分布事务管理器的性能优化

作为应用服务器重要组成部分的分布管理器主要用于提供分布环境下的事务处理能力，它的性能是制约应用服务器上分布事务吞吐能力的重要因素。讨论了在J2EE应用服务器框架下如何进行分布事务管理器的性能优化，同时给出了分布事务管理器ISTX的性能优化实现。实验数据表明，对ISTX的性能优化能在保证分布事务可靠性的前提下，较大幅度地提高了分布事务处理的性能。     

JPernLite: extensible transaction services for the WWW

Concurrency control is one of the key problems in design and implementation of collaborative systems such as hypertext/hypermedia systems, CAD/CAM systems, and software development environments. Most existing systems store data in specialized databases with built-in concurrency control policies, usually implemented via locking. It is desirable to construct such collaborative systems on top of the World Wide Web, but most Web servers do not support even conventional transactions, let alone distributed (multi-Website) transactions or flexible concurrency control mechanisms oriented toward team work-such as event notification, shared locks, and fine granularity locks. We present a transaction server that operates independently of Web servers or the collaborative systems, to fill the concurrency control gap. By default, the transaction server enforces the conventional atomic transaction model, where sets of operations are performed in an all-or-nothing fashion and isolated from concurrent users. The server can be tailored dynamically to apply more sophisticated concurrency control policies appropriate for collaboration. The transaction server also supports applications employing information resources other than Web servers, such as legacy databases, CORBA objects, and other hypermedia systems. Our implementation permits a wide range of system architecture styles     

Communication facilities for distributed transaction-processingsystems

The study of communication designs in the context of the Raid system, a robust and adaptable distributed database system for transaction processing, is discussed. Related research work on local interprocess communication, remote interprocess communication, and communication protocols for both local area and wide area networks is briefly summarized. A series of experiments on the performance of the facilities available for building the Raid communication software is described. Raid's communication software, called Raidcomm, has evolved as a result of the knowledge gained both from other systems and from the authors' experiments. Several communication services and mechanisms that can be used to make Raid efficient are identified     

Fault detection for Byzantine quorum systems

In this paper, we explore techniques to detect Byzantine server failures in asynchronous replicated data services. Our goal is to detect arbitrary failures of data servers in a system where each client accesses the replicated data at only a subset (quorum) of servers in each operation. In such a system, some correct servers can be out-of-date after a write and can therefore, return values other than the most up-to-date value in response to a client's read request, thus complicating the task of determining the number of faulty servers in the system at any point in time. We initiate the study of detecting server failures in this context, and propose two statistical approaches for estimating the risk posed by faulty servers based on responses to read requests     

A paracasting model for concurrent access to replicated Internet content

In this paper, we develop a model to study how to effectively download a document from a set of replicated servers. We propose a generalized application-layer anycasting protocol, known as paracasting, to advocate concurrent access of a subset of replicated servers to cooperatively satisfy a client's request. Each participating server satisfies the request in part by transmitting a subset of the requested file to the client. The client can recover the complete file when different parts of the file sent from the participating servers are received. This model allows us to estimate the average time to download a file from the set of homogeneous replicated servers, and the request blocking probability when each server can accept and serve a finite number of concurrent requests. Our results show that the file download time drops when a request is served concurrently by a larger number of homogeneous replicated servers, although the performance improvement quickly saturates when the number of servers increases. If the total number of requests that a server can handle simultaneously is finite, the request blocking probability increases with the number of replicated servers used to serve a request concurrently. Therefore, paracasting is effective when a small number of servers, say, up to four, are used to serve a request concurrently.     

支持复杂事务模式的Web应用服务器复制机制

当前普遍采用的复制技术和事务处理技术都无法满足应用的End-to-End可靠性需求,前者通过前向错误恢复来保证应用操作的存活性,后者通过后向错误恢复来保证应用数据的安全性.如何融合这两种技术以实现End-to-End可靠性保证,成为目前研究的热点问题.然而,已有的方法都是基于简单事务模式的假设,即只有中间层应用服务器上的容器发起事务,而很少考虑应用中普遍存在的复杂事务模式,如客户事务和嵌套事务.为了解决这个问题,首先识别出了几种典型的事务模式.针对这些事务模式,基于状态同步点概念提出了一种能够统一提供End-to-End可靠性保证的Web应用服务器复制机制RSCTP(replication scheme for complex transaction pattern).RSCTP机制采取primary-backup方式来复制EJB组件以保证业务逻辑的高可用性,同时采取primary-backup方式复制事务协调者来消除分布式事务处理中两阶段提交协议可能出现的阻塞问题.通过在不同事务模式下的失效分析,说明了该机制的有效性.已经实现了RSCTP机制并集成到了遵循J2EE规范的Web应用服务器OnceAS中.性能评价显示,该机制带来的系统开销较小.     

Java based replicated server objects and their remote method invocations in a distributed environment

A distributed system is said to be fault‐tolerant if it is able to provide important services despite partial failures of the computers or software objects in the system. These systems are needed to support applications such as remote access and control, virtual mobile offices and wide area collaborative systems where there are chances of failures in the network and software objects. Fault‐tolerance is usually achieved by replicating the objects in the system. Traditional distributed applications constructed using Java RMI (remote method invocation) are not fault‐tolerant because of the lack of support of object replication. The objective of the present work is to design a remote method invocation that supports server object replication. To provide a fault‐tolerant service to the remote client objects, server objects are actively replicated. The problems associated with the method invocation in the context of active server object replication are presented and solutions are discussed and implemented. Copyright © 2001 John Wiley & Sons, Ltd.     

WCSR:一个弱一致性的复制数据库系统

介绍了一个弱一致性的复制数据库系统ＷＣＳＲ。ＷＣＳＲ系统支持服务的移动性与断接,采用Ｒｅａｄ－Ａｎｙ－Ｗｒｉｔｅ－Ａｎｙ复制范型,通过周期同步方式维护系统的一致性,     

Utility servers in charlotte

Most distributed operating systems are built with a kernel replicated in each machine that supports only basic interprocess communication (IPC) and process control. All other system services, such as memory management, file system, and name service, are distributed in a set of utility servers, which are ordinary processes (except perhaps for some privileges) residing at various machines. Design and implementation of such utility servers in distributed environments are far different from those in a centralized system. This paper presents our experience in building utility servers in Charlotte, a message-based distributed operating system running on a loosely-coupled multicomputer. Utility services in Charlotte are provided by server squads. Each member in a squad covers services to its own community. The squad as a whole co-operatively provides services to the entire system. These servers are designed with the goals of simplicity, efficiency and robustness. They are intended to support a multiprogramming system for the development of distributed algorithms and other distributed applications. We address several major issues in developing a utility server, including the server structure, the management of message buffers, deadlock, and the robustness of server processes. Several utility servers in the Charlotte system are discussed as real examples.     

On the Design, Development, Deployment, and Network Survivability Analysis of the Dynamic Routing System Protocol

With the ever-increasing demands on server applications, reliability is of paramount importance. Often these services are implemented using a distributed server cluster architecture where many servers act together providing end user services. We evaluated one hundred deployed systems and found that over a one-year period, thirteen percent of the hardware failures were network related. To reliably provide end-user services, the server clusters must guarantee server-to-server communication in the presence of these network failures. We describe a protocol designed to provide proactive dynamic routing for server clusters architectures called the Dynamic Routing System (DRS) protocol and present analysis to its survivability in the presence of network failure. Our experiments show that, for an eight-node server cluster with three concurrent network failures, the DRS provides a 267% improvement in the probability of server to server communication over a traditional network topology. Additionally, the proactive routing approach of the DRS performs better than traditional routing systems by fixing network problems before they affect application communication.     

面向内存数据网格的分布式事务保障机制

在线事务处理（online transaction processing,OLTP）应用面临并发量和数据量持续增长的问题,并且高并发读写操作使得后台数据库成为瓶颈。内存数据网格（in-memory data grid,IMDG）是基于内存的新型分布式数据访问平台,是解决系统数据库写操作瓶颈的有效技术途径之一。然而内存数据网格中数据访问操作涉及的数据分布是不可预知的,需要提供分布式事务保障。针对内存数据网格的系统特点,提出了一种分布式事务保障机制,设计实现了事务处理模型、请求处理和数据定位方法以及事务保障协议,并规范化地定义了客户端与服务器端以及服务器端之间的操作接口。在事务处理基准测试TPC-W上的实验结果表明,新机制可以提高在线事务应用的处理速度,并具备良好的扩展性。     

A performance evaluation study of pipeline TMR systems

A distributed system in which a job can be broken into a number of subjobs which are processed sequentially at various processors is considered. The performance of such a system is then compared to the replicated (triple modular redundant, or TMR) version of the system in which each subjob will require concurrent replicated processing with majority voting. The effect of voting times and processor failure rates on the performance of the system is investigated with analytical approximations and computer simulations. The accuracy of the former is examined. The results indicate the possible existence of a threshold voting time, below which the TMR system performs better than the unreplicated one, and above which the situation is reversed. Such thresholds are observed, where possible, in systems with repairable servers, as well as in those with nonrepairable servers     

Unification of transactions and replication in three-tier architectures based on CORBA

In this paper, we describe a software infrastructure that unifies transactions and replication in three-tier architectures and provides data consistency and high availability for enterprise applications. The infrastructure uses transactions based on the CORBA object transaction service to protect the application data in databases on stable storage, using a roll-backward recovery strategy, and replication based on the fault tolerant CORBA standard to protect the middle-tier servers, using a roll-forward recovery strategy. The infrastructure replicates the middle-tier servers to protect the application business logic processing. In addition, it replicates the transaction coordinator, which renders the two-phase commit protocol nonblocking and, thus, avoids potentially long service disruptions caused by failure of the coordinator. The infrastructure handles the interactions between the replicated middle-tier servers and the database servers through replicated gateways that prevent duplicate requests from reaching the database servers. It implements automatic client-side failover mechanisms, which guarantee that clients know the outcome of the requests that they have made, and retries aborted transactions automatically on behalf of the clients.     

Web应用服务器研究综述

Web应用服务器是Web计算环境下产生的新型中间件,为创建、部署、运行、集成和管理事务性Web应用提供一个跨平台的运行环境,被认为是自关系型数据库以来最令人激动的企业应用技术.诸多IT企业纷纷推出其各自的Web应用服务器产品和系统,学术界也对这种热门领域产生了浓厚的兴趣.在分析Web计算环境下传统中间件发展所遇到的问题的基础上,介绍Web应用服务器的起源和发展、运行模式以及评测基准,然后对Web应用服务器研究现状进行综述,主要包括Web应用服务器的定义、体系结构、组件容器、分布事务处理、负载平衡、高速缓存、Web Service等研究热点和关键技术.根据评测基准,对若干主流Web应用服务器从功能和性能两个方面进行分析和比较.此外,还指出了Web应用服务器目前存在的不足以及未来的发展趋势.     

移动数据库广播的并发控制算法研究

在基于数据广播的移动信息服务中,由于更新数据和广播数据两个过程是并发执行的,所以移动客户将面临接收数据不一致性问题。为此,该文提出了一种基于规则的并发控制算法来解决以上问题。在允许同步更新的情况下采用事件-条件-操作规则来进行无冲突的数据广播。该算法对移动机和数据库服务器的影响很小,同时又为基于广播的移动计算提供了数据一致性服务。     

Adaptive metadata rebalance in exascale file system

This paper presents an effective method of metadata rebalance in exascale distributed file systems. Exponential data growth has led to the need for an adaptive and robust distributed file system whose typical architecture is composed of a large cluster of metadata servers and data servers. Though each metadata server can have an equally divided subset from the entire metadata set at first, there will eventually be a global imbalance in the placement of metadata among metadata servers, and this imbalance worsens over time. To ensure that disproportionate metadata placement will not have a negative effect on the intrinsic performance of a metadata server cluster, it is necessary to recover the balanced performance of the cluster periodically. However, this cannot be easily done because rebalancing seriously hampers the normal operation of a file system. This situation continues to get worse with both an ever-present heavy workload on the file system and frequent failures of server components at exascale. As one of the primary reasons for such a degraded performance, file system clients frequently fail to look up metadata from the metadata server cluster during the period of metadata rebalance; thus, metadata operations cannot proceed at their normal speed. We propose a metadata rebalance model that minimizes failures of metadata operations during the metadata rebalance period and validate the proposed model through a cost analysis. The analysis results demonstrate that our model supports the feasibility of online metadata rebalance without the normal operation obstruction and increases the chances of maintaining balance in a huge cluster of metadata servers.     

多复制服务器间无阻塞的数据更新

在Client/Server系统中,服务器的可用性是提高整个系统可用性的关键,采用多复制服务器是提高系统可用性的最有希望的手段.但是,复制数据更新过程中的阻塞问题是整个系统性能的一个瓶颈.本文提出一种无阻塞的多服务器独立提交的复制数据更新方法,对于因失效不能完成更新的服务器采用协调机制使其达到相同的最终状态.     

Modeling and performance comparison of reliability strategies fordistributed video servers

Large scale video servers are typically based on disk arrays that comprise multiple nodes and many hard disks. Due to the large number of components, disk arrays are susceptible to disk and node failures that can affect the server reliability. Therefore, fault tolerance must be already addressed in the design of the video server. For fault tolerance, we consider parity-based as well as mirroring-based techniques with various distribution granularities of the redundant data. We identify several reliability schemes and compare them in terms of the server reliability and per stream cost. To compute the server reliability, we use continuous time Markov chains that are evaluated using the SHARPE software package. Our study covers independent disk failures and dependent component failures. We propose a new mirroring scheme called Grouped One-to-One scheme that achieves the highest reliability among all schemes considered. The results of this paper indicate that dividing the server into independent groups achieves the best compromise between the server reliability and the cost per stream. We further find that the smaller the group size, the better the trade-off between a high server reliability and a low per stream cost     

One multiserver mixed queueing system with the limited waiting time

In this paper, a M/G/n/c multiserver queueing system with basic and standby servers is studied. Customers servicing is disturbed by failures of servers that make up a simplest flow. After the failure, the server needs a random time for renewal. It is also assumed that customers have limited, exponentially distributed waiting time in the system. The system is studied in both stationary and nonstationary modes.     

基于标记的缓存协作分布式Web服务器系统

介绍了提高Web服务器性能的前沿技术--分布式Web服务器系统,讨论了现有各种方案的优缺点,在此基础上提出一种新的分布式Web服务器系统.该系统使用基于标记的缓存协作用户请求分发方法(tag based cache cooperative Web requests distribution,简称TB-CCRD),通过前端机把系统中各个Web服务器的缓存组织成一个大的虚拟缓存系统,提高系统的整体缓存命中率,缩短了请求的响应时间;通过分布式处理TCP连接转交来消除前端机的性能瓶颈;利用标记通告URL在缓存中的位置,避免了额外的系统内部通信.从而得到了一个可扩展的高性能分布式Web服务器系统.