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。     

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.     

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。     

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.     

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.     

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.     

Performance analysis of location-dependent cache invalidation schemes for mobile environments

Mobile location-dependent information services are gaining increasing interest in both academic and industrial communities. In these services, data values depend on their locations. Caching frequently accessed data on mobile clients can help save wireless bandwidth and improve system performance. However, since client location changes constantly, location-dependent data may become obsolete not only due to updates performed on data items but also because of client movements across the network. To the best of the authors' knowledge, previous work on cache invalidation issues focused on data updates only. This paper considers data inconsistency caused by client movements and proposes three location-dependent cache invalidation schemes. The performance for the proposed schemes is investigated by both analytical study and simulation experiments in a scenario where temporal- and location-dependent updates coexist. Both analytical and experimental results show that, in most cases, the proposed methods substantially outperform the NSI scheme, which drops the entire cache contents when hand-off is performed.     

Data Consistency for Cooperative Caching in Mobile Environments

The trend toward wireless communications and advances in mobile technologies are increasing consumer demand for ubiquitous access to Internet-based information and services. A 3D framework provides a basis for designing, analyzing, and evaluating strategies to address data consistency issues in mobile wireless environments. A proposed relay-peer-based cache consistency protocol offers a generic and flexible method for carrying out cache invalidation     

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.     

A mobility-aware dynamic database caching scheme for wireless mobile computing and communications

This paper describes a mobility-aware dynamic database caching scheme for wireless mobile computing and communications. A mobile-floating agent scheme is proposed, in which caching techniques are cognizant of the mobile nature of mobile users and the location-sensitive nature of mobile systems. The mobile-floating agent maintains a second class cache in the fixed network and employs Barbara's invalidation reports broadcasting cache consistency strategies to maintain a dynamic cache consistent with the first class cache in the mobile client. The invalidation reports broadcasting scheme is combined with knowledge of the mobility behavior of each individual mobile user and broadcasts of invalidation reports only occur within the user's mobility area. The evaluation results show that, for a large system (200 cells), this scheme can reduce the system cost by more than 87%, for even highly mobile users, compared with a fully replicated database system.Recommended by: Daniel Barbara, Ravi Jain and Narayanan Krishnakumar     

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.     

Sleepers and workaholics: Caching strategies in mobile environments (Extended version)

In the mobile wireless computing environment of the future, a large number of users, equipped with low-powered palmtop machines, will query databases over wireless communication channels. Palmtop-based units will often be disconnected for prolonged periods of time, due to battery power saving measures; palmtops also will frequently relocate between different cells, and will connect to different data servers at different times. Caching of frequently accessed data items will be an important technique that will reduce contention on the narrow-bandwidth, wireless channel. However, cache individualization strategies will be severely affected by the disconnection and mobility of the clients. The server may no longer know which clients are currently residing under its cell, and which of them are currently on. We propose a taxonomy of different cache invalidation strategies, and study the impact of clients' disconnection times on their performance. We study ways to improve further the efficiency of the invalidation techniques described. We also describe how our techniques can be implemented over different network environments.     

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.     

Energy saving strategies for cooperative cache replacement in mobile ad hoc networks

Data caching on mobile clients is widely seen as an effective solution to improve system performance. In particular, cooperative caching, based on the idea of sharing and coordination of cache data among multiple users, can be particularly effective for information access in mobile ad hoc networks where mobile clients are moving frequently and network topology is changing dynamically. Most existing cache strategies perform replacement independently, and they seldom consider coordinated replacement and energy saving issues in the context of a mobile ad hoc network. In this paper, we analyse the impact of energy on designing a cache replacement policy and formulate the Energy-efficient COordinated cache Replacement Problem (ECORP) as a 0-1 knapsack problem. A dynamic programming algorithm called ECORP-DP and a heuristic algorithm called ECORP-Greedy are presented to solve the problem. Simulations, using both synthetic workload traces and real workload traces in our experiments, show that the proposed policies can significantly reduce energy consumption and access latency when compared to other replacement policies.     

A Scalable Asynchronous Cache Consistency Scheme (SACCS) for mobile environments

In the literature, there exit two types of cache consistency maintenance algorithms for mobile computing environments: stateless and stateful. In a stateless approach, the server is unaware of the cache contents at a mobile user (MU). Even though stateless approaches employ simple database management schemes, they lack scalability and ability to support user disconnectedness and mobility. On the other hand, a stateful approach is scalable for large database systems at the cost of nontrivial overhead due to server database management. We propose a novel algorithm, called Scalable Asynchronous Cache Consistency Scheme (SACCS), which inherits the positive features of both stateless and stateful approaches. SACCS provides a weak cache consistency for unreliable communication (e.g., wireless mobile) environments with small stale cache hit probability. It is also a highly scalable algorithm with minimum database management overhead. The properties are accomplished through the use of flag bits at the server cache (SC) and MU cache (MUC), an identifier (ID) in MUC for each entry after its invalidation, and estimated time-to-live (TTL) for each cached entry, as well as rendering of all valid entries of MUC to uncertain state when an MU wakes up. The stale cache hit probability is analyzed and also simulated under the Rayleigh fading model of error-prone wireless channels. Comprehensive simulation results show that the performance of SACCS is superior to those of other existing stateful and stateless algorithms in both single and multicell mobile environments.     

一种基于移动环境的语义缓存一致性维护技术

在深入研究缓存失效广播技术和语义缓存的基础上，提出了一种新的基于移动环境的语义缓存一致性维护技术——基于语义缓存的异步有状态（BSCAS）技术。BSCAS技术可以支持移动客户的各种断接方式，减少无线通信的开销，让移动客户有更好的自治性。     

移动数据库技术的缓存管理策略研究

移动数据库的缓存技术使客户机通过缓存少量的服务器数据项,便能拥有较高的处理速度,而且,它也是解决移动数据库频繁断接性问题的关键技术。文章研究了缓存管理策略在移动数据库中的应用,重点研究了缓存管理策略三大关键问题：缓存粒度、缓存一致性以及缓存替换,提出了一种基于统计与优先级原则解决缓存一致性问题的新思路。     

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.     

A scalable Web cache sharing scheme

A new Web cache sharing scheme is presented. Our scheme reduces the duplicated copies of the same objects in global shared Web caches. It also reduces the message overhead of existing schemes significantly. Trace-driven simulations with actual Web cache logs show that the proposed scheme performs better than the two well-known Web cache sharing schemes, the Internet Cache Protocol and the Cache Array Routing Protocol.     

Compiler-directed cache management in multiprocessors

The necessity of finding alternatives to hardware-based cache coherence strategies for large-scale multiprocessor systems is discussed. Three different software-based strategies sharing the same goals and general approach are presented. They consist of a simple invalidation approach, a fast selective invalidation scheme, and a version control scheme. The strategies are suitable for shared-memory multiprocessor systems with interconnection networks and a large number of processors. Results of trace driven simulations conducted on numerical benchmark routines to compare the performance of the three schemes are presented