Allocating Isolation Levels to Transactions Alan Fekete   View Paper (PDF)   Return to Research Session 5: Data Mining / Transaction Management Abstract Serializability is a key property for executions of OLTP systems; without this, integrity constraints on the data can be violated due to concurrent activity. Serializability can be guaranteed regardless of application logic, by using a serializable concurrency control mechanism such as strict twophase locking (S2PL); however the reduction in concurrency from this is often too great, and so a DBMS o ers the DBA the opportunity to use di erent concurrency control mechanisms for some transactions, if it is safe to do so. However, little theory has existed to decide when it is safe! In this paper, we discuss the problem of taking a collection of transactions, and allocating each to run at an appropriate isolation level (and thus use a particular concurrency control mechanism), while still ensuring that every execution will be conflict serializable. When each transaction can use either S2PL, or snapshot isolation, we characterize exactly the acceptable allocations, and provide a simple graph-based algorithm which determines the weakest acceptable allocation. ©2006 Association for Computing Machinery