The mobile computing system based on wireless communication technology relaxes constraints regarding to time and space in data use to a degree that users are able to access data objects stored in remote database sites wherever they are and even while they are moving through wireless media at any time. In this paper, we propose a new locking scheme for transaction scheduling in client-server DBMS architecture for such a mobile computing environment. In the mobile computing environment, all messages between clients and a server are accomplished through wireless channel, however, are frequently delayed and unpredictably disconnected due to the adverse characteristics of the wireless channel, e.g., narrow bandwidth and unreliableness. As a result, the previous client-server locking schemes based on two-phase locking(2PL) and generally shown a excellent performance could suffer from serious performance degradation. It is because the probability of communication delay could be increased due to a large number of message exchanges for locking process and transaction concurrency could be limited in the case of long lock-holding time resulting from such a delay in the data transfer for a locking (or unlocking) process.
In this thesis, we proposed 2PL with commitment prioritization (2PL/CP) for mobile database systems. 2PL/CP minimizes the number of messages needed for locking process and improves transaction concurrency by (1) allowing a transaction Ti to be committed while other transactions are reading the data that will be updated by Ti, (2) letting read lock be compatible with write lock and delaying read-write conflict check up to the commit time, (3) does not nearly consider read-only transactions in transaction scheduling.
We examined the performance evaluation of 2PL/CP compared to the previous representative locking schemes. Our experimental results show that our scheme of 2PL/CP outperforms those schemes under a wide variety of write ratios and the number of clients.
데이타 접근능력과 초시간성이 최대화되어질 수 있는 이동환경하의 이동자료기지체계(MDBS)의 기본구조로 클라이언트-서버 방식이 추천되어진다. 이러한 접근방식을 위해 여러 로킹 기법이 트랜잭션 동시성 제어 기법으로서 제안되었으며 그것들의 대부분은 캐쉬 일관성을 유지하기 위해 사본 제어 기법을 가진 두단계로킹(2PL)을 사용한다. 그러한 기법들은 정적 클라이언트-서버 DBMS구조에서는 좋은 성능을 보이지만 MDBS에서는 여러 역요인, 예를 들어 협소한 대역폭과 신뢰할 수 없는 무선 채널에 의한 자료 전송 지연으로 인해 심각한 성능 감소를 보일 수 있다. 우리는 앞서 발표된 로킹 기법이 이동 환경에 직접 적용되어질 때의 문제점을 제시하고. 트랜잭션 스케쥴링에서 적은 수의 메세지 통신으로 역효과를 최소화하는 새로운 로킹기법인 CT2PL을 제안한다. CT2PL은 (1)선행 트랜잭션의 지연으로 인한 역영향없이 완료 요구가 도착하는 순서에 따라 트랜잭션들을 완료함 (2)읽기로크와 쓰기로크를 양립시킴 (3)트랜잭션 스케쥴링에서 읽기전용 로크를 제외함으로써 트랜잭션 동시성을 향상시킨다.
