Ludger Fiege

A peer-to-peer approach to content-based publish/subscribe

Publish/subscribe systems are successfully used to decouple distributed applications. However, their efficiency is closely tied to the topology of the underlying network, the design of which has been neglected. Peer-to-peer network topologies can offer inherently bounded delivery depth, load sharing, and self-organisation. In this paper, we present a content-based publish/subscribe system routed over a peer-to-peer topology graph. The implications of combining these approaches are explored and a particular implementation using elements from Rebeca and Chord is proven correct.

Supporting mobility in content-based publish/subscribe middleware

Publish/subscribe (pub/sub) is considered a valuable middleware architecture that proliferates loose coupling and leverages reconfigurability and evolution. Up to now, existing pub/sub middleware was optimized for static systems where users as well as the underlying system structure were rather fixed. We study the question whether existing pub/sub middleware can be extended to support mobile and location-dependent applications. We first analyze the requirements of such applications and distinguish two orthogonal forms of mobility: the system-centric physical mobility and an application-centric logical mobility (where users are aware that they are changing location). We introduce location-dependent subscriptions as a suitable means to exploit the power of the event-based paradigm in mobile applications. Briefly spoken, location-dependency refines a subscription to accept only events related to a mobile users current location. Implementations for both forms of mobility are presented within the content-based pub/sub middleware Rebeca, drawing from its refined routing capabilities (namely, covering and merging).

Technologies for E-Services

In the traditional application model, services are tightly coupled with the processes they support. For example, whenever a server’s process changes, existing clients using that process must also be updated. However, electronic commerce is moving toward e-service based interactions, where corporate enterprises use e-services to interact with each other dynamically, and a service in one enterprise could spontaneously decide to engage a service fronted by another enterprise. We clarify here the relationship between currently developing standards such as UDDI, WSDL, and WSCL, and propose a conversation controller mechanism that leverages such standards to direct services in their conversations. We can thus treat services as pools of methods, independent of the conversations they support. Even method names can be decided on independently of the conversations. Services can spontaneously discover each other and then engage in complicated interactions without the services themselves having to explicitly support conversational logic. The dynamism and flexibility enabled by this decoupling is the essential difference between applications offered over the web and e-services.

Filter Similarities in Content-Based Publish/Subscribe Systems

Matching notifications to subscriptions and routing notifications from producers to interested consumers are the main problems in large-scale publish/subscribe systems.Most previously proposed distributed notification services either use flooding or, if filtering is performed, they assume that each event broker has global knowledge about all active subscriptions. Both approaches degrade the scalability of notification services as the former wastes network resources and the latter generates overly large routing tables.In this paper we describe content-based routing algorithms that exploit filter similarities in order to reduce the size of routing tables and the number of control messages that are exchanged among the brokers in order to keep the routing tables up-to-date. In particular, the proposed algorithms do not assume global knowledge about all active subscriptions. Furthermore, we describe how these optimizations can be supported if the underlying data and filter model is based on structured records.

Evaluating advanced routing algorithms for content-based publish/subscribe systems

We present an evaluation of advanced routing algorithms for content-based publish/subscribe systems that focuses on the inherent characteristics of routing algorithms (routing table sizes and filter forwarding overhead) instead of system-specific parameters (CPU load etc.). The evaluation is based on a working prototype instead of simulations and compares several routing algorithms to each other. Moreover, the effects of locality among the interests of the consumers are investigated. The results offer new insights into the behavior of content-based routing algorithms. Firstly, advanced routing algorithms can be considered mandatory in large-scale publish/subscribe systems. Secondly, the use of advertisements considerably improves scalability. Thirdly, advanced routing algorithms operate efficiently in more dynamic environments than was previously thought. Finally, the good behavior of the algorithms improves even if the interests of the consumers are not evenly distributed, which can be expected in practice.

Towards multi-purpose wireless sensor networks

Current wireless sensor network (WSN) architectures are based on the assumption that all sensor nodes are participating in a single global task. In many scenarios, however it will be desirable to use a single sensor network for multiple concurrent applications. In order to enable such multipurpose WSNs efficiently, delimiting each application to its specific set of relevant nodes is one of the key issues that needs to be solved. We present scoping as a general concept for the creation and maintenance of network-wide node subsets and describe a flexible and modular architecture that meets the requirements of multi-purpose WSNs.

Looking into the past: enhancing mobile publish/subscribe middleware

Publish/subscribe (pub/sub) middleware facilitates loosely coupled cooperation and fits well the needs of spontaneous, ad-hoc interaction. However, newly started mobile applications have to be bootstrapped to interpret the current flow of notifications correctly and commence normal operation. This problem is aggravated in mobile environments where disconnections and context changes occur frequently.In this paper, we propose two forms of subscriptions that allow consumers to subscribe to past events to improve the bootstrapping process. The first form uses logical mobility to harness possible client movements and subscribe in future locations to bootstrap virtual counterparts before the application needs the data. The second form is based on buffers and offers a way to integrate data repositories distributed in the network.

Modular event-based systems

Event-based systems are developed and used to integrate components in loosely coupled systems. Research and product development have focused so far on efficiency issues but neglected methodological support to build such systems. In this article, the modular design and implementation of an event system is presented which supports scopes and event mappings, two new and powerful structuring methods that facilitate engineering and coordination of components in event-based systems. We give a formal specification of scopes and event mappings within a trace-based formalism adapted from temporal logic. This is complemented by a comprehensive introduction to the event-based style, its benefits and requirements.

A modular approach to build structured event-based systems

Event-based systems are developed and used as a coordination model to integrate components in loosely coupled systems. Research and product development focused so far on efficiency issues but neglected methodological support to build such systems. In this paper, we present the modular design and implementation of an event system which supports scopes and event mappings, two new and powerful structuring methods that facilitate engineering and coordination of components in event-based systems. The approach is based on a trace-based specification method adapted from temporal logic.

DREAM: Distributed Reliable Event-Based Application Management

New applications and the convergence of technologies, ranging from sensor networks to ubiquitous computing and from autonomic systems to event-driven supply chain management, require new middleware platforms that support proactive event notification. We present a system overview and discuss the principles of Dream, a reactive middleware platform that integrates event detection and composition mechanisms in a highly distributed environment; fault-tolerant and scalable event notification that exploits a variety of filter placement strategies; content-based notification to formulate powerful filters and concept-based notification to extend content-based filtering to heterogeneous environments; middleware-mediated transactions that integrate notifications and transactions; and scopes, which are administration primitives for both deployment- and runtime configurability, as well as for the management of policies. We discuss four prototypes that were implemented as proof-of-concept systems and present lessons learned from them.