• The KIPS Transactions:PartD
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• v.8D no.3
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• pp.221-232
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• 2001

Due to rapid advance of wireless communication technology, demand on data services in mobile environment is gradually increasing. Caching at a mobile client could reduce bandwidth consumption and query response time, and yet a mobile client must maintain cache consistency. It could be efficient for the server to broadcast a periodic cache invalidation report for cache consistency in a cell. In case that long period of disconnection prevents a mobile client from checking validity of its cache based solely on the invalidation report received, the mobile client could request the server to check cache validity. In doing so, some schemes may be more efficient than others depending on the number of available channels and the mobile clients involved. In this paper, we propose new cache consistency schemes, effects, efficient especially (1) when channel capacity is enough to deal with the mobile clients involved or (2) when that is not the case, and evaluate their performance.

• The Journal of the Korea Contents Association
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• v.4 no.3
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• pp.71-80
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• 2004

In distributed file-systems, cooperative caching algorithm which owns the data cached at each node jointly is used to reduce an expense of disk access. Cooperative caching algorithm is the method that increases a cache hit-ratio and decrease a disk access as it holds the cache information of distributed systems in common and makes cache larger virtually. Recently, several cooperative caching algorithms decrease the message costs by using approximate information of the cache and increase the cache hit-ratio by using local and global cache fields dynamically. And they have an advantage that increases the whole field hit-ratio by sending a replaced buffer to the idle node on buffers replacement in order to maintain the replaced cache in the cache field. However the wrong approximate information deteriorates the performance, the consistency maintenance goes to great expense to exchange messages and the cost that manages Age-information of each node to choose the idle node increases. In this thesis, we propose a cooperative cache algorithm that maintains correct cache information, minimizes the maintenance cost for consistency and the management cost for buffer Age-information. Also, we show the superiority of our algorithm through the performance evaluation.

• The Journal of Information Technology
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• v.3 no.3
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• pp.117-134
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• 2000

Client-server DBMS based on a data-skipping model can exploit client resources effectively by allowing inter-transaction caching. However, inter-transaction caching raises the need of transactional cache consistency maintenance(TCCM) protocol, since each client is able to cache a portion of the database dynamically. Detection-based TCCM schemes can reduce the message overhead required for cache consistency if they validate clients replica asynchronously, and thus they cm show high throughput rates. However, they tend to show high ratios of transaction abort since transactions can access invalid replica. For coping with this drawback, this paper develops a new notion of shadow transaction, which is a backup-purpose one that is kept ready to replace an aborted transaction. This paper proposes a new detection-based TCCM scheme named DL-ST on the basis of the notion of shadow transaction. Using a simulation model, this paper evaluates the effect of shadow transaction in terms of transaction through rate and abort ratio.

• Journal of KIISE:Computer Systems and Theory
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• v.33 no.5
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• pp.282-296
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• 2006

In a wireless client server environment, data broadcasting is an efficient data dissemination method; a server broadcasts data, and some of broadcasted data are cached in a mobile client's cache to save the narrow communication bandwidth, limited resources, and data access time. A server also broadcasts invalidation reports to maintain the consistency between server data and a client's cached data. Most of existing works on the cache consistency problems simply purge the entire cache when the disconnection time is long enough to miss the certain amount(window size) of IRs. This paper presents a cache invalidation method to increase mobile clients' cache reusability in case of a long disconnection. Instead of simply dropping the entire cache regardless of its consistency, a client estimates the cost of purging all the data with the cost of selective purge. If the cost of dropping entire cache is higher, a client maintains the cache and selectively purge inconsistent data using uplink bandwidth for validation request. The simulation results show that this scheme increases the cache reusability since it effectively considers the update rates and the broadcast frequencies of cached data in estimating the cost of cache maintenance.

• The Transactions of the Korea Information Processing Society
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• v.7 no.8
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• pp.2310-2326
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• 2000

Client-server DBMS based on a data-shipping model can exploit e1ient resources effectively by allowing inter-transaction caching. However, inter-transaction caching raises the need of transactional cache consistency maintenancetTCCM protocol. since each client is able to cache a portion of the database dynamically. Deferred locking(DL) is a new detection-based TCCM scheme designed on the basis of a primary copy locking algorithm. In DL, a number of lock ,ujuests and a data shipping request are combined into a single message packet to minimize the communication overhead required for consistency checking. Lsing a simulation model. the performance of the prolxlsed scheme is compared with those of two representative detection based schemes, the adaptive optimistic concurrency control and the caching two-phase locking. The performance results indicate that DL improves the overall system throughput with a reasonable transaction abort ratio over other detection - based schemes.

• Proceedings of the Korea Contents Association Conference
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• 2003.11a
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• pp.234-244
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• 2003

In distributed file-systems, cooperative caching algorithm which owns the data cached at each node jointly is used to reduce an expense of disk access. Cooperative caching algorithm is the method that increases a cache hit-ratio and decrease a disk access as it holds the cache information of distributed systems in common and makes cache larger virtually. Recently, several cooperative caching algorithms decrease the message costs by using approximate information of the cache and increase the cache hit-ratio by using local and global cache fields dynamically. And they have an advantage that increases the whole field hit-ratio by sending a replaced block to the idel node on cache replacement in order to maintain the replaced block in the cache field. However the wrong approximate information deteriorates the performance, the concistency maintenance goes to great expense to exchange messeges and the cost that manages Age-information of each node to choose the idle node increases. In this thesis, we propose a cooperative cache algorithm that maintains correct cache information, minimizes the maintance cost for consistency and the management cost for cache Age-information. Also, we show the superiority of our algorithm through the performance evaluation.

• Journal of KIISE:Databases
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• v.32 no.3
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• pp.276-284
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• 2005

In the mobile computing environment, caching data at the client's side is a useful technique for improving the performance. For caching to be effective, the cache contents must be consistent with those stored in the server. That is, unfortunately, difficult to enforce due to the frequent disconnection and mobility of clients. In the literature, the basic approach adopted is for the sever to periodically broadcast invalidation reports(IRs) that contain information about objects that have been updated recently. However IR-based cache maintenance scheme has a main drawback. Any client who has been disconnected longer than w seconds has to discard all cached items even some of them may still be valid. In this paper, we propose two main schemes which can adapt clients' losing IR messages by long disconnection. First, the client sends its disconnection time when connection is re-established, and the server uses this information to decide the size of broadcast window w dynamically. Second, the server maintains IR messages for a given period of time and a client who has been disconnected longer than w seconds can verify its cache consistency individually. Compared to previous IR-based schemes, our schemes can significantly reduce cache missing and uplink request and improve the throughput by reducing response time.

• Proceedings of the Korea Information Processing Society Conference
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• 2003.05b
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• pp.779-782
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• 2003

이동 컴퓨팅 환경에서 이동 호스트가 사용하는 캐쉬는 이동 호스트의 자치성을 증대시킴으로서 전력의 한계와 제한된 대역폭이라는 단점을 보완할 수 있는 중요한 장치로 사용된다. 하지만 서버 데이터와의 일관성 유지를 해야하는 부담을 가지게 된다. 본 연구에서는 기존의 일관성 유지 기법이 가지는 불필요한 데이터의 방송 문제점을 보완하고 오랜 접속 단절 후에 연결된 이동 호스트에 대해서는 서버가 보관하고 있던 지난 방송 정보를 전달함으로서 대역폭을 효율적으로 이용하면서 이동 호스트의 자치성을 증대시키는 캐쉬 유지 기법을 제안한다.

• Journal of KIISE:Databases
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• v.28 no.1
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• pp.115-130
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• 2001

트랜잭션간 캐슁을 허용하는 데디터전송 시스템은 각 클라이언트가 데이터베이스의 일부분을 동적으로 캐슁하므로 트랜잭션 캐쉬 일관성 유지 기법의 필요성을 야기한다. 지연 로킹 기법은 클라이언트가 액세스한 데이터에 대한 로크 설정 및 유효성 검사가 비동기적으로 이루어지는 검사기반 기법이다. 지연 로킹 기법은 매우 낮은 통신부담을 보이므로 높은 성능을 발휘할 수 있지만, 트랜잭션 철회율이 높은 단점이 있다. 이 단점에 대처하기 위하여 본 논문에서는 철회되는 트랜잭션 대신에 실행되기 위하여 관리되는 백업 목적의 트랜잭션인 그림자 트랜잭션의 개념을 제안한다. 그리고 이 개념과 지연 로킹 기법을 통합하여 그림자 트렌잭션을 이용한 새로운 트랜잭션 캐쉬 일관성 유지 기법을 제안한다. 그리고 모의실험을 통하여 제안된 기법의 성능과 검사기반 기법 중 가정 대표적인 적응적 낙관적 동시성 제어 기법과 캐슁 두단계 로킹 기법과의 성능을 비교한다.

• Proceedings of the Korea Information Processing Society Conference
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• 2000.04a
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• pp.244-247
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• 2000

이동 통신 기술의 발전으로, 이동컴퓨팅 환경에서의 데이타 서비스에 대한 수요가 증가하고 있다. 이동 호스트 내에 캐쉬가 존재할 경우, 대역폭의 절약 및 빠른 응답시간을 제공할 수 있지만, 캐쉬 일관성을 유지해야하는 부담이 생긴다. 한 셀 내에 존재하는 수 많은 이동 호스트들의 캐쉬 일관성 유지를 위해서 서버(MSS)가 캐쉬 무효화 보고를 일정시간마다 주기적으로 방송하는 방법은 효과적일 수 있다. 기존에 제시된 무효화 보고에서는 갱신된 데이타의 식별자가 사용되지만, 좀 더 적은 양의 대역폭을 소모하는 효율적인 무효화 보고가 되기 위해서는 데이타들을 그룹핑할 수 있는 방안이 연구되어야 한다. 본 논문에서는 서버의 데이타베이스를 데이타의 식별자 값을 기준으로 여러 구역으로 나누었을 때, 데이타의 갱신이 데이타베이스 내 인근 구역에서 많이 발생할 경우, 이진 트리를 기반으로 하여 효율적인 무효화 보고를 생성하는 방법을 제안하고 그 성능을 평가한다.