Distributed Lock Management for Mobile Transactions

In a mobile computing environment, a user carrying a portable computer can execute a mobile transaction by submitting the operations of the transaction to distributed data servers from different locations. The mobility of transaction hosts introduces new issues concerning the efficiency of transaction lock management in the distributed data servers. For example, even in a fully replicated database environment, read locks of a transaction may be executed at different servers, because these operations may be submitted from different locations. The distribution of read locks implies that extra messages are required to release these locks when the mobile host decides to commit the transaction. In this paper, we present a new lock management scheme which allows a read unlock for an item to be executed at any copy site of that item; the site may be different from the copy site on which the read lock is set. The scheme, therefore, utilizes the replicated copies of data items to reduce the message costs incurred by the mobility of the transaction host. We demonstrate this idea in an optimistic locking algorithm called 02PL-MT. Like its counterpart algorithm 02PL, presented in [4] for a conventional distributed database system, 02PL-MT grants read locks immediately on demand and defers write locks until the commitment time. However, 02PL-MT requires the transmission of fewer messages than 02PL in a mobile environment in which data items are replicated. The idea presented in this paper can also be used to improve the efficiency of other distributed lock protocols (e.g., pessimistic locking) in a mobile environment, if the number of read operations dominates that of write operations.