Speculation-based protocols for improving the performance of read-only transactions

In the emerging web databases and e-commerce scenario, information systems have to meet intensive information requirements from a large number of users. These information systems receive both update transactions (UTs) and read-only transactions (ROTs). A UT contains both read and write operations and an ROT contains only read operations. Designing efficient protocols to process ROTs is a research issue. There are three aspects for designing a concurrency control protocol for processing ROTs: correctness, data currency and performance. The protocol should process transactions by satisfying serializability criteria. Also, the protocol should process transactions without any data currency issues. Note that “data currency” is another important aspect of the protocol. A protocol should process the ROTs without missing the effect of any preceding UTs. Finally, the protocol should give high performance. If both the UTs and ROTs are processed with the popular two-phase locking (2PL) protocol, the performance degrades with the data contention as the UTs have to wait for ROTs and vice versa in case of a conflict. On the other hand, it can be noted that, the 2PL processes ROTs without any correctness and data currency issues. So, efforts are being made to propose improved protocols by exploiting the special property of ROT that “it does not modify the data” and by compromising on correctness and data currency. There are efforts to improve the performance by processing ROTs with multi-version based approaches, at lower isolation levels, and by compromising correctness. One of the popular protocols is snapshot isolation (SI)-based protocol, in which an ROT reads the snapshot of the database ignoring modifications done by preceding UTs. In SI-based protocols, even though performance is improved, both correctness and data currency aspects are compromised. The 2PL protocol suffers from the performance problems and SI-based protocol suffers from both correctness and data currency problems. So, the research challenge is to develop efficient concurrency control protocols to improve the performance of ROTs without correctness and data currency problems. In this thesis, we have proposed speculationand semantics-based protocols for processing ROTs that provide high performance and process transactions without any correctness and data currency issues. Speculation-based protocols have been proposed in the literature to improve the performance of UTs. Basically, under speculation, a transaction carries out multiple executions by reading the uncommitted values of preceding conflicting transactions. A transaction retains one of the speculative executions based on the termination decisions of preceding transactions. Speculation improves the parallelism but requires extra processing resources. Also, the number of speculative executions and number of object versions explode with data contention. In this thesis, the notion of speculation is being extended to propose improved protocols for processing ROTs. In the proposed protocols, ROTs are processed with speculation and UTs are processed with 2PL. The ROTs access after-images produced by UTs and carry out speculative executions. As a result, the following advantages have been identified: (i) an ROT can carry out speculative executions and commit independently based on the status of preceding UTs, and (ii) the number of speculative executions and data versions is significantly reduced. We have proposed two speculation-based protocols for processing ROTs. In the synchronous approach, the speculative executions of an ROT can be carried out in a synchronous manner, i.e., all the speculative executions of an ROT are carried out at the same pace. In the asynchronous approach, the speculative executions of an ROT are carried out at different pace. It was observed that asynchronous approach improves the performance over the synchronous approach. In the proposed speculation-based protocols for ROTs, UTs follow 2PL and are made to wait if they conflict with concurrent UTs and ROTs. To reduce the waiting time of UTs, we have proposed a semanticsbased approach by proposing the notion of “compensatability”. In this protocol, a UT need not wait for an ROT, if the computation of that ROT is compensatable. However, the compensatable ROT has to perform compensating computations before its commitment. We have proposed enhanced protocols by extending semantics to the proposed speculation-based protocols. The proposed protocols bring significant benefits. The ROTs are processed without violating correctness and compromising data currency. The performance results show that the proposed protocols significantly improve the performance over 2PL and SI-based protocols with manageable extra processing resources.