移动计算环境中缓存失效策略的归类研究法

缓存失效报告法是在移动计算环境中普遍采用的缓存方案，即服务器定期广播缓存失效报告，该报告中包含了最近被更新的对象，根据报告内容，客户可使缓存中被更新对象失效，以保证缓存的有效性。缓存失效报告技术可采用多种策略，本文在研究多种缓存策略的基础上，提出了一种针对缓存失效方案的归类研究法，可对具体策略加以分析和评估，在此基础上对其性能进行改良，以形成更适应特定移动计算环境要求的缓存方案。     

动态调整移动数据库中失效报告时间窗口的大小

在移动计算环境中，如何维护移动客户机中数据缓存的一致性，是研究的一个热点．广播失效报告是一种有效的方法，但由于不同的环境，不同的时期各种参数的变化，失效报告时间窗口ω的大小如何确定是一个难点．时间窗口ω应随各种参数的变化而不断地进行调整，才能保证网络系统的传输工作在最有效的状态．从理论上推导了失效报告时间窗口ω的计算公式，分析了ω的合理取值，并给出了具体实现的算法．     

A scalable low-latency cache invalidation strategy for mobile environments

Caching frequently accessed data items on the client side is an effective technique for improving performance in a mobile environment. Classical cache invalidation strategies are not suitable for mobile environments due to frequent disconnections and mobility of the clients. One attractive cache invalidation technique is based on invalidation reports (IRs). However, the IR-based cache invalidation solution has two major drawbacks, which have not been addressed in previous research. First, there is a long query latency associated with this solution since a client cannot answer the query until the next IR interval. Second, when the server updates a hot data item, all clients have to query the server and get the data from the server separately, which wastes a large amount of bandwidth. In this paper, we propose an IR-based cache invalidation algorithm, which can significantly reduce the query latency and efficiently utilize the broadcast bandwidth. Detailed analytical analysis and simulation experiments are carried out to evaluate the proposed methodology. Compared to previous IR-based schemes, our scheme can significantly improve the throughput and reduce the query latency, the number of uplink request, and the broadcast bandwidth requirements.     

在移动计算环境中基于移动代理的缓存失效方案

1 引言缓存技术是分布式计算环境中的重要技术,它可以改善系统的整体性能(如查询响应时间、吞吐量等),而移动计算的网络环境是一种特殊的分布式环境,与传统的分布式系统相比,它具有鲜明的特点:移动性、断接性、带宽多样性、可伸缩性、弱可靠性、网络通信的非对称性、电源能力局限性等等。这些特点使得缓存技术在移动计算环境中尤为重要。因为缓存能有效减少带宽需求,并能节省移动计算机的能耗。     

A cache invalidation strategy for mobile networks

The diversity of services delivered over wireless channels has increased people's desire in ubiquitously accessing these services from their mobile devices. However, a ubiquitous mobile computing environment faces several challenges such as scarce bandwidth, limited energy resources, and frequent disconnection of the server and mobile devices. Caching frequently accessed data is an effective technique to improve the network performance because it reduces the network congestion, the query delay, and the power consumption. When caching is used, maintaining cache consistency becomes a major challenge since data items that are updated on the server should be also updated in the cache of the mobile devices. In this paper we propose a new cache invalidation scheme called Selective Adaptive Sorted (SAS) cache invalidation strategy that overcomes the false invalidation problem that exists in most of the invalidation strategies found in the literature. The performance of the proposed strategy is evaluated and compared with the selective cache invalidation strategy and the updated invalidation report startegy found in the literature. Results showed that a significant cost reduction can be obtained with the proposed strategy when measuring performance metrics such as delay, bandwidth, and energy.     

Gossip-based cooperative caching for mobile applications in mobile wireless networks

Cooperative caching is an efficient way to improve the performance of data access in mobile wireless networks, by cache nodes selecting different data items in their limited storage in order to reduce total access delay. With more demands on sharing a video or other data, especially for mobile applications in an Internet-based Mobile Ad Hoc Network, considering the relations among data items in cooperative caching becomes more important than before. However, most of the existing works do not consider these inherent relations among data items, such as the logical, temporal, or spatial relations. In this paper, we present a novel solution, Gossip-based Cooperative Caching (GosCC) to address the cache placement problem, and consider the sequential relation among data items. Each mobile node stores the IDs of data items cached locally and the ID of the data item in use into its progress report. Each mobile node also makes use of these progress reports to determine whether a data item should be cached locally. These progress reports are propagated within the network in a gossip-based way. To improve the user experience, GosCC aims to provide users with an uninterrupted data access service. Simulation results show that GosCC achieves better performance than Benefit-based Data Caching and HybridCache, in terms of average interruption intervals and average interruption times, while sacrificing message cost to a certain degree.     

Energy-Efficient Mobile Cache Invalidation

Caching data in a wireless mobile computer can significantly reduce the bandwidth requirement. However, due to battery power limitation, a wireless mobile computer may often be forced to operate in a doze or even totally disconnected mode. As a result, the mobile computer may miss some cache invalidation reports. In this paper, we present an energy-efficient cache invalidation method for a wireless mobile computer. The new cache invalidation scheme is called grouping with cold update-set retention (GCORE). Upon waking up, a mobile computer checks its cache validity with the server. To reduce the bandwidth requirement for validity checking, data objects are partitioned into groups. However, instead of simply invalidating a group if any of the objects in the group has been updated, GCORE retains the cold update set of objects in a group if possible. We present an efficient implementation of GCORE and conduct simulations to evaluate its caching effectiveness. The results show that GCORE can substantially improve mobile caching by reducing the communication bandwidth (thus energy consumption) for query processing.     

基于多时间窗口的移动数据库失效报告技术的研究

如何维护移动环境下的客户端缓存中数据的一致性,是移动数据库中的关键技术.而数据广播技术则是利用无线通信网络不对称的特点,使移动客户端数据和服务器端数据保持一致性的最为实用的技术.但由于不同的环境,不同的时段各种参数的变化,失效报告时间窗口ω的大小如何确定是一个难点.根据移动数据库中数据更新的时间间隔,提出了基于多时间窗口的失效报告技术.     

Scalable cache invalidation algorithms for mobile data access

In this paper, we address the problem of cache invalidation in mobile and wireless client/server environments. We present cache invalidation techniques that can scale not only to a large number of mobile clients, but also to a large number of data items that can be cached in the mobile clients. We propose two scalable algorithms: the Multidimensional Bit-Sequence (MD-BS) algorithm and the Multilevel Bit-Sequence (ML-BS) algorithm. Both algorithms are based on our prior work on the Basic Bit-Sequences (BS) algorithm. Our study shows that the proposed algorithms are effective for a large number of cached data items with low update rates. The study also illustrates that the algorithms ran be used with other complementary techniques to address the problem of cache invalidation for data items with varied update and access rates.     

An evaluation of cache invalidation strategies in wirelessenvironments

Caching can reduce the bandwidth requirement in a wireless computing environment as well as minimize the energy consumption of wireless portable computers. To facilitate mobile clients in ascertaining the validity of their cache content, servers periodically broadcast cache invalidation reports that contain information of data that has been updated. However, as mobile clients may operate in a doze or even totally disconnected mode (to conserve energy), it is possible that some reports may be missed and the clients are forced to discard the entire cache content. In this paper, we reexamine the issue of designing cache invalidation strategies. We identify the basic issues in designing cache invalidation strategies. From the solutions to these issues, a large set of cache invalidation schemes can be constructed. We evaluate the performance of four representative algorithms-two of which are known algorithms (i.e., Dual-Report Cache Invalidation and Bit-Sequences) while the other two are their counterparts that exploit selective tuning (namely, Selective Dual-Report Cache Invalidation and Bit-Sequences with Bit Count). Our study shows that the two proposed schemes are not only effective in salvaging the cache content but consume significantly less energy than their counterparts. While the Selective Dual-Report Cache Invalidation scheme performs best in most cases, it is inferior to the Bit-Sequences with the Bit-Count scheme under high update rates     

Cache invalidation strategies for Internet-based vehicular ad hoc networks

Internet-based vehicular ad hoc network (Ivanet) is an emerging technique that combines a wired Internet and a vehicular ad hoc network (Vanet) for developing an ubiquitous communication infrastructure and improving universal information and service accessibility. A key design optimization technique in Ivanets is to cache the frequently accessed data items in a local storage of vehicles. Since vehicles are not critically limited by the storage/memory space and power consumption, selecting proper data items for caching is not very critical. Rather, an important design issue is how to keep the cached copies valid when the original data items are updated. This is essential to provide fast access to valid data for fast moving vehicles. In this paper, we propose a cooperative cache invalidation (CCI) scheme and its enhancement (ECCI) that take advantage of the underlying location management scheme to reduce the number of broadcast operations and the corresponding query delay. We develop an analytical model for CCI and ECCI techniques for fasthand estimate of performance trends and critical design parameters. Then, we modify two prior cache invalidation techniques to work in Ivanets: a poll-each-read (PER) scheme, and an extended asynchronous (EAS) scheme. We compare the performance of four cache invalidation schemes as a function of query interval, cache update interval, and data size through extensive simulation. Our simulation results indicate that the proposed schemes can reduce the query delay up to 69% and increase the cache hit rate up to 57%, and have the lowest communication overhead compared to the prior PER and EAS schemes.     

A concurrency control scheme for mobile transactions in broadcast disk environments

Broadcast disk technique has been often used to disseminate frequently requested data efficiently to a large volume of mobile clients over wireless channels. In broadcast disk environments, a server often broadcasts different data items with differing frequencies to reflect the skewed data access patterns of mobile clients. Previously proposed concurrency control methods for mobile transactions in wireless broadcast environments are focused on the mobile transactions with uniform data access patterns. These protocols perform poorly in broadcast disk environments where the data access patterns of mobile transactions are skewed. In broadcast disk environments, the time length of a broadcast cycle usually becomes large to reflect the skewed data access patterns. This will often cause read-only transactions to access old data items rather than the latest data items. Furthermore, updating mobile transactions will be frequently aborted and restarted in the final validation stage due to the update conflict of the same data items with high access frequencies. This problem will increase the average response time of the update mobile transactions and waste the uplink communication bandwidth. In this paper, we extend the existing FBOCC concurrency control method to efficiently handle mobile transactions with skewed data access patterns in broadcast disk environments. Our method allows read-only transactions to access the more updated data, and reduces the average response time of updating transactions through early aborts and restarts. Our method also reduces the amount of uplink communication bandwidth for the final validation of the update transactions. We present an in-depth experimental analysis of our method by comparing with existing concurrency control protocols. Our performance analysis shows that it significantly decreases the average response time and the amount of uplink bandwidths over existing methods.     

Concurrency control strategies for ordered data broadcast in mobile computing systems

Although data broadcast has been shown to be an efficient method for disseminating data items in mobile computing systems, the issue on how to ensure consistency and currency of data items provided to mobile transactions (MT), which are generated by mobile clients, has not been examined adequately. While data items are being broadcast, update transactions may install new values for them. If the executions of update transactions and the broadcast of data items are interleaved without any control, mobile transactions may observe inconsistent data values. The problem will be more complex if the mobile clients maintain some cached data items for their mobile transactions. In this paper, we propose a concurrency control method, called ordered update first with order (OUFO), for the mobile computing systems where a mobile transaction consists of a sequence of read operations and each MT is associated with a time constraint on its completion time. Besides ensuring data consistency and maximizing currency of data to mobile transactions, OUFO also aims at reducing data access delay of mobile transactions using client caches. A hybrid re-broadcast/invalidation report (IR) mechanism is designed in OUFO for checking the validity of cached data items so as to improve cache consistency and minimize the overhead of transaction restarts due to data conflicts. This is highly important to the performance of the mobile computing systems where the mobile transactions are associated with a deadline constraint on their completion times. Extensive simulation experiments have been performed to compare the performance of OUFO with two other efficient schemes, the multi-version broadcast method and the periodic IR method. The performance results show that OUFO offers better performance in most aspects, even when network disconnection is common.     

An energy efficient re-access scheme for data caching in data broadcast of a mobile computing environment

Data caching is used to improve the response time and the power consumption of a mobile client in a mobile computing environment. To enhance the performance of data caching, one needs to improve the hit ratio and to reduce the cost in processing a cache miss. In a mobile computing environment, a cached data item of a mobile client needs to remain up-to-date with respect to its corresponding data item in the server. A cached data item which is out of date is called a cached invalidated data item. Accessing a cached invalidated data item can be regarded as processing a cache miss. To access a cached invalidated data item, a mobile client needs to download the new content of the data item from the broadcast channel. This operation is called a re-access operation in this paper. Re-accessing a cached invalidated data item incurs large tuning time overhead. In this paper, we propose a re-access scheme that reduces this overhead by allowing a mobile client to access a cached invalidated data item from the broadcast channel without accessing indices. We analyze the performance of the proposed scheme and validate the analysis through experiments. The experiments showed that the proposed scheme significantly reduces the tuning time of a mobile client. Furthermore, the proposed scheme is robust in the sense that it allows changes on the broadcast structure in data broadcasting.     

A cache invalidation scheme for continuous partial match queries in mobile computing environments

The continuous partial match query is a partial match query whose result remains consistently in the client’s memory. Conventional cache invalidation methods for mobile clients are record ID-based. However, since the partial match query uses content-based retrieval, the conventional ID-based approaches cannot efficiently manage the cache consistency of mobile clients. In this paper, we propose a predicate-based cache invalidation scheme for continuous partial match queries in mobile computing environments. We represent the cache state of a mobile client as a predicate, and also construct a cache invalidation report (CIR), which the server broadcasts to clients for cache management, with predicates. In order to reduce the amount of information that is needed for cache management, we propose a set of methods for CIR construction (in the server) and identification of invalidated data (in the client). Through experiments, we show that the predicate-based approach is very effective for the cache management of mobile clients.     

Broadcast-based group invalidation: An energy-efficient cache invalidation strategy

In a wireless environment, mobile clients may cache frequently accessed data to reduce the contention on the narrow bandwidth of the wireless channel. However, to minimize energy consumption, mobile clients may also often operate in a disconnected mode. As a result, the clients may miss some cache invalidation reports broadcast by a server. Thus, upon reconnection, a cache invalidation scheme must be employed to ensure the validity of the cached data. Existing techniques either require the clients to discard the cached data entirely, or require the clients to transmit uplink messages to a server. While the former eliminates the benefits of caching, the latter may lead to high energy consumption, poor channel utilization, and costs. In this paper, we present a new cache invalidation scheme, called Broadcast-Based Group Invalidation (BGI), that retains the benefits of caching while avoiding unnecessary transmissions (which translates to energy saving, better channel utilization, and lower costs). Under BGI, a pair of invalidation reports is broadcast periodically. While the object invalidation report enables the clients to salvage as many recently cached objects as possible, the group invalidation report cuts down on false invalidation. We conduct extensive studies based on a simulation model. The simulation results show that BGI consumes less energy and is superior over existing techniques.     

一种用户任务级别上的数据库入侵检测方法

通常，更新数据库中一个数据项之前可能读或者写一些数据项，更新一个数据项后，可能也有一些数据项被更新。依据数据之间的这种依赖关系，本文提出一种用户任务级别 的数据依赖挖掘算法，从数据库日志中挖掘出用户任务事务中存在的数据依赖关系。如果在检测阶段发现同一用户任务中有事务与挖掘出的数据依赖关系不符合，则可判断此
此用户任务中有异常活动出现。     

Information access and QoS issues in a mobile computing environment

Efficient access to coherent information files is an important issue in mobile computing. In this paper we consider performance analysis of information access in a mobile computing environment. We employ a distributed directory scheme that permits uninterrupted use under disconnected and low bandwidth connections and minimizes invalidation and false sharing. This scheme allows for two invalidation schemes that avoid false sharing, and a mechanism for efficient maintenance of file details. We analyse the performance of the directory scheme in terms of time taken and energy consumed in executing operations such as read/write, connection, disconnection, and hoarding in mobile environments. The directory scheme is employed for facilitating efficient transfer of partially updated files in hierarchical networks. We also investigate quality of service issues related to file accesses in the mobile computing environment discussed in the paper. However, the scheme can be extended to any mobile computing environment.     

Streaming data reduction using low-memory factored representations

Many special purpose algorithms exist for extracting information from streaming data. Constraints are imposed on the total memory and on the average processing time per data item. These constraints are usually satisfied by deciding in advance the kind of information one wishes to extract, and then extracting only the data relevant for that goal. Here, we propose a general data representation that can be computed using modest memory requirements with limited processing power per data item, and yet permits the application of an arbitrary data mining algorithm chosen and/or adjusted after the data collection process has begun. The new representation allows for the at-once analysis of a significantly larger number of data items than would be possible using the original representation of the data. The method depends on a rapid computation of a factored form of the original data set. The method is illustrated with two real datasets, one with dense and one with sparse attribute values.     

Efficient cache invalidation schemes for mobile data accesses

This paper presents two cache invalidation schemes to maintain data consistency between the server and clients in a mobile environment. Designed according to real situations like MANET, the Adaptive Data Dividing (ADD) scheme divides data into groups of different utilization ratios and varies group broadcast intervals to reduce data access time and bandwidth consumption. The Data Validity Defining (DVD) scheme aims to solve the validity problem of cached data items, which usually happens after clients disconnect from the server. Experimental evaluations and performance analyses exhibit that the two schemes outperform most existing cache invalidation schemes in terms of data access time, cache miss ratios, query uplink ratios and bandwidth consumption.