Establishing Service Communities in Peer-to-Peer Networks1
Carsten Jacob*, Heiko Pfeffer†, Lei Zhang††, and Stephan Steglich†
*
Fraunhofer Institute for Open Communication Systems (FOKUS),
Kaiserin-Augusta-Allee 31, 10589 Berlin, Germany
carsten.jacob@fokus.fraunhofer.de
†
Technische Universität Berlin,
Sekr. FR 5-14, Franklinstraße 28/29, 10587 Berlin, Germany
{heiko.pfeffer|stephan.steglich}@tu-berlin.de
††
Wireless Technology Innovation (WTI) Institute
Beijing University of Posts and Telecommunications
Beijing, P.R.China
zlscorpion@gmail.com
the effort for defining complex descriptions, e.g., for service
interfaces or user preferences.
Our approach therefore focuses on so-called Universal
Service Terminals that can be organized in communities that
commonly fulfill semantically defined tasks and are able to
exchange data in an autonomous, context-aware, and
user-centric manner. Hereby, particular attention was put on
the active information exchange and the utilization of
semantics to reduce the usage complexity by minimizing the
need for extensive service descriptions. Special emphasis is put
on the integration of handheld devices also reducing the
provided functionality to cover as many device types as
possible.
The remainder of this paper is organized as follows.
Section II describes related work in terms of service
composition approaches, semantic description languages, and
P2P technologies. Section III defines the notion of Universal
Service Terminals (USTs) also examining the basic
architecture approach. UST Service Communities as we define
them are discussed in Section IV. Here, the focus is put on the
utilization of a distributed blackboard approach for
information sharing. In Section V an initial realization
approach is depicted. Finally, Section VI concludes the paper.
Abstract—The increasing device heterogeneity and mobility in
Peer-to-Peer (P2P) networks make flexible and context-aware
middleware approaches necessary to enable the user to benefit
from this trend. Current approaches regarding ad hoc service or
device interworking basically focus on seamlessly connecting
discovered device functionality that is normally defined in
respective static descriptions. At the same time, Web 2.0
technologies exemplify the success of collaboration based work,
whereas the considered community aspects and interworking
remain on user and data level only.
In this paper, we propose an approach that expands the
integration of community aspects to the service layer to enable a
seamless interworking of different devices and services by
applying semantics and Web2.0 principles to P2P environments.
The notion of Universal Service Terminals is presented being able
to process semantic user requests by means of multiple
cooperating devices. Regarding this, a distributed blackboard
approach is utilized to facilitate an active information exchange
enabling a best-effort treatment of the user’s goal and enhancing
common functionality such as service discovery. Special emphasis
is put on the support of handheld devices.
Keywords—Universal Service Terminal; service communities;
context-awareness; Peer-to-Peer networks
I. INTRODUCTION
II. BASICS AND RELATED WORK
Current approaches regarding ad hoc service or device
interworking basically focus on seamlessly connecting
discovered device functionality that is normally defined in
respective static descriptions. The increasing user mobility and
information overload demand for approaches that are
additionally able to intelligently manage and disseminate
available data and provide services in P2P environments in a
context-aware manner. Therefore, enhanced interaction and
service models are needed that, above all, allow for minimizing
As far as device and service interworking approaches are
concerned, service discovery and composition are particular
important aspects that determine the quality of the outcome.
II.A. Service Discovery and Composition
Typically, resources that can be potentially used over the
network are modeled as services. The services are normally
associated with metadata describing them and their usage
pattern. Many services that are available within the
environment are discovered, e.g., using a specific service
discovery mechanism [1], [2], [3]. The descriptions of
discovered services are stored in a centralized directory as in
SLP [2] or in a distributed directory structure as in Jini [1].
1
This work was supported by the German Federal Ministry of Education
and Research within the “Universal Service Terminal Enhancement (UST+)”
project.
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1-4244-1457-1/08/$25.00 © IEEE
This full text paper was peer reviewed at the direction of IEEE Communications Society subject matter experts for publication in the IEEE CCNC 2008 proceedings.
Applications may access the directory to locate one or more
needed services. The directory performs a query on the
registered services and returns a matching service, if found. In
systems such as SpiderNet [6], Anamika [5], and the recent
web services infrastructure [4], where service composition is
supported, requesting applications are accommodated by
locating complex combinations of simple services not having
to discover multiple services on their own.
idea of Universal Service Terminals (USTs) that basically
addresses the vision of combining three important aspects on
the service level.
III.A. The UST vision
First, service communication with regard to the use of
interfaces, data exchange, and access is supposed to be realized
in a simple and consistent manner. Second, based on the
successful communication, service cooperation is enabled.
Here, interface descriptions, a distributed use of services,
composition, and resource management are of particular
interest. Third, service cooperation should be take place in an
organized manner, which is especially addressed by the use of
semantics, the incorporation of context information, and
effective recovery mechanisms.
These three aspects, if regarded in a user-centric and
target-oriented way, basically define the vision the UST
approach aims at and introduce important benefits for the user.
First, he or she addresses a desired functionality, task, or
general goal instead of a particular service. Second, the user
does not have to define complex descriptions before being able
to use context-aware services. Because of the active
information exchange and data dissemination in mobile UST
environments, he or she is furthermore able to make use of
information that would not be locally available otherwise.
Third, the user does not have to care for particular aspects, e.g.,
recovery or security, but has one simplified GUI.
In summary, the UST approach aims at a semantically
enhanced solution for ad-hoc and infrastructure based
environments facilitating a context-aware and autonomic
inter-device communication and data exchange. Additionally,
handheld devices are especially regarded to address mobile ad
hoc environments.
II.B. Semantic Description Languages and Frameworks
Semantic description languages are utilized to define
metadata that can be used to make information
machine-interpretable and to infer additional implicit facts to
benefit discovery and composition mechanisms.
For example, the Web Ontology Language (OWL) [7] is a
description language based on XML that can be used to
semantically describe entities and properties. OWL-S [8] uses
OWL to define semantic service descriptions, which can be
grounded to the WSDL [9] of related Web Services. Besides
OWL-S, semantic web frameworks such as WSMO [10] and
SWSF [11] define their own ontologies and rule languages
describing various aspects of services in the web.
Regarding the composition of services, a number of
middleware approaches have been proposed [12], [13], [14],
[18] for web services that exploit the semantic information
associated with service descriptions.
II.C. Device Interoperability Approaches
In the area of middleware technologies incorporating
multiple different devices and communication technologies
there are many promising approaches having distinct focal
points.
For home, for example, UPnP [15] facilitates data exchange
among devices in local area networks defining respective
mechanisms, e.g., for addressing, discovery, and description.
JXTA [17] is a P2P technology developed at Sun
Microsystems with the objective of providing interoperability
in data sharing, platform independence (language/OS) in
service sharing, and ubiquity across the range of devices
covered. JXTA consists of the core, which provides security,
monitoring, and group services among peers. The JXTA
services layer provides a range of functionality such as
indexing, searching, and sharing. The JXTA applications layer
allows users to build P2P applications in the JXTA framework.
Services and information are ‘advertised’ in XML descriptors.
These descriptors are searched using a peer discovery protocol.
Access to these services is provided by the JXTA services
layer.
In [16] the notion of Virtual Devices is introduced
encapsulating and hiding multiple devices in the Personal Area
Network (PAN) of the user to provide one interface.
Furthermore, a middleware is proposed for the realization of
Virtual Device.
III.B. Basic Architecture
Figure 1 depicts the basic architecture incorporating a
Universal Service Layer with multiple modules for different
tasks. Those modules are supported by enabler entities
providing functionality used by some of the modules.
Application
GUI
Application Logic
Bootstrapping
Universal Service
Request Processing
Description&Discovery
Composition
State Management
Recovery
Event Notification
Security&Privacy
Configuration
Data Exchange
Data Transport
Interaction
P&S
SDS
...
Invocation Style
Transport Protocol
Network Protocol
Figure 1: Basic architecture approach
III.B.1. Layered Approach
III. UNIVERSAL SERVICE TERMINALS (USTS)
The data exchange layer defines the style or protocol that is
used to invoke methods and represent data in a proper way,
e.g., REST or RPC. The interaction layer is used in addition to
To allow for a seamless information exchange and use of
distributed services in P2P environments we developed the
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the universal service layer to define interaction models services
may stick to, e.g., publish-subscribe or Semantic Data Spaces
[19]. This layer can be used to harmonize service interaction
having a common representation and messaging paradigm. The
universal service layer provides all the functionality that is
needed for creating and administering Universal Service
Terminals. Desired functionalities comprise state management,
service composition and recovery, security and privacy,
service discovery and descriptions, system configuration,
request processing, and event notification. Bootstrapping is
considered as the process initiating and preparing the UST
device by properly loading and configuring needed modules.
During this phase, the data exchange, interaction, and universal
service layer are supposed to be setup.
composition data managed by the composition module, usage
policies utilized by the security module, and context
information used by multiple modules, e.g., the request
processing module.
To address this aspect, the UST Distributed Blackboard
(UDB) is introduced as one mediator enabler to facilitate a
community-like communication among distinct devices. This
blackboard is an optional building block in the UST layer of
variable size that can be used by local or remote services for
data exchange, i.e., a local or remote module can add or
remove data as desired to/from the blackboard. This
functionality can therefore be used to propagate a piece of
information in highly dynamic networks of UST devices. To
increase security aspects the blackboard is used as an
information base, but is not intended for mobile code. To
facilitate the later use of exchanged data, any added piece of
information is expected to be semantically tagged. Indeed,
because of the open information sharing the usefulness of data
is not guaranteed. The local description&discovery module is
responsible for administering the blackboard, e.g., removing
data that is not used for a long time. The security&privacy
module may be used to restrict the UDB access for particular
remote UST devices. The easiest way to realize the UDB idea
is a cached semantic model that reasoning processes can
interpret while also providing a simple interface to external
nodes to add and remove concepts and properties, e.g., defined
in OWL.
III.B.2. Universal Service Layer (USL) Modules
The Universal Service Layer defines a number of conceptual
modules that are responsible for specific tasks, but are not
necessarily separated in this manner as far as the actual
implementation is concerned.
The request processing module receives a user request
stemming from the local device or a service request from
another device and is responsible for processing it, i.e., calling
the other modules accordingly and executing services. The
description and discovery module administers service
descriptions (its own description as well as discovered ones)
and finds suitable services of other devices. The composition
module is responsible for finding and composing services to
form a proper community fulfilling a particular user request.
The state management module is responsible for
synchronization issues, e.g., if the current device is
incorporated in a composition that requires the maintenance of
a common state. The recovery module is closely connected to
the composition module being responsible for finding and
integrating a suitable service in case another service of a
composition disappears. Event notification describes the
process of informing other UST+ devices or being informed by
them about state changes while a process is actually running.
This event may cause the currently executed service or
composition to be altered or stopped. Security and privacy
ensures the proper use of the device services as intended by the
developer, vendor, and user. The module is therefore
responsible for configuring the services offered by the device
according to defined policies. The configuration module is
used by the user to configure the UST+ layer and related
services according to his or her wishes. For example, in some
cases it may be necessary to use a particular technology or
service instead of those the UST middleware normally utilizes.
Here, it is also possible to alter device, service, or user
information.
IV. UST SERVICE COMMUNITIES
Special regard in our approach with respect to Universal
Service Terminals is put on the community-like behavior of
distinct UST devices in P2P networks enabled by the UDB.
IV.A. Web 2.0 and mobile P2P environments
We identified four aspects that are especially interesting and
relevant in this context and that are often characteristic for user
communities in the Web 2.0 domain.
First, parties taking part in such kinds of communities work
commonly with respect to a particular goal, which is often set
by the domain or topic of the community, e.g., students or
sports. Second, active and open information sharing is a very
typical aspect, i.e., community members are allowed to
contribute to a common knowledge base exchanging
information with each other. Third, the participating parties
normally possess complementary information or services
relevant for others, which, in our case, may also be
functionalities. Fourth, the data exchange is enabled by an ad
hoc and loosely coupled grouping or interaction in terms of a
simple way of addressing and being connected to other parties.
With respect to Web 2.0 communities web portals are one
common means of easily connecting people.
Each one of these four aspects is supposed to be addressed
by our approach to facilitate a community-like communication
for UST services and devices. The use of semantics for
describing data and services is also applied for the definition of
a common goal that is automatically interpretable by each
participating device. The idea of a sharable UDB aims at an
III.B.3. USL Module Enablers
Multiple modules described above rely on data that is
provided by external services, but may be cached internally
depending on the device capabilities. These data may comprise
service descriptions administered by the discovery module,
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active and open way for sharing information. The Universal
Service Layer enables the use of functionality provided by
distinct and distributed devices. Here, the assumed device
mobility increases the diversity, and semantics can be applied
to reasonably combine available services and data. Finally, the
loosely coupled way of communication is addressed by
introducing suitable service interaction models such as the
Semantic Data Space [19] based on REST [20] principles, but
also by the Universal Service Layer that autonomously
processes provided data.
a new goal provided by the user, .e.g., achieving the semantic
effect “bright” in the current location, a new task ti is generated
by the request processing module and sent together with a
pointer to its blackboard space by broadcast to all nodes.
Descriptor
Functionality
serviceID
Denotes the unique ID of the service, e.g. its
REST URI.
Identifies the ID of the Blackboard.
Represents the number of parallel executions
of the service still possible. For instance, a
service that can be used in four service
compositions in parallel at most and which is
already involved in three, has an execState
of 1.
Holds a list of all IDs of Service Compositions
the service is involved in.
blackboardID
execState
IV.B. The UDB and handheld devices
Regarding the realization of the UDB idea emphasis is put on
supporting nodes with different and limited capabilities.
Restrictions can be made according to a number of aspects,
e.g.:
• The kind of data stored in the blackboard and the
respective lifetime to address limited storage capacity.
• The reasoning complexity to address limited computing
capabilities.
To especially regard for handheld devices in our approach we
decided to support three classes of nodes. Tiny nodes are
normally sensors only providing services that can be used by
other nodes, but have no interaction models or mediators
installed. Basic nodes support restricted interaction models and
mediators. Full nodes provide full support for available
interaction models and mediators.
We assigned most common handheld devices into the basic
class because of limited storage, computing capabilities, and
energy supply. The boundary between basic and full support is
currently determined by Java CDC since we decided for Java
ME as the implementation platform. With regard to the
blackboard idea we defined four classes:
• (0) no blackboard supported
• (1) plain semantic model (RDFS) and simplified interface
to add and retrieve data
• (2) enhanced model (OWL Lite or DL) and basic
operations supported (e.g., get the superclass of a
concept)
• (3) enhanced model and full reasoning support.
Tiny nodes are supposed to support classes 0 or 1, basic
nodes classes 1 or 2, and full nodes classes 2 or 3. Hence, the
blackboard version for basic nodes does not provide extensive
reasoning mechanisms as, for example, provided by Pellet [23]
for OWL and supported by fully equipped nodes such as
desktop PCs.
Each service running on an UST device is able to access a
particular part of the UDB. To identify the respective
blackboard an ID is added to the service’s specific state that is
administered by the State Management module. The four main
descriptions of the service state are listed in Table 1.
The state of a node is uniquely defined by a finite set of
service states from the services it hosts and an ID nodeID.
USTs are built dynamically when a user’s goal requires the
collaboration of services from different nodes.
The supporting use of the UDB can be illustrated by means
of a simple service execution example. In case a node receives
compState
Table 1: Service State Descriptors
In case a node receiving the task finds a service si matching
the task ti based on semantic service descriptions, e.g., a lamp
that can be switched on, the node sends a message and si is
bound for solving ti. Every node that is not able to find an
appropriate service si for ti scans the blackboards of the
services it is hosting in order to search for knowledge related to
this task. For instance, one service may be able to relate similar
tasks ti to ti ’. This node uses the Blackboard pointer
blackboardID from the requesting node in order to create a
new Blackboard space for ti and inserting its according
knowledge. In case the requesting node does not receive an
appropriate service si matching ti within a certain timespan
(decided by a timeout), the requesting node may check its
blackboard whether knew knowledge on ti exists. After parsing
the knowledge that ti may be similar to ti ’, it can decide to
request a service matching ti’ instead, probably receiving a
binding for service si ’, e.g., open the curtain if the sun is
shining. Although the service may not be exactly the type of
service striven, it can help to provide a best effort solution to
the user. In case neither an appropriate service can be found for
ti nor additional knowledge is available to modify the task, the
composition attempt has failed.
In summary, the blackboard is intended for supporting the
other modules of the universal service layer such as the service
discovery module. The provided additional information may
be incomplete and most likely does not cover a complete
domain, but only parts of it. Because of the semantic
representation interested nodes are able to relate existing
information and make use of it. Data exchange and node
cooperation is enabled by suitable interaction and data
evaluation models, e.g., as illustrated in [22]. In addition,
gossiping algorithms [24] envision the random propagation of
data in P2P environments being easily applied to the
blackboard approach.
V. REALIZATION
Our implementation approach utilizes a REST-based [20]
data exchange layer in the Java programming language
providing interfaces for both the Universal Service Layer and
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the Interaction Layer. The Interaction Layer currently supports
the Semantic Data Space, whereas the basic concepts are
described in [19]. A simplified Service Discovery Mediator is
implemented whereas participating nodes are able to
synchronize administered service lists by means of UDP
multicast messages. As far as semantics are concerned, we
apply OWL for the description of requests and responses being
processed with the help of the Jena Semantic Web
Framework [21]. Further modules are under development.
As mentioned in Section IV.B, we developed a thin client in
J2ME for basic nodes based on the CDC framework and
Foundation Profile to also support handhelds not being
necessarily able to take part, for example, in expensive
reasoning processes. The current size of the core framework’s
jar file is about 100 kb containing 40 classes. Additional
libraries are needed, e.g., to enable the REST-conform
communication, resulting in an executable jar file. Challenges
in the implementation process aiming at the support of basic
nodes especially pertained to API restrictions such as, for
example, limited functionality of available classes, e.g., the
String class, the constricted class library, or type conversion
constraints.
We are currently enhancing our blackboard approach and
also focus our work on a proper request processing module,
whereas user preferences and context information are
represented in OWL to ease the matching process. For the
UDB implementation we plan to combine work done in [22]
referring to data dissemination and evaluation with a more
generic SDS approach not only being able to cope with service
requests, but general data.
[4]
[5]
[6]
[7]
[8]
[9]
[10]
[11]
[12]
[13]
[14]
[15]
[16]
VI. CONCLUSION AND OUTLOOK
In this paper we presented basic concepts regarding service
communities for Universal Service Terminals. Based on an
initial architecture definition, we described communication,
cooperation, and organization aspects with regard to
distributed service interworking. Hereby, the focus was put on
the idea of a distributed blackboard for information exchange
and service discovery support.
In our ongoing work we advance our current
implementation and combine it with aspects derived from data
dissemination mechanisms. Special attendance is put on
service composition and the mapping to other approaches, e.g.,
UPnP, and infrastructure-based environments.
Further work comprises the introduction of trust principles
in our approach to be able to apply data dissemination and
access policies.
[17]
[18]
[19]
[20]
[21]
[22]
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