Group Consistency for Read-only Transactions in Mobile Environments

In broadcast environments, communication bandwidth asymmetry is a distinguishing feature. The low bandwidth available for clients to communicate with the broadcast server makes it expensive and inefficient to maintain global serializability among all the transactions that execute at distributed mobile clients. In this paper, investigation is made to relax the requirements of global serializability among transactions while data consistency is still achieved. The NRS-Algorithm is proposed to process read-only transactions separately from update transactions to reduce the degree of data contention between the two kinds of transactions. Moreover, each read-only transaction running at the clients is guaranteed to read consistent data values autonomously without the need to do validation at the server. This is extremely beneficial to the concurrency control in broadcast environments where the upstream communication capacity is limited. To generalize the idea, two kinds of group consistency requirements for read-only transactions are defined. Group strong consistency requires a group of read-only transactions to be conflict serializable with respect to the update transactions. A weaker form of consistency, group view consistency, is also defined to allow a group of read-only transactions to be view serializable with respect to some update transactions. The NRS-Algorithm is then extended to preserve the two kinds of group consistency within a group of read-only transactions.