Abstract. Dynamic loading of code is a crucial and often neglected part of today's distributed systems that face increasing dynamics, complexity and heterogeneity. Ubiquitous computing and mobile computing even strengthen this trend. As the local availability of suitable code cannot be assumed in such environments, we propose a generic, decentralised code loading infrastructure. The whole process of publication, look-up, implementation selection and the final loading of platform-specific code is decentralised and requires only basic peerto-peer functionality. In contrast to previous work, our infrastructure allows any peer participating in the network to offer and to obtain platform-specific code in a dynamic and heterogeneous environment. By building on our generic concept, we present a JXTA-based service for dynamic code loading, which is realised by extending and improving JXTA-built-in mechanisms for dynamic service integration. Subsequently, we show the practical application of our infrastructure by an integration into our CORBA middleware and an implementation of mobile objects and mobile web services.

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SUMMARYWikis-being major applications of the Web 2.0-are used for a large number of purposes, such as encyclopedias, project documentation, and coordination, both in open communities and in enterprises. At the application level, users are targeted as both consumers and producers of dynamic content. Yet, this kind of peer-to-peer (P2P) principle is not used at the technical level being still dominated by traditional client-server architectures. What lacks is a generic platform that combines the scalability of the P2P approach with, for example, a wiki's requirements for consistent content management in a highly concurrent environment. This paper presents a flexible content repository system that is intended to close the gap by using a hybrid P2P overlay to support scalable, fault-tolerant, consistent, and efficient data operations for the dynamic content of wikis. On the one hand, this paper introduces the generic, overall architecture of the content repository. On the other hand, it describes the major building blocks to enable P2P data management at the system's persistent storage layer, and how these may be used to implement a P2P-based wiki application: (i) a P2P back-end administrates a wiki's actual content resources. (ii) On top, P2P service groups act as indexing groups to implement a wiki's search index.

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At times when the IT manager's best friend is systems consolidation (which is a euphemism for centralisation), it may come somewhat as a surprise for you that this book investigates decentralisation in the context of content management systems. It may seem quite obvious that content will and should be managed by the party who creates and owns the content, and hence should be held in a-somewhat-centralised and managed location. However, over the past few years, we have been witnesses of some important trends and developments which call for novel ways of thinking about content management and maybe even broader, about computer systems in general.First, ongoing business globalization creates natural distribution of information at a corporate level, as well as decentralization of control over business resources and business processes. Changing alliances with partners require flexible architectures for content management that can adapt to changing constellations, roles, and access rights. Second, the need for outsourcing and resource efficiency has brought about concepts of virtualization, recently culminating in the cloud computing buzzword. Virtualization of content management services requires extremely scalable and flexible underlying information and communication architectures. These kinds of solutions are theoretically and practically impossible to implement based on centralised client-server architectures. Third, we are currently experiencing a dramatic shift in the roles of consumers in the Internet. The times have gone when quality content was only delivered by publishers and news agencies. Wikis and other Web 2.0 tools empower consumers to produce and publish their personal content. Twitter and citizen journalism may serve as two examples. To summarize, future content management solutions face challenges caused by business globalization, outsourcing and virtualisation, and the shift from consumers to prosumers.Peer-to-peer computing is a key computing paradigm to tackle these challenges. Far from being a new idea, the first generation of peer-to-peer networks date back to the origin of the Internet. The principle of packet switching which is implemented in the Internet Protocol is truly peer-to-peer: nodes in an IP network make routing decisions in a decentralized way, based on local information. This peer-to-peer architecture guarantees the enormous resilience and robustness of the Internet.About ten years ago, the second generation of peer-to-peer networks emerged: Internet file sharing. Decentrally organized systems and protocols like Napster and Gnutella were the basis of a scalable infrastructure for file exchange with the ability to provide acceptable quality of service for users looking for, e.g., mp3 files in the dynamic environment of the Internet. File sharing was followed by peer-to-peer IP telephony (Skype) and peer-to-peer solutions for the delivery of huge digital objects such as software updates (exemplified by BitTorrent).The third generation of peer-to-peer networks and applications employs p...