Processing Read-Only Transactions in Hybrid Data Delivery Environments with Consistency and Currency Guarantees

摘要

Different isolation levels are required to ensure various degrees of data consistency and currency to read-only transactions. Current definitions of isolation levels such as Conflict Serializability, Update Serializability or External Consistency/Update Consistency are not appropriate for processing read-only transactions since they lack any currency guarantees. To resolve this problem, we propose four new isolation levels which incorporate data consistency and currency guarantees. Further, we present efficient implementations of the proposed isolation levels. Our concurrency control protocols are envisaged to be used in a hybrid mobile data delivery environment in which broadcast push technology is utilized to disseminate database objects to a large number of mobile clients and conventional point-to-point technology is applied to satisfy on-demand requests. The paper also presents the results of a simulation study conducted to evaluate the performance of our protocols. According to the simulation results the costs imposed by the MVCC-SFBS protocol, which ensures Serializability to read-only transactions are moderate relative to those imposed by the MVCC-SFBUS and MVCC-SFBVC protocols, which provide weaker consistency guarantees. A comparison study reveals that the MVCC-SFBVC scheme outperforms all other investigated concurrency control schemes suitable for mobile database systems.