08transaction.ppt - Introduction to Transaction Processing Gang Qian Department of Computer Science University of Central Oklahoma 1 Overview(Chapter 20

08transaction.ppt - Introduction to Transaction Processing...

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1 Introduction to Transaction Processing Gang Qian Department of Computer Science University of Central Oklahoma
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2 Overview (Chapter 20) Introduction to Transaction Processing Transaction Concepts Transaction Schedule Concepts Conflict Serializability View Serializability
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3 Introduction to Transaction Processing Data is accessed concurrently For efficiency purpose The problem is how can we maintain the consistency in the data The purpose of transaction processing is to allow maximum concurrent data processing while producing the same effect as if the tasks are processed one by one
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4 Example 1 Transfer $500 from saving to checking and log All statements in the transaction need to be performed Any problem should cause all operations to be cancelled (rollback) Insufficient fund, invalid account, hardware failure, etc. Atomicity of transactions
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5 Example 2 Current balance $ 1000 Transaction 1: add $100 to balance Transaction 2: add $200 to balance We have interleaved operations as shown in next slide What is the final balance?
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7 Example 3 Tran1: Transfer $ 50 from saving to checking account Tran 2: Withdraw $10 from the same checking account T1 crashes before “write sbal-50” Need to rollback Cascading effect on T2
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8 The System Log Log or Journal : The log keeps track of all transaction operations that affect the values of database items. The log is kept on disk, so it is not affected by any type of failure except for disk or catastrophic failure In addition, the log is periodically backed up to archival storage (tape) to guard against such catastrophic failures It is used for transaction recovery
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9 Transactions A Transaction : Logical unit of database processing that includes one or more access operations: Read: retrieval Write: insert or update, delete A transaction (a set of operations) may be interactive SQL commands, or may be embedded within a program Transaction boundaries : Begin and End transaction Oracle: The completion of one transaction starts the next one
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10 Properties of Transactions (ACID) Atomicity : A transaction is an atomic unit of processing; it is either performed in its entirety or not performed at all Consistency preservation : A correct execution of the transaction must take the database from one consistent state to another. Isolation : A transaction should not make its updates visible to other transactions until it is committed Enforcing this property will make cascading rollbacks of transactions unnecessary Durability or permanency : Once a transaction changes the database and the changes are committed, these changes must never be lost because of subsequent failure
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11 A SIMPLE MODEL OF A DATABASE (for purposes of discussing transactions): A database is a collection of named data items Granularity of data - a field, a record , or a whole disk block (Concepts are independent of granularity)
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