Franz J. Hauck

Locating objects in wide-area systems

Locating mobile objects in a worldwide system requires a scalable location service. An object can be a telephone or a notebook computer, but also a software or data object, such as a file or an electronic document. Our service strictly separates an objects name from the addresses where it can be contacted. This is done by introducing a location-independent object handle. An objects name is bound to its unique object handle, which, in turn, is mapped to the addresses where the object can be contacted. To locate an object, we need only its object handle. We present a scalable location service based on a worldwide distributed search tree that adapts dynamically to an objects migration pattern to optimize lookups and updates.

Promondia: a Java-based framework for real-time group communication in the Web

The World Wide Web has evolved from a distributed hypertext system to a platform-independent graphical user interface that integrates many network services. So far, its technology has restricted it mainly to applications for information retrieval. As networks become ubiquitious and more and more users have a permanent connection, there is an increasing demand for other network services, such as real-time data feeds, group communication, and teleconferencing. So far, these services have been provided by various proprietary software systems, which were hard to set up and use, and thus not very successful. Integrating real-time group communication services into the World Wide Web is a natural way to make them more accessible and will take the Web a step further on its way to becoming the universal network application. In this paper, we describe functionalities required for these services and present an implementation based on Sun Microsystems Java2 programming language. We focus on the high-level functionalities and abstractions, but also describe an object-oriented programming model for group communication systems.

Algorithmic Design of the Globe Wide-Area Location Service

We describe the algorithmic design of a worldwide location service for distributed objects. A distributed object can reside at multiple locations at the same time, and offers a set of addresses to allow client processes to contact it. Objects may be highly mobile like, for example, software agents or Web applets. The proposed location service supports regular updates of an objects set of contact addresses, as well as efficient look-up operations. Our design is based on a worldwide distributed search tree in which addresses are stored at different levels, depending on the migration pattern of the object. By exploiting an objects relative stability with respect to a region, combined with the use of pointer caches, look-up operations can be made highly efficient.

Inheritance modeled with explicit bindings: an approach to typed inheritance

While there are suitable type-systems for aggregation that allow the exchange of objects and classes, the configuration of systems, and dynamic binding, there are no type-systems for inheritance relationships. Thus, maintenance of class libraries becomes difficult or even impossible because changes in a class library can cause changes in the inheriting classes in an application program. Modeling inheritance with explicit and parametrical bindings introduces a typed interface for inheritance which allows type-safe changes in class libraries. Furthermore the approach opens new possibilities for composition and makes programming easier.

Aspectix: A Quality-Aware, Object-Based Middleware Architecture

Quality of service is becoming more and more important in distributed systems. Current middleware systems lack quality-of-service support on the application and on the system level. AspectIX is a CORBA-compliant middleware platform that defines generic interfaces to control quality-of-service and an infrastructure for quality implementations. AspectIX is based on a fragmented object model that can provide transparent client-side quality implementations. Quality implementations can be weaved into functional fragments using a hierarchy of Weavelets which are modular code-transforming software components. A distributed policy decision engine allows administrators to influence object-internal decisions, e.g., decisions about how to implement the current quality-of-service requirements.

Supporting hierarchical guided tours in the World Wide Web

Abstract Guided tours are well known from hypertext systems as a means to collect hypertext documents in a sequence with a certain meaning. This can be followed by readers using next and previous links. As there is almost no support for guided tours in the World Wide Web so far, these structures have to be built up manually, which not only causes a lot of work on updates but is also almost impossible in the case of non-local documents. This paper presents a simple server-side concept for building guided tours using any existing Web pages. The pages used within a tour need not be aware of being used, and thus, need no special preparation at all. Pages may occur multiple times within several tours or even within one tour. We present a server-based implementation demonstrating the concept.

Meta objects for access control: extending capability-based security

Object-based programming is becoming more and more popular and is currently conquering the world of distributed programming models. In object-based systems, access control is often based on capabilities, as capability-based security is a well-known paradigm. It has been extended by means to restrict, revoke, and expire capabilities. On the other hand, capabilities have serious drawbacks. First, in object-based systems, programming is based on the frequent exchange of object references (i.e., capabilities). Thus, it is hard to check which parts of an application are able to gain control of a certain capability. This becomes even harder if we consider distributed object-based systems like Java RMI and CORBA. Second. a capability usually cannot prevent method invocations from leaking unprotected references as return values. Transitive access control is not possible in a transparent way, which is independent of the code describing the invocation. We present a new security paradigm based on meta objects. Meta objects can be attached to object references and control access to the corresponding objects. Meta objects offer the same functionality as capability-based security. In addition. they can be used for implicit and transitive access control of object references passed as a parameter or as a result. Such a reference can be automatically protected by the meta object by attaching itself or another meta object to the reference before passing it on. Meta objects can implement arbitrary and user-defined security policies. They help to separate security policies from application code, and thus support reuse.

SAMProc: middleware for self-adaptive mobile processes in heterogeneous ubiquitous environments

Software in Ubiquitous Computing is faced with a dynamic and heterogeneous environment. For tapping the environments full potential, software must be able to adapt dynamically and react to the environment in a platform-and language-independent manner. However, developing mobile and adaptive applications is a complex and error-prone task. This work presents the design of SAMProc, a new middleware that supports developers in creating ubiquitous applications. We introduce the novel concept of self-adaptive mobile processes, which allows an abstract high-level specification of an applications lifecycle and its distribution aspects. In our system, an application may dynamically migrate with an adaptation of its interface, state and implementation at runtime, while transparently and permanently remaining addressable. The SAMProc middleware uses an MDA-like approach, in which the developer creates a self-adaptive mobile process description. A middleware tool automatically maps these descriptions to our infrastructure, i.e., either self-adaptive migratable CORBA objects or Web services. The developer focuses on application logic; the tool generates migration and adaptation code. We demonstrate the feasibility by presenting an example application in which mobile reporters publish data in a basic workflow.

Support for Mobility and Replication in the AspectIX Architecture

Unlike CORBA with its static client-server relationship, AspectIX uses the concept of distributed objects. Distributed objects consist of so called fragments, that communicate with other fragments to synthesize the desired behaviour. The local fragment can carry more semantics than a simple CORBA stub and can be replaced at runtime by another fragment to fulfill the application’s requirements. AspectIX therefore provides a single mechanism that is especially suited to realize both: mobility and replication.

COSCA: an easy-to-use component-based PaaS cloud system for common applications

The emergence of cloud computing marks a significant change in the way computers are used in both enterprise and personal environments. Yet, as a young technology, cloud computing is far from being mature. Platform-as-a-service (PaaS) clouds promise to reduce maintenance and administration costs, but current frameworks lack crucial features for supporting a broad range of applications. Especially rigid constraints of the current PaaS programming models limit broader usage. Based on this observation we compiled eleven requirements of typical business applications such as programming model, placement, scalability, routing, isolation, load balancing, accounting, adaptability and modularity. We further observe that none of current platforms support a majority of these requested features. As a result, we present our own PaaS system, called COSCA that meets all of these requirements. COSCAs component-based design especially supports adaptability and modularity. We believe that our requirements and architecture may serve as a valuable guide for PaaS designers, implementers, and providers.