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Concurrency Control Objectives • • • • • Management of Databases Concurrency Control Database Recovery Database Security Database Administration Transactions • ACID – Atomic, Consistent, Isolation, Durable • Transactions – completely succeed or completely fail ACID attributes • Atomic – Group one or more changes into an all or nothing operation • Consistency – during a transaction it may violate integrity rules, but no other transaction can see it and it eliminated before ending • Isolation- Appear that transaction is running by itself (invisible to all other) • Durability – once commited, changes persists even in event of system failure Database Design issues Because of simultaneous updates consider: • Concurrency Control – mechanism to apply ACID attributes to transactions • Transaction • Security Lost Update Anomaly Inconsistent retrieval anomaly Database Design Issues • Lost updates • Inconsistent reads DBMS components • • • • Transactions Transaction Manager Data Manager Data Operations for Transactions • Read – read value of X • Write – write new value of X • Begin – start of a private workspace for transaction • End – end of transaction and attempt to transfer information from private workspace to database Two Phase Commit The movement of transaction data from private workspace to stored database 1. First Phase – try writing private workspace to a secure storage (not the stored DB). If OK then go to 2nd phase, otherwise lose changes but no harm done to DB. 2. Second Phase – data is moved from private workspace to stored database. If anything happens, the DB can recover from values in secure storage. Control Scheme – sharing fields • Pessimistic scheme – at least two users going for same resource, one gets there first and locks the object until they are done. • Optimistic scheme – use when likely one user will access field at a time. At transaction commit, make sure one has accessed the field since your transaction began via timestamp. Control Scheme Goal • Purpose of both types is DB in consistent state after transactions. • To achieve consistent state, transactions must behave as performing one after another – serializability • Locks are one method to enforce serializability to concurrent transactions Database recovery • Consistency – return state of DB after a failure (ACID) • Failure – what kinds of failure to anticipate • Transaction – atomic (succeed or fail) Database Failures • Transaction failures – DB intact – Various reason a transaction fails – User aborts – DBMS aborts because of deadlock • System failures (main memory lost) – Bug in DBMS – OS fault – Hardware fault • Media failure (previous content lost) – – – – Bug in OS Hardware errors in channel or disk controller Head crash Magnetic decay Recovery actions • Depends on type of failure • Transaction and system failures – previous content intact then use log files that contain the latest changes to the last consistent state of DB • Media failure – recover with a redundant copy of the database Direct Page and Indirect Page