ics321-20091105-transaction

Ics321-20091105-tran - ICS 321 Fall 2009 Overview of Transaction Management Asst Prof Lipyeow Lim Information Computer Science Department

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ICS 321 Fall 2009 Overview of Transaction Management Asst. Prof. Lipyeow Lim University of Hawaii at Manoa 11/5/2009 1 Lipyeow Lim -- University of Hawaii at Manoa
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Transactions A transaction is the DBMS’s abstract view of a user program: a sequence of reads and writes. A user’s program may carry out many operations on the data retrieved from the database, but the DBMS is only concerned about what data is read/written from/to the database. A DBMS supports multiple users, ie, multiple transactions may be running concurrently Concurrent executions can be exploited for DBMS performance. Because disk accesses are frequent, and relatively slow, it is important to keep the CPU humming by working on several user programs concurrently. 11/5/2009 Lipyeow Lim -- University of Hawaii at Manoa 2
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Concurrency in a DBMS Users submit transactions, and can think of each transaction as executing by itself. Concurrency is achieved by the DBMS, which interleaves actions (reads/writes of DB objects) of various transactions. Each transaction must leave the database in a consistent state if the DB is consistent when the transaction begins. DBMS will enforce some ICs, depending on the ICs declared in CREATE TABLE statements. Beyond this, the DBMS does not really understand the semantics of the data. (e.g., it does not understand how the interest on a bank account is computed). Issues: Effect of interleaving transactions, and crashes 11/5/2009 Lipyeow Lim -- University of Hawaii at Manoa 3
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ACID Properties 4 important properties of transactions Atomicity : all or nothing Users regard execution of a transaction as atomic No worries about incomplete transactions Consistency : a transaction must leave the database in a good state
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This note was uploaded on 11/15/2010 for the course ICS 321 taught by Professor Lim during the Fall '09 term at University of Hawaii, Manoa.

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Ics321-20091105-tran - ICS 321 Fall 2009 Overview of Transaction Management Asst Prof Lipyeow Lim Information Computer Science Department

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