MDBSTrans - Multidatabase Transaction Management COP5711...

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Multidatabase Transaction Management COP5711
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Multidatabase Transaction Management Outline Review - Transaction Processing Multidatabase Transaction Management Issues Global Serialization Techniques Global Atomicity and Recovery Problems Global Deadlock Problem
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Multidatabase Transaction Management ACID Property A tomicity : A transaction is either performed in its entirety or not performed at all C onsistency : A correct execution of the transaction must take the database from one consistent state to another I solation : A transaction should not make its updates visible to other transaction until it is committed D urability : Once a transaction changes the database and changes are committed, these changes must never be lost because of subsequent failure
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Multidatabase Transaction Management Transaction Histories (Schedules) A history lists the order in which actions of a set of transactions were successfully completed. r 1 (a) c w 3 (a) r 2 (b) c w 3 (b) c
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Multidatabase Transaction Management Serial History The simplest histories first run all the actions of one transaction, then run all the actions of another to completion, and so on. Such one-transaction-at-a-time histories are called serial histories . serial histories have no concurrency-induced inconsistency and no transaction sees dirty data ( They are correct !)
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Multidatabase Transaction Management Locking constraints the set of allowed histories. Histories are not constructed, they are a byproduct of the system behavior. Histories that obey the locking constraints are called Legal . Legal Histories
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Multidatabase Transaction Management Legal Histories - Examples Histories are not constructed, they are a byproduct of the system behavior. Conflict !
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Multidatabase Transaction Management Isolated Histories A history implies a dependency relation (time order) among the transactions r 1 (a) c w 3 (a) r 2 (b) c w 3 (b) c
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Multidatabase Transaction Management Isolation Theory A transaction should: 1. Be well-formed: it should cover all actions with locks 2. Set XLOCK on any data it writes. 3. Be 2-phase: it should not release locks until it knows it needs no more locks. 4. Hold XLOCKs until COMMIT or ROLLBACK. If these rules are followed, the execution history will give each transaction the illusion it is running in isolation.
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Multidatabase Transaction Management Local vs. Global Transactions Local Transactions: Access data managed by only a single DBMS Executed outside of MDBS control Global Transactions: Consists of a number of subtransactions Subtransactions are processed as local transactions
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Multidatabase Transaction Management Mutidatabase Environment Each local DBMS ensures the ACID properties at its site Consistency & Isolation: Each local DBMS generates a serializable schedule consisting of operations of local and global transactions that were executed at its site Atomicity and Durability: Each local DBMS uses some form of recovery scheme, e.g.,
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This note was uploaded on 04/06/2010 for the course CS COP 6371 taught by Professor Dr.hua during the Spring '10 term at University of Central Florida.

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MDBSTrans - Multidatabase Transaction Management COP5711...

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