7-2高级事务处理讲义.ppt

* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * Concurrency Control Correctness without serializability: Correctness depends on the specific consistency constraints for the databases. Correctness depends on the properties of operations performed by each transaction. Use database consistency constraints as to split the database into subdatabases on which concurrency can be managed separately. Treat some operations besides read and write as fundamental low-level operations and extend concurrency control to deal with them. Concurrency Control (Cont.) A non-conflict-serializable schedule that preserves the sum of A + B Nested and Multilevel Transactions A nested or multilevel transaction T is represented by a set T = {t1, t2, ..., tn} of subtransactions and a partial order P on T. A subtransaction ti in T may abort without forcing T to abort. Instead, T may either restart ti, or simply choose not to run ti. If ti commits, this action does not make ti, permanent (unlike the situation in Chapter 15). Instead, ti, commits to T, and may still abort (or require compensation) if T aborts. An execution of T must not violate the partial order P, i.e., if an edge ti ? ti appears in the precedence graph, then ti ? ti must not be in the transitive closure of P. Nested and Multilevel Transactions (Cont.) Subtransactions can themselves be nested/multilevel transactions. Lowest level of nesting: standard read and write operations. Nesting can create higher-level operations that may enhance concurrency. Types of nested/ multilevel transactions: Multilevel transaction: subtransaction of T is permitted to release locks on completion. Saga: multilevel long-duration transaction. Nested transaction: locks held by a subtransaction ti of T are automatically assign to T on completion of ti. Example of Nesting Rewrite transaction T1 using subtransactions Ta and Tb that perform increment or decrement operations: T1 consists of T1,1, which subt