Chapter 15: Transactions Transaction Concept Transaction State Implementation of Atomicity and Durability Concurrent executions Serializability Recoverability Implementation of Isolation Transaction Definition in SQL Testing for Serializability 标 Database System Concepts 15.1 @Silberschatz, Korth and Sudarshan
Database System Concepts 15.1 ©Silberschatz, Korth and Sudarshan Chapter 15: Transactions Transaction Concept Transaction State Implementation of Atomicity and Durability Concurrent Executions Serializability Recoverability Implementation of Isolation Transaction Definition in SQL Testing for Serializability
Transaction Concept A transaction is a unit of program execution that accesses and possibly updates various data items A transaction must see a consistent database During transaction execution the database may be inconsistent When the transaction is committed the database must be consistent Two main issues to deal with x Failures of various kinds, such as hardware failures and system crashes x Concurrent execution of multiple transactions 标 Database System Concepts 15.2 @Silberschatz, Korth and Sudarshan
Database System Concepts 15.2 ©Silberschatz, Korth and Sudarshan Transaction Concept A transaction is a unit of program execution that accesses and possibly updates various data items. A transaction must see a consistent database. During transaction execution the database may be inconsistent. When the transaction is committed, the database must be consistent. Two main issues to deal with: Failures of various kinds, such as hardware failures and system crashes Concurrent execution of multiple transactions
ACID Properties To preserve integrity of data, the database system must ensure Atomicity. Either all operations of the transaction are properly reflected in the database or none are Consistency. EXecution of a transaction in isolation preserves the consistency of the database Isolation. Although multiple transactions may execute concurrently, each transaction must be unaware of other concurrently executing transactions. Intermediate transaction results must be hidden from other concurrently executed transactions t That is, for every pair of transactions T; and T, it appears to Ti that either Ti, finished execution before Ti started, or Ti started execution after T finished Durability. After a transaction completes successfully, the changes it has made to the database persist, even if there are system failures Database System Concepts 15.3 OSilberschatz. Korth and Sudarshan
Database System Concepts 15.3 ©Silberschatz, Korth and Sudarshan ACID Properties Atomicity. Either all operations of the transaction are properly reflected in the database or none are. Consistency. Execution of a transaction in isolation preserves the consistency of the database. Isolation. Although multiple transactions may execute concurrently, each transaction must be unaware of other concurrently executing transactions. Intermediate transaction results must be hidden from other concurrently executed transactions. That is, for every pair of transactions Ti and Tj , it appears to Ti that either Tj , finished execution before Ti started, or Tj started execution after Ti finished. Durability. After a transaction completes successfully, the changes it has made to the database persist, even if there are system failures. To preserve integrity of data, the database system must ensure:
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 Consistency requirement -the sum of A and B is unchanged by the execution of the transaction Atomicity requirement -if the transaction fails after step 3 and before step 6, the system should ensure that its updates are not reflected in the database, else an inconsistency wil result 标 Database System Concepts 15.4 OSilberschatz. Korth and Sudarshan
Database System Concepts 15.4 ©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) Consistency requirement – the sum of A and B is unchanged by the execution of the transaction. Atomicity requirement — if the transaction fails after step 3 and before step 6, the system should ensure that its updates are not reflected in the database, else an inconsistency will result
Example of Fund Transfer(Cont 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 despite failures Isolation requirement--if between steps 3 and 6, another transaction is allowed to access the partially updated database, it will see an inconsistent database (the sum A+ B will be less than it should be Can be ensured trivially by running transactions serially, that is one after the other. However, executing multiple transactions concurrently has significant benefits, as we will see 标 Database System Concepts 15.5 OSilberschatz. Korth and Sudarshan
Database System Concepts 15.5 ©Silberschatz, Korth and Sudarshan Example of Fund Transfer (Cont.) 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 despite failures. Isolation requirement — if between steps 3 and 6, another transaction is allowed to access the partially updated database, it will see an inconsistent database (the sum A + B will be less than it should be). Can be ensured trivially by running transactions serially, that is one after the other. However, executing multiple transactions concurrently has significant benefits, as we will see