CS411 - Transaction Management - Note 1 - 2

So who coined acid correctness of transactions 12 of

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Unformatted text preview: ial for recovery What’s on the Log File? • Transaction starts/stops • DB writes: "before" and/or "after" images of DB records • befores can be used to rollback an aborted transaction • afters can be used to redo a transaction (recovery from catastrophe) • COMMITs and ABORTs The log itself is as critical as the DB! Correctness of Transactions (8 of 12) Without log files recovery will have no idea where and what went wrong! Transaction Management (19 of 61) 2 Big Main Issues: The Big TP Issues • • Taking action to restore the DB to a consistent state • Concurrency Control • Making sure simultaneous transactions don't interfere with one another Correctness of Transactions (9 of 12) Transaction Management (20 of 61) Recovery • • Recovery Concurrency Control: How to run multiple transactions? Concurrency control can be a key factor in measuring performance of database system. E.g.: How many users can access Facebook? How many people can use bank accounts? The ACID Properties Famous Acronym To Remember: 4 properties transaction management will satisfy • Atomicity • Consistency Preservation • Isolation • Durability Atomicity Consistency Preservation Isolation Durability Correctness of Transactions (10 of 12) Transaction Management (21 of 61) The ACID Properties: From Oracle Wiki Atomicity – All or nothing which means either all actions must be completed or no transactions will be completed. Consistency – Starting from a state that satisfies all constraints, then must end in a state that also satisfies all the constraints. Correctness of Transactions (11 of 12) Transaction Management (22 of 61) Isolation – Although a database is capable of doing millions of transaction, before it commits the transactions are not to be visible. In other words, transactions must not affect, and must not be affected by any other transactions. Durability – Committed transactions are permanent, no one should be allowed to be change transaction and should survive system failure no matter what Question: Of the 4, which of these is a programmer’s responsibility? Answer: Consistency Behind the Scene: It’s Your Turn! So, who coined “ACID”? Correctness of Transactions (12 of 12) Transaction Management (23 of 61) Refer to http:...
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