Carlo Marchetti

Content-Based Publish-Subscribe over Structured Overlay Networks

This paper introduces a novel architecture for implementing content-based pub/sub communications on top of structured overlay networks. This architecture overcomes some well-known limitations of existing infrastructures, i.e. lack of self-configuration and of adaptiveness to dynamic changes. This is achieved by devising a mediator stratum between the rich event subscription semantics of content-based pub/sub systems and the standard logical addressing scheme of overlays. The paper describes details of the design and provides considerations in selecting the subscription-to-node and event-to-node mappings suitable for the solution. We identify the lack of native support for one-to-many communication by overlay networks as the main impediment for efficient system operation. The paper introduces a novel primitive for one-to-many message delivery, showing through simulation how this can improve performance of the architecture. The simulation study also shows performance comparison between the different mappings proposed as well as evaluation of other optimizations discussed in the paper

The DaQuinCIS architecture: a platform for exchanging and improving data quality in cooperative information systems

In cooperative information systems, the quality of data exchanged and provided by different data sources is extremely important. A lack of attention to data quality can imply data of low quality to spread all over the cooperative system. At the same time, improvement can be based on comparing data, correcting them and thus disseminating high quality data. In this paper, we present an architecture for managing data quality in cooperative information systems, by focusiag on two specific modules, the Data Quality Broker and the Quality Notification Service. The Data Quality Broker allows for querying and improving data quality values. The Quality Notification Service is specifically targeted to the dissemination of changes on data quality values.

Active software replication through a three-tier approach

A replication logic is the set of protocols and mechanisms implementing a software replication technique. A three-tier approach to replication consists in separating the replication logic from both clients and replicated servers by embedding such logic in a middle-tier In this paper we first introduce the fundamental concepts underlying three-tier replication. This approach has two main practical advantages: (i) it allows to maintain consistency among the state of server replicas deployed within an asynchronous distributed system and (ii) it supports very thin clients. Then we present the Interoperable Replication Logic (IRL) architecture, which is a Fault Tolerant CORBA compliant infrastructure exploiting a three-tier approach to replicate stateful deterministic CORBA objects. In this context, we illustrate the three-tier replication protocol currently implemented in our IRL prototype and a performance analysis that shows the feasibility of the three-tier approach to software replication.

A quality model for multichannel adaptive information

The ongoing diffusion of novel and mobile devices offers new ways to provide services across a growing set of network technologies. As a consequence, traditional information systems evolve to multichannel systems in which services are provided through different channels, being a channel the abstraction of a device and a network. This work proposes a quality model suitable for capturing and reasoning about quality aspects of multichannel information systems. In particular, the model enables a clear separation of modeling aspects of services, networks, and devices. Further, it embeds rules enabling the evaluation of end-to-end quality, which can be used to select services according to the actual quality perceived by users.

CORBA request portable interceptors: a performance analysis

Interceptors are a mean to add specific network-oriented capabilities (such as authentication, flow control, caching etc.) to a distributed application which runs over a middleware without changing either the application code or the middleware. However, interceptors could be nonintuitive and this could in turn limit their use on a large scale. We present results of an investigation on CORBA portable interceptors in Java on various CORBA platforms. This study includes the identification of the basic mechanisms provided by an interceptor, of their limitations, a proxy-based technique to overcome some of these limitations and a performance analysis. We also release fragments of Java code used for experiments on Interceptor.

Three-tier replication for FT-CORBA infrastructures

Enforcing strong replica consistency among a set of replicas of a service deployed across an asynchronous distributed system in the presence of crash failures is a real practical challenge. If each replica runs the consistency protocol bundled with the actual service implementation, this target cannot be achieved, as replicas need to be located over a partially synchronous distributed system to solve the distributed agreement problems underlying strong replica consistency.

CORBA request portable interceptors: analysis and applications

Interceptors are an emerging middleware technology enabling the addition of specific network‐oriented capabilities to distributed applications. By exploiting interceptors, developers can register code within interception points, extending the basic middleware mechanisms with specific functionality, e.g. authentication, flow control, caching, etc. Notably, these extensions can be achieved without modifying either the application or the middleware code.

Enabling data quality notification in cooperative information systems through a Web-service based architecture

Cooperative information systems (CISs) are often characterized by a high degree of data replication; as an example, in an e-government scenario, the personal data of citizens are stored by almost all administrations. In such scenarios, organizations typically provide the same information with distinct quality levels and this enables providing users with data of the highest available quality. Furthermore, the comparison of data values might be used to enforce a general improvement of data quality in all organizations. In the DaQuinCIS project (Aguilera et al., 1999), we propose an architecture for the management of data quality in CISs; this architecture allows the diffusion of data and related quality and exploits data replication to improve the overall quality of cooperative data. In this paper, we present an overview of a component of the DaQuinCIS architecture, namely the quality notification service (QNS), which is used to inform interested users when changes in quality values occur within the CIS. QNS can be used to control the quality of critical data, e.g. to keep track of quality changes and to be always aware when quality degrades under a certain threshold. The interaction between the QNS and its users follows the publish/subscribe paradigm: a user willing to be notified for quality changes subscribes to the QNS by submitting the features of the events to be notified for, through a specific subscription language. When a change in quality occurs, an event is published by the QNS i.e., all the users which have a consistent subscription receive a notification. However, as shown in the paper by Marchetti et al. (2003), currently available pub/sub infrastructures do not allow to meet all the requirements that a QNS implementation should satisfy, in particular scaling to a large number of users and coping with platform heterogeneity. QNS addresses both these problems through a layered architecture that: (i) encapsulates the technological infrastructure specific of each organization; (ii) adopts the standard Web-service technology to implement inter-organization communications; and (iii) embeds solutions and algorithms (namely, merge subscriptions and diffusion trees) to reduce the use of physical and computational resources. The remainder of this paper is organized as follows: we first introduce some preliminary concepts and the QNS specification; then we motivate and describe the internal architecture of the service. Due to the lack of space, explanations are given at a very high abstraction level. Interested readers can find technical and formal details, as well as running examples, in the paper by Scannapieco et al. (2003).

Total order communications: a practical analysis

Total Order (TO) broadcast is a widely used communication abstraction that has been deeply investigated during the last decade. As such, the amount of relevant works may leave practitioners wondering how to select the TO implementation that best fits the requirements of their applications. Different implementations are indeed available, each providing distinct safety guarantees and performance. These aspects must be considered together in order to build a correct and sufficiently performing application. To this end, this paper analyzes six TO implementations embedded in three freely-distributed group communication systems, namely Ensemble, Spread and JavaGroups. Implementations are first classified according to the enforced specifications, which is given using a framework for specification tailored to total order communications. Then, implementations are compared under the performance viewpoint in a simple yet meaningful deployment scenario. In our opinion, this structured information should assist practitioners (i) in deeply understanding the ways in which implementations may differ (specifications, performance) and (ii) in quickly relating a set of total order algorithms to their specifications, implementations and performance.

A classification of total order specifications and its application to fixed sequencer-based implementations

During the last two decades the design and development of total order (TO) communications has been one of the main research topics in dependable distributed computing. The huge amount of research work has produced several TO specifications and a wide variety of TO implementations with different guarantees whose differences are often left hidden or unclear. This paper presents a systematic classification of six distinct TO specifications based on a well-defined formal framework. The classification allows us (i) to define in a formal way the differences among the behaviors of faulty and correct processes admitted by each specification, and (ii) to easily match TO implementations with respect to their enforced specification. The classification is applied to study the properties of eight variations of TO implementations based on a fixed sequencer given in a well-known context, namely primary component group communication systems.