ACID Properties Need Fast Relief: Relaxing Consistency Using Epsilon Serializability

Epsilon Serializability (ESR) is a generalization of classic serializability (SR). ESR supports more concurrency and autonomy than SR by allowing a bounded amount of inconsistency in transactions that can tolerate it. ESR has three main advantages over previous \weak consistency" models: (1) ESR is a general framework, applicable to a wide range of application semantics; (2) ESR is upward-compatible, since it reduces to SR as ! 0; and (3) ESR has number of eecient algorithms that support it. We brieey describe the concept of ESR, summarize the published work on ESR as well as ongoing research, outline an implementation on Transarc Encina, and sketch the current research eeorts. 1 Limitations of Serializability Serializability (SR) has been recognized as being too restrictive for even classic transaction processing. It is well known that many DB2 installations use level 2 consistency instead of level 3 (SR). Many optimization techniques have been used to relax SR, for example, Escrow Method 12] and the many proposals for semantics-based concurrency control. For new applications such as CAD/CAM, scientiic data management, and long transactions, the ACID properties 10] of SR can really get in the way. For example, long transactions usually want to relax atomicity, since atomically aborting the entire transaction would mean signiicant loss of work. Similarly, in Sagas 8] compensation transactions were used to alleviate the durability problems when transactions commit, but later we wish to undo their eeects. Applications that use level 2 consistency show that consistency is not always required. Finally, for collaborative work, where the exchange of intermediate information is desirable and necessary, the isolation property needs to be modiied. Many proposals have tried to weaken the ACID properties of classic transactions. Typically, they are not compatible with SR and this incompatibility limits the applicability of these proposals. Also, many of these proposals introduce too much run-time overhead compared to optimized classic transaction processing systems, making them less attractive. Epsilon Serializability (ESR) relaxes the atomicity, consistency, and isolation properties in a controlled way that makes it compatible with SR. In addition, ESR can be supported with very little additional overhead compared to classic transaction processing systems.