Conditional Conflict Serializability - an Application-Oriented Correctness Criterion

Standard transaction management ensures the ACID properties for transactions (atomicity, consistency, isolation, and durability; Härder & Reuter 1983, Gray & Reuter 1993). Of primary interest here is the isolation property, which corresponds to the correctness criterion serializability. It is generally acknowledged in the database research community that serializability is unsuitable for several application domains (Elmagarmid 1992, Kaiser 1995, Ramamritham & Chrysanthis 1996), in particular where transaction duration is long relative to response time requirements. This paper presents conditional conflict serializability (CCSR), a correctness criterion capable of supporting serializability, dirty reads, and almost any degree of concurrency in between those two extremes. This makes CCSR suitable for almost any application domain where read-write and write-read conflicts need to be handled in a more lenient way than with unconditional conflict serializability. In particular, CCSR is appropriate for applications that need long-lasting transactions. An important motivation behind CCSR is the need for a correctness criterion that is both general and simple. There is a need for a framework that offers application programmers easy-to-use customization of concurrency control, without affecting programmers who need their applications to be fully isolated from others. Another major motivation behind CCSR has been to enable arbitrary levels of concurrency without forcing programmers to perform recovery by means of compensating actions. The larger context for the results presented in this paper is Apotram (which stands for application-oriented transaction model; Anfindsen 1997). In particular, it should be noted that while CCSR has nothing to offer in terms of handling writewrite conflicts, it is part of a framework where this is taken care of. The Apotram mechanism for handling write-write conflicts is called nested databases (Anfindsen 1996; 1997, 45-52), and the corresponding correctness criterion is called nested conflict serializability (NCSR). NCSR and CCSR can be combined into a single correctness criterion.