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Chapter 14 - Transactions. This chapter focuses on the fundamentals of a transaction-processing system: atomicity, consistency, isolation, and durability. It provides an overview of the methods used to ensure these properties, including locking and snapshot isolation. | Chapter 14: Transactions Database System Concepts, 6th Ed. ©Silberschatz, Korth and Sudarshan See www.db-book.com for conditions on re-use Chapter 14: Transactions Transaction Concept Transaction State Concurrent Executions Serializability Recoverability Implementation of Isolation Transaction Definition in SQL Testing for Serializability. Database System Concepts - 6th Edition 14.2 ©Silberschatz, Korth and Sudarshan Transaction Concept A transaction is a unit of program execution that accesses and possibly updates various data items. E.g. transaction to transfer $50 from account A to account B: 1. read(A) 2. A := A – 50 3. write(A) 4. read(B) 5. B := B + 50 6. write(B) Two main issues to deal with: Failures of various kinds, such as hardware failures and system crashes Concurrent execution of multiple transactions Database System Concepts - 6th Edition 14.3 ©Silberschatz, Korth and Sudarshan Example of Fund Transfer Transaction to transfer $50 from account A to account B: 1. read(A) 2. A := A – 50 3. write(A) 4. read(B) 5. B := B + 50 6. write(B) Atomicity requirement if the transaction fails after step 3 and before step 6, money will be “lost” leading to an inconsistent database state Failure could be due to software or hardware the system should ensure that updates of a partially executed transaction are not reflected in the database Durability requirement — once the user has been notified that the transaction has completed (i.e., the transfer of the $50 has taken place), the updates to the database by the transaction must persist even if there are software or hardware failures. Database System Concepts - 6th Edition 14.4 ©Silberschatz, Korth and Sudarshan Example of Fund Transfer (Cont.) Transaction to transfer $50 from account A to account B: 1. 2. 3. 4. 5. 6. read(A) A := A – 50 write(A) read(B) B := B + 50 write(B) Consistency requirement in above example: the sum of A and B is
