lect-11 - Computer Science 425 Distributed Systems Lecture...

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2006, 2007, M. T. Harandi Lecture 11-1 Computer Science 425 Distributed Systems Lecture 11 Transaction Processing
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2006, 2007, M. T. Harandi Lecture 11-2 Transactions Sequences of operations that forms a single step, transforming the server data from one consistent state to another. All or nothing principle: a transaction either completes successfully, and the effects are recorded in the objects, or it has no effect at all. Transactions’ operations are indivisible from the point of view of other transactions No access to intermediate results/states Free from interference by other operations Transactions may be distributed Transactions could run concurrently
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2006, 2007, M. T. Harandi Lecture 11-3 Banking transaction for a customer (ATM) Transfer $100 from saving to checking account; Transfer $200 from money-market to checking account; Withdraw $400 from checking account. Transaction: 1. savings.deduct(100) /* includes verification */ 2. checking.add(100) /* depends on success of 1 */ 3. mnymkt.deduct(200) /* includes verification */ 4. checking.add(200) /* depends on success of 3 */ 5. checking.deduct(400) /* includes verification */ 6. commit 7. dispense(400) Example Transaction
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2006, 2007, M. T. Harandi Lecture 11-4 Transaction: 1. savings.deduct(100) 2. checking.add(100) 3. mnymkt.deduct(200) 4. checking.add(200) 5. checking.deduct(400) 6. commit Transaction Failure Modes A failure at these points means the customer looses money, we need to restore A failure at these points does not cause lost money, but the transaction cannot be repeated This is the point of no return A failure after the commit point (ATM crashes) needs corrective action, no undoing possible.
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2006, 2007, M. T. Harandi Lecture 11-5 Properties of Transactions (ACID) A tomicity: All or nothing C onsistency: starting in a consistent state, the transaction ends in a consistent state.
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This note was uploaded on 07/01/2009 for the course CS CS421 taught by Professor Kirillmechitov during the Summer '09 term at University of Illinois, Urbana Champaign.

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lect-11 - Computer Science 425 Distributed Systems Lecture...

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