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

Different isolation levels are required to ensure various degrees of data consistency and currency to read-only transactions. Current definitions of isolation levels like Conflict Serializability, Update Serializability or External Consistency/Update Consistency are not appropriate for processing read-only transactions since they lack any currency guarantees. To correct the problem, we propose four new isolation levels that 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. The results show that the costs of providing serializability to read-only transactions relative to other protocols that provide weaker consistency guarantees are moderate if read-only transactions are required to observe the updates of transactions that committed after their starting points as long as the serializability requirement is not violated.