Transaction State Active-the initial state;the transaction stays in this state while it is executing Partially committed-after the final statement has been executed. Failed--after the discovery that normal execution can no longer proceed. Aborted-after the transaction has been rolled back and the database restored to its state prior to the start of the transaction. Two options after it has been aborted: restart the transaction;can be done only if no internal logical error kill the transaction Committed -after successful completion. Database System Concepts-5th Edition,Sep 10,2005. 15.7 @Silberschatz,Korth and Sudarshan
Database System Concepts - 5 15.7 ©Silberschatz, Korth and Sudarshan th Edition, Sep 10, 2005. Transaction State Active – the initial state; the transaction stays in this state while it is executing Partially committed – after the final statement has been executed. Failed -- after the discovery that normal execution can no longer proceed. Aborted – after the transaction has been rolled back and the database restored to its state prior to the start of the transaction. Two options after it has been aborted: restart the transaction; can be done only if no internal logical error kill the transaction Committed – after successful completion
Transaction State (Cont.) partially committed committed active failed aborted Database System Concepts-5th Edition,Sep 10,2005. 15.8 @Silberschatz,Korth and Sudarshan
Database System Concepts - 5 15.8 ©Silberschatz, Korth and Sudarshan th Edition, Sep 10, 2005. Transaction State (Cont.)
Implementation of Atomicity and Durability The recovery-management component of a database system implements the support for atomicity and durability. The shadow-database scheme: assume that only one transaction is active at a time. a pointer called db_pointer always points to the current consistent copy of the database. all updates are made on a shadow copy of the database,and db_pointer is made to point to the updated shadow copy only after the transaction reaches partial commit and all updated pages have been flushed to disk. in case transaction fails,old consistent copy pointed to by db pointer can be used,and the shadow copy can be deleted. Database System Concepts-5th Edition,Sep 10,2005. 15.9 ©Silberschat乜，Korth and Sudarshan
Database System Concepts - 5 15.9 ©Silberschatz, Korth and Sudarshan th Edition, Sep 10, 2005. Implementation of Atomicity and Durability The recovery-management component of a database system implements the support for atomicity and durability. The shadow-database scheme: assume that only one transaction is active at a time. a pointer called db_pointer always points to the current consistent copy of the database. all updates are made on a shadow copy of the database, and db_pointer is made to point to the updated shadow copy only after the transaction reaches partial commit and all updated pages have been flushed to disk. in case transaction fails, old consistent copy pointed to by db_pointer can be used, and the shadow copy can be deleted
Implementation of Atomicity and Durability (Cont.) The shadow-database scheme: db-pointer db-pointer old copy of old copy of database new copy of database database (to be deleted) (a)Before update (b)After update Assumes disks do not fail Useful for text editors,but extremely inefficient for large databases(why?) Does not handle concurrent transactions Will study better schemes in Chapter 17. Database System Concepts-5th Edition,Sep 10,2005. 15.10 ©Silberschat乜，Korth and Sudarshan
Database System Concepts - 5 15.10 ©Silberschatz, Korth and Sudarshan th Edition, Sep 10, 2005. Implementation of Atomicity and Durability (Cont.) Assumes disks do not fail Useful for text editors, but extremely inefficient for large databases (why?) Does not handle concurrent transactions Will study better schemes in Chapter 17. The shadow-database scheme:
Concurrent Executions Multiple transactions are allowed to run concurrently in the system. Advantages are: increased processor and disk utilization,leading to better transaction throughput:one transaction can be using the CPU while another is reading from or writing to the disk reduced average response time for transactions:short transactions need not wait behind long ones. Concurrency control schemes-mechanisms to achieve isolation;that is,to control the interaction among the concurrent transactions in order to prevent them from destroying the consistency of the database Will study in Chapter 16,after studying notion of correctness of concurrent executions. Database System Concepts-5th Edition,Sep 10,2005. 15.11 ©Silberschat乜，Korth and Sudarshan
Database System Concepts - 5 15.11 ©Silberschatz, Korth and Sudarshan th Edition, Sep 10, 2005. Concurrent Executions Multiple transactions are allowed to run concurrently in the system. Advantages are: increased processor and disk utilization, leading to better transaction throughput: one transaction can be using the CPU while another is reading from or writing to the disk reduced average response time for transactions: short transactions need not wait behind long ones. Concurrency control schemes – mechanisms to achieve isolation; that is, to control the interaction among the concurrent transactions in order to prevent them from destroying the consistency of the database Will study in Chapter 16, after studying notion of correctness of concurrent executions