction Recovery (continued) • Write-through – Database is immediately updated by transaction operations during the transaction’s execution, even before the transaction reaches its commit point 55 10 A Transaction Log for Transaction Recovery Examples 56 10 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 57 10 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 58 10 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 59