A distributed publish, subscribe notification service for pervasive environments [Elektronische Ressource] / von Andreas Zeidler

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• Informatik

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• Publié par: technischen_universitat_darmstadt
• Publié le: 01 janvier 2005
• Nombre de lectures: 8
• Langue: English
• Poids de l'ouvrage: 1 Mo

Extrait

ADistributedPublish/Subscribe
NotificationServiceforPervasive
Environments
Vom Fachbereich Informatik
der Technischen Universität Darmstadt
genehmigte
Dissertation
zur Erlangung des akademischen Grades
eines Doktor-Ingenieurs (Dr.-Ing.)
von Diplom-Informatiker
AndreasZeidler
aus Mülheim an der Ruhr
Referent: Prof. A. Buchmann, PhD, TU Darmstadt
Korreferent: Prof. Dr. K. Geihs, TU Berlin
Tag der Einreichung: 01.09.2004
Tag der mündlichen Prüfung: 04.11.2004
Darmstädter Dissertationen D17Summary
Based on the success of mobile telephony in the recent past, many observers expect mobility in con-
junction with mobile devices to open up a wide field for novel applications. Many experts predict the
arrival of new services, such as mobile commerce, location-based services, multimedia messaging,
and mobile gaming. They claim that this new class of mobile applications will constitute a main
driving-force for technological advancements at least for the next decade. Also, in the third gener-
ation of mobile telephony, we expect to observe the logical next step: the convergence of mobile
appliances into a new generation of smart devices, such as smartphones. Today, we can distinguish
three basic categories of mobile devices: (i) specialized devices, such as special-purpose systems
or mobile phones, (ii) devices that are more flexible, such as personal digital assistants (PDAs),
and (iii) real general-purpose devices, such as laptop computers. The next generation of devices
will be unifying distinct features of the categories named above. Such devices will become flexible,
lightweight, and mobile at the same time. This meets a basic requirement found in the vision of
ubiquitous and pervasive computing. The latter vision places its main focus on smart mobile devices
as the enabling technology for interaction of mobile users with the surrounding infrastructure.
Another trend expected is based on the observation that more and more artifacts in the infrastruc-
ture will be equipped with processors and—more importantly—networking interfaces. Therefore,
they are able to generate as well as receive data. The original thesis of ubiquitous computing ex-
pects a mobile user to be embedded into surroundings filled with communicating and interacting
artefacts, all serving the spontaneous needs of the user. Moreover, we are convinced that the interac-
tion between users and the surroundings in highly mobile and dynamic settings has to be mediated
by a common middleware platform, together with personalized devices and specialized services,
facilitating the needs of mobile users.
This basic system model of nomadic users and smart infrastructures poses a number of challenges
for such middleware support. First of all, mobility by itself requires different paradigms for inter-
action than those found in classical distributed systems. Many paradigms, well-established in static
distributed systems are likely to fail when applied to these new settings. One prominent example
among many is the request/reply paradigm, which is too static and tight-coupled to be successful in
dynamic mobile settings. Here, different paradigms, like loose-coupling and data-centric computing,
are more likely to succeed.
The next key challenge for middleware is to support mobile applications to react “smartly” to
changes of their execution environment. Users of such applications obviously expect their electronic
helpers to adapt themselves to the current situation they are used in. A well-known example is to
turn off the ringer tones of a mobile phone when the user is in a meeting situation. Such adaptation is
part of what usually is called context- or situation-aware computing. The challenge for middleware
support lies here in providing means to retrieve context information from the environment on a
syntactic and semantic level. Here we face issues of heterogeneity, together with efficient filtering
of large volumes of information available.
iSummary ii
Another rationale—as well as challenge—for middleware support in dynamic and mobile scenar-
ios is the need to decouple producers and consumers of data in the system in time and space.For
the systems considered in this thesis, often it is not feasible for producers and consumers to “know”
each other, especially due to the number of participants or resource constraints. Effective means for
anonymous interaction are therefore essential. Moreover, for mobile clients the receiver cannot be
assumed to be online at the same time the sender produces the data. Again, a middleware solution
can provide facilities for buffering and access to past information.
The scale of pervasive systems we envision is also a challenge. On the one hand, systems will
grow in physical size, like spanning a whole city. On the other hand, systems also can be rather
small in size, but dense in the number of processors and applications contained within. Thus, the
key challenge is to provide a communication infrastructure in which data and information is still
manageable even for small devices while communication remains efficient and scalable.
Altogether, we are convinced that this constitutes a strong demand for a mediator between pro-
ducers and consumers of data, i.e., a middleware solution.
This thesis presents solutions to the challenges listed above using mechanisms that are based on a
distributed publish/subscribe notification service. The main contributions are:
Requirement analysis. As a basis for assessing the novelty of pervasive computing environments in
comparison to conventional distributed systems, we provide a thourough analysis of the problem do-
main. The main result is a taxonomy and a number of requirements on which we built and assess our
own solutions. Among the requirements, the need for proper support for mobility and environment-
awareness is of outstanding importance. Moreover, we compare several different communication
paradigms for distributed systems to identify one which will serve best as the basis for extensions
needed in pervasive systems. We identify the well-established publish/subscribe paradigm as a suit-
able basis for such extensions. It already addresses a number of requirements, therefore forming a
sound foundation, but falls short to meet others as detailed below.
Mobility support. As a first step towards a content-based publish/subscribe notification infrastructure
for pervasive systems, we introduce an important mechanism for transparency of mobility. This is
a common requirement for clients of the infrastructure that roam freely. Certain aspects of the han-
dling of this issue are located in the and are opaque to the client. This can be beneficial
for a client either because it is not aware of its own mobility, e.g., together with legacy applications,
or deliberately wants to delegate some aspects into the infrastructure. Therefore, we devised a re-
location algorithm that facilitates location transparency, offering the possibility to transfer existent
event-based applications seamlessly into mobile environments. The algorithm extends the exist-
ing content-based routing infrastructure to support non-interrupted, sender-FIFO ordered delivery of
notifications in the mobile case, without having a client even to be aware of this extension.
Location-dependent subscriptions and notifications. The next logical step is to provide means for
mobility-aware applications to express their interest in events and data related to their current en-
vironment. To do so, we choose location as a well-understood and rich indexing scheme on such
information. First, most information can be related to some location and next, we need strong
selection criteria to distinguish relevant from irrelevant information. However, to make location us-
able together with a content-based publish/subscribe notification service, we introduced a special
location model. It serves as the foundation for location-dependent subscriptions and notifications,
respectively. The challenge from the point of view of the publish/subscribe infrastructure is twofold:
first, hiding the details and burdens of adaptation of subscriptions to the current
position of a client. Second, due to the uncertainty of the client position and movement, to keep
delivery of information timely and accurate and to keep the network load for the client bearable. WeSummary iii
introduce an adaptive algorithmic solution that addresses both challenges. It offers delivery guar-
antees, normally only found together with network flooding, but contrary to flooding, leverages the
information about the client subscription’s relevancy in space to restrict the degree to which uncer-
tainty of location related message delivery is necessary. Thereby, an effective means to express and
implement location-awareness is introduced.
Decoupling in space and time. To a large degree the previous solutions, together with the basic pub-
lish/subscribe paradigm, already decouple sender and receipient of data in space and time. However,
an inherent danger of asynchronous, anonymous communication is the unpredictability of when
data is generated. This can be harmful in cases where a mobile client needs a certain number of
notifications to reach a consistent state, from which its execution can commence. Here, we propose
techniques to access past information. We devise mechanisms in the infrastructure, enabling a client
to minimize the time-span it h