Jul 7th, 2015
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To preserve integrity of data, the database system must ensure: Atomicity. Either all operations of the transaction are properly reflected in the database or none are. Consistency.Execution of a transaction in isolation preserves the consistency of the database.

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TransactionTransaction Concept A transaction is a unit of program execution thataccesses and possibly updates various data items. A transaction must see a consistent database. During transaction execution the database may beinconsistent. When the transaction is committed, the databasemust be consistent. Two main issues to deal with: Failures of various kinds, such as hardwarefailures and system crashes Concurrent execution of multiple transactionsACID Properties To preserve integrity of data, the database systemmust ensure:Atomicity. Either all operations of the transaction areproperly reflected in the database or none are.Consistency. Execution of a transaction in isolationpreserves the consistency of the database.Cont. Isolation. Although multiple transactions mayexecute concurrently, each transaction must beunaware of other concurrently executingtransactions. Intermediate transaction results mustbe hidden from other concurrently executedtransactions. That is, for every pair of transactions Ti and Tj, itappears to Ti that either Tj, finished executionbefore Ti started, or Tj started execution after Tifinished. Durability. After a transaction completessuccessfully, the changes it has made to thedatabase persist, even if there are system failures.Transaction Notation Transactions access data using two operations:1) read(X), which transfers the data item X fromthe database to a local buffer belon

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