Consistency Control Systems for Remote Object Replication

Current group communication services have mostly been implemented on a homogeneous, distributed computing environment. This limits their applicability because most modern distributed computing environment are heterogeneous in nature. This paper describes the design, implementation, and performance evaluation of a CORBA group communication service. Using CORBA to implement a group communication service enables that group communication service to operate in a heterogeneous, distributed computing environment. To evaluate the effect of CORBA on the performance of a group communication service, this paper provides a detailed comparison of the performance measured from three implementations of an atomic broadcast protocol and a group membership protocol. Two of these implementations use CORBA, while the third uses UDP sockets for interprocess communication. The main conclusion is that heterogeneity can be achieved in group communication services by implementing them using CORBA, but there is a substantial performance cost. This performance cost can be reduced to a certain extent by carefully choosing a design and tuning various protocol parameters such as buffer sizes and timer values.

Group communication services typically generate large multicast data streams. Delivering such massive data streams to the end system nodes at the edge of the Internet has been a challenging problem in terms of high stress on the network links and high demand on network resources and routing node capacities. Most of existing research has been dedicated on geo-distance based routing with various optimizations to alleviate the performance impact on geo-distance based routing due to unpredictable network dynamics. Most representative techniques are targeted at reducing the delivery path length or optimizing routing path by utilizing network locality. In this paper, we identify the inefficiency of geo-distance based routing protocols in many existing multicast overlay networks in terms of both resource utilization and group communication efficiency. To address this issue, we develop a utility-based routing scheme (UDR) that can provide efficient group communication services in a decentralized geographical overlay network. Our approach makes three unique contributions. First, we introduce a utility function to refine the geo-distance based routing in such a way that the routing path selection can carefully incorporate both geo-distance based metric and the network latency. Second, we enhance our utility driven routing scheme with self-adaptive capability by considering the nodes' state and network density. Thus, nodes in the multicast network can dynamically accommodate the changes of network conditions based solely on their local knowledge about the network. Third, we devise a suite of optimization techniques to minimize the maintenance cost and computational complexity of our self-adaptive and utility-drive routing scheme. We evaluate our approach through extensive experiments based on a realistic network topology model and show that the UDR method is highly scalable and it effectively enhances the multicast delivery efficiency for large scale group communication services compared to existing geo-distance based routing protocols.

We discuss a group protocol which supports applications with group communication service when QoS supported by networks or required by applications is changed. An autonomic group protocol is realised by cooperation of multiple autonomous agents. Each agent autonomously takes a class of each protocol function so as to support only and all QoS required. A group is composed of views for scalability. Each view is a subset of the agents where the agents autonomously take protocol classes consistent with each other. We make clear what combination of classes can be autonomously taken by agents in a view.

In distributed systems, high service availability can be achieved by letting a group of servers replicate the service state; if some servers fail, the surviving ones know the service state and can continue to provide the service. Group communication services, such as membership and atomic broadcast, have been proposed to solve the problem of maintaining server state replica consistency. Group membership achieves agreement on the history of server groups that provide the service over time, while atomic broadcast achieves agreement on the history of state updates performed in each group.

Java RMI, Jini and CORBA provide effective mechanisms for implementing a distributed computing system. Recently many numeral libraries have been developed that take advantage of Java as an object‐oriented and portable language. The widely‐used client‐server method limits the extent to which the benefits of the object‐oriented approach can be exploited because of the difficulties arising when a remote object is the argument or return value of a remote or local method. In this paper this problem is solved by introducing a data object that stores the data structure of the remote object and related access methods. By using this data object, the client can easily instantiate a remote object, and use it as the argument or return value of either a local or remote method. Copyright © 2003 John Wiley & Sons, Ltd.

There are increasing demands for making multiple sensors/devices accessible over a wireless local area network (WLAN) for measurement, control, and monitoring purposes. Such a system comprises a client, and multiple small embedded computer-based servers deployed in remote places. To apply the advantage of distributed object models, such as Java Remote Method Invocation (RMI), the model cannot work on an embedded computer because it demands sufficient resources, including the processing power of the central processing unit, memory, and an operating system. The model provides synchronous operations using the Transmission Control Protocol/Internet Protocol with heavy overheads, which results in low throughput compared with the lightweight User Datagram Protocol. A simple asynchronous remote object protocol for embedded computers (eASROP) is designed and implemented to enable an embedded computer and its acquired data to masquerade as an RMI-compatible remote object on a server. The client is not blocked after it makes a remote method call to a server, and can continue its job. The client can asynchronously receive a return object that contains status data and control data. The results of performance measurements obtained in experiments demonstrated that for the eASROP on an embedded computer-based server (48 MHz), the extra execution time that the client experienced was 1.67–15.4% of that for the RMI on a personal-computer-based server (3.1 GHz, 4 cores) on a WLAN.

Software based Distributed Shared Memory (DSM) systems have been the focus of considerable research effort, primarily in improving performance and consistency protocols. Unfortunately, computer clusters present a number of challenges for any DSM systems that are not solvable through consistency protocols alone. These challenges relate to the ability of DSM systems to adjust to load fluctuations, computers being added/removed from the cluster, to deal with faults, and the ability to use DSM objects larger than the available physical memory. We present here a proposal for the Synergy Distributed Shared Memory System and its integration with the virtual memory, group communication and process migration services of the Genesis Cluster Operating System.

This paper presents several distributed algorithms that cause a token to continually circulate through all the nodes of a mobile ad hoc network. An important application of such algorithms is to ensure total order of message delivery in a group communication service. Some of the proposed algorithms are aware of, and adapt to changes in, the ad hoc network topology. When using a token circulation algorithm, a round is, said to complete when every node has been visited at least once. Criteria for comparing the algorithms include the average time required to complete a round, number of bytes sent per round, and number of nodes visited per round. Comparison between the proposed algorithms is performed using simulation results obtained from a detailed simulation model (with ns-2 simulator).

Developing JAVA Card Application with RMI API

The increasing mobility of Internet users and the growing need to collaborate with staff travelling on business belong to the key characteristics of the future Internet. Mobile collaborative applications require novel schemes for group communication that actively support collaborations in wireless scenarios. An intrinsic problem of collaborations in wireless scenarios is the temporary connection loss with the group due to user mobility and network issues. Existing group communication protocols do not sufficiently support this situation. In this paper, we present a new peer-to-peer group communication protocol, called Moversight, that was specifically designed to support closed group cooperation in mobile environments with varying churn rates. It applies a new group communication paradigm, called mobile optimistic virtual synchrony, to handle churn-related peer failures. It enhances the virtual synchrony paradigm for mobile scenarios. The paper introduces the paradigm and describes the main protocol features of Moversight. Finally we present simulation results to demonstrate the applicability of the protocol.

This chapter provides details of group communication services as specified in 3GPP Release 12. This includes the architectural framework adopted for unicast and broadcast delivery of voice and data in order to effectively support group communication. It also highlights different use cases and how group communication solutions apply for these use cases.

Service resilience, defined as the continued availability of a service despite failures and other negative changes in its environment, is vital in many systems. It is typically achieved by state machine replication using group communication middleware for coordination of service replicas. In this paper we focus on systems that represent critical data as Web resources identified by Uniform Resource Identifiers (URIs). The best examples of such systems are RESTful web services. We present RESTGroups : a group communication front-end for RESTful web services. Our system is based on Spread --- a popular group communication toolkit. Contrary to Spread and other such toolkits, we represent group communication services as resources on the Web, addressed by URIs. In the paper, we describe the system architecture and the API.

We have developed a cost-effective regional computer communication network system and examined its implementation. The system provides a LAN environment and group communication services to the customers. The main system components are: the intelligent bridge circuit, called the low-end card (LEC), the center brouter (bridging router), and the management server. The LEC is set up in the subscriber's household and provides the popular Ethernet interface (10 Base-T). The leased circuits are connected between the LEC and the center brouter. The LEC and the brouter are controlled by the management server to function as the group communication system. The brouter, which has a multiplex subscriber-port interface, is required to accommodate a lot of customers cost effectively. This paper presents the system design of a large-capacity brouter having a SDH/SONET multiplex-port interface and its operation. The brouter can accommodate up to approximately 2700 subscribers. Ethernet packets are transferred through each subscriber channel between the LEC and the brouter using the HDLC protocol. A group communication mechanism based on packet filtering via the MAC addresses of PCs is presented. The Ethernet packets are filtered by the brouter to eliminate all packets except those from/to the same group members. The group management operation among the center brouter, the LEC, and the management server is discussed.

Object replication is a technique to enhance fault tolerance and high availability. Group communication is a useful mechanism guaranteeing the consistency among replicated objects. We propose a generic group communication framework that allows transparent plug-in of various group communication protocols with no modification of existing CORBA. For this, we extend the Open Communications Interface (OCI) to support interoperability, reusability of existing group communication, and independency on ORB and OS. The proposed scheme is composed of group membership, group IOR, and group multicast. Experimental results show that group object invocation using the proposed scheme performs substantially better than group object invocations using IIOP as the number of members in a group increases.

Mobility becomes very important in distributed Computing. Thus, many works deal with process strong mobility, but few of them deal with mobility of shared objects. In this paper, we present a mobility approach based on source code transformation, which allows strong mobile processes to communicate via remote shared objects. This approach enables shared object strong mobility. We demonstrate how both strong mobile processes and strong mobile shared objects can migrate at the same time while maintaining consistency of the application execution. We implemented our approach on Java Threads, using Java RMI. The evaluation of execution times shows that the overhead is very small compared to the original execution time.