Plastic

June, 2006 PLASTIC Consortium

4 Conceptual Model
In this chapter we describe a first version of the PLASTIC conceptual model. It builds upon
previous work of the consortium partners in the area of mobile distributed computing platforms
and serves as input to the development of technical work packages WP2-4. The model (as
planned in the PLASTIC DoW [23]) will be incrementally revised at each reached milestone on
the basis of the enhanced knowledge acquired in the domain of mobile context-aware
services.
According to the PLASTIC DoW [23] as described in Task 1.3 a component model is the key
element in a service-oriented software architecture. In the PLASTIC setting we devised a
conceptual model of components and services that takes explicitly into account platform
heterogeneity by means of the notion of resource/context awareness. Resources may range
from hardware devices to software interaction protocols, whereas awareness drives adaptation
of the component/services to the characteristics of the execution context, in order to provide a
best effort service adaptation with respect to QoS attributes. The conceptual model
representation we present in the following is described by means of a UML-like notation.
The main role of the PLASTIC conceptual model is to build a common vocabulary on which all
the partners can base their modeling and development tasks. The effort spent to achieve this
goal will make communication among all the partners easier and unambiguous.
Recently, several approaches to conceptualize the world of services have been proposed.
Our model is inspired by the SeCSE conceptual model [13]. This model aims at providing a
common terminology across the SeCSE IST Strep project [19] for letting all the involved
people communicate with each other using a common language and a common semantics.
The model has been conceived to be extensible since it can be easily adapted in order to
accommodate needs coming from different service-oriented domain. The SeCSE conceptual
model was initially inspired by the Web Service Architecture [22] (WSA) drafted by the W3C to
introduce a unified model in the domain of service-oriented applications. However, the SeCSE
model can be seen as complementary to the one of WSA since SeCSE tries to clarify and
detail the concept of service more than the WSA.
We extend the conceptual model of SeCSE by introducing new concepts and relations. In
particular we introduce the concepts of: context, location, adaptation, resources available at
the consumer side, service level agreement. In addition, we introduce into our service model
the concepts related to software components. The need of this integration comes from the
adaptive nature of B3G applications, as we discuss in Section 4.3.2. To the best of our
knowledge, the conceptual model we propose is one of the first attempt to explicitly specify the
relationships between Service Oriented Architectures and Component-Based Software worlds.
In the following, before introducing the description of the conceptual model (Section 4.4), we
briefly introduce relevant related work (Section 4.1), we discuss the meaning of services in
real-world situations (Section 4.2), and how this is reflected in the software domain. Then we
discuss differences and similarities of component orientation and service orientation (Section
4.3). This issue is particularly relevant in our context since PLASTIC services rely on
component-based applications. In Section 4.3.2 we discuss how these two approaches can be
combined in order to produce service oriented component models .
We remark that this chapter reports our attempt to conceptualize the world of services within
the PLASTIC project. However, we believe that, although addressing the specific domain of
services in the B3G domain, the conceptual model we propose may be suitable for any
domain where adaptation with respect to QoS and context characteristics is central.

PLASTIC IST-26955 80/98


4.1 Related work
In this Section we synthetically introduce the work that has been considered in order to define
the initial conceptual model for PLASTIC.
The WSA conceptual model [22] is structured in four areas, each focusing on a specific
aspect: (i) Service Model for service concept relationships; (ii) Message Oriented Model for
message transmission, processing and the relationship between message senders and
receivers; (iii) Resource Oriented Model for uniquely identified, owned resources that are
relevant to Web services; (iv) Policy Model for modeling aspects of the architecture that relate
to constraints, security and QoS.
The SeCSE model marginally considers the message oriented, resource oriented, and policy
models of the WSA, but it tries to clarify and detail more than the WSA does for the concept of
service, and service-related activities (i.e., publication, discovery, composition, and
monitoring).
Also, the WSA message models are not concerned with any semantic significance of the
content of a message or relationships to other messages; the SeCSE conceptual model
makes clear the relationships between service description, semantics, and service interface.
The work in [15] proposes a SOA reference model for identifying the entities involved in a
service-oriented environment, for understanding the roles played by them and for establishing
relationships between them. The proposed reference model is a truly abstract framework that
provides a higher level commonality by supplying a large set of definitions that any SOA based
initiative should take into account. The model is not tied to any particular implementation
details, standards and technologies. Focusing on software architectures, it intends to be the
basis for describing reference architectures and patterns that can be used to define particular
SOA.

4.2 Services and Service Oriented Architecture
Bringing together the vision of the academic world and the vision of SUN [21], IBM [12] and
Microsoft [6] about Service Oriented Architecture (SOA), in this section we discuss the basic
questions arising in the world of services and we describe the SOA vision by highlighting
principles and primary involved entities. The questions we address are what are services? and
what are software services?
In the survey [6] the authors collect and classify a set of terms related to the concept of
service. They distinguish between terms - such as IT-services, information services, public
services, governmental services, etc - that consider domain-related contents of the service
and terms - such as service, e-service, Web services, Real-World services, commercial
services, etc - that are called “service terms'' and that strictly relate to the definition and
essence of a service itself.
Focusing on service terms, the authors point out how these terms have multiple interpretations
for the different communities of business science, information science and computer science,
resulting in confusion. That is, starting from the awareness of the existence of differing
terminologies among different research communities, they explain how these different
interpretations are related and how they can be combined in order to achieve a common
understanding and a shared terminology for a profitable collaboration.
Concentrating on the terms services, e-service and Web-service (i.e., most commonly used
’service terms’) and surveying visions of the above mentioned communities, the authors collect
definitions for these terms.
The business science community makes use of the term services (with no prefix) to refer to
“business activities that often result in intangible outcomes or benefits; they are offered by a
service provider to its environment''. The same community gives three different - yet similar -


interpretations for the term e-service. According to two of them, an e-service is “an Internet-
based version of a traditional service'': one interpretation is more extensive since it mentions
customer relationships or business processes; the other one does not. The third interpretation
includes the first one but it is much extensive since it is not limited to the Web but it allows for
the “provision of service over generic electronic networks''.
Web service is not considered a business term, and when it is used one of the definitions from
the computer science community is adopted. Many definitions exist within the computer
science community for the term Web Service. Bringing together elements such as
software/applications, software functionalities and Internet, in [6] the authors notice that all
the definitions agree on the fact that Web services are software/applications to be used on the
Internet. Most of them explicitly recognize “the existence of functionalities behind the software
but not the existence of business processes or business functionalities''.
There is no general agreement for the term e-service. Sometimes it refers to “the realization of
federated and dynamic e-business components in the Internet environment'' but often it is
considered a Web service synonym. Moreover, the term service is also used as a synonym for
both Web service and e-service. Some, on the other hand, give to the term service a business-
based interpretation defining it as an intangible product, benefit, good.
Finally, in [6] the authors conclude the comparison by stating that, for the information science
community, the term Web service is used like in computer science and the term service is
mostly used like in business science. In the same manner, e-services are interpreted either as
an Internet-based version of traditional services (similar to many researchers from business
science), or as Web services (similar to many computer scientists). Other terms such as
commercial service and real-world service are sometimes used to refer to services in their
business meaning.
According to the W3C Web Services Architecture (WSA) [22], in [18] an abstract conceptual
architecture for semantic web services is presented. In this case, the definition of the
requirements on the architecture came from analyzing a set of case studies. Even though the
author concentrates on semantic web-service, the discussion on how the word service can be
used in several different ways in different domains is interesting.
Recognizing the importance of three main aspects such as (i) “value'' in some domain, (ii)
implementation of entities able to provide what is needed, and (iii) interactions involved, three
usages of the word service are given.
(i) Service considered as provision of value in some domain: distinguishing between
the particular provision of the value itself and the general capability to provide, the
author refers to the former as a concrete service and to the latter as an abstract
service. According to this distinction, “a concrete service is the provision of something
of value, in the context of some application domain, by one party to another''; “an
abstract service is defined as the capacity to perform something of value, in the context
of some application domain''.
(ii) Service considered as a software entity able to provide something of value:
commonly used in the computer science and IT community. In this domain the author
prefers to define a service as “a software entity being (part of) a service provider
agent''. Rather than speaking about sending messages to services, receiving results
from services, executing services etc, the author prefers to maintain generality in order
to avoid confusion when mixing the usage of this definition with the previous one.
(iii) Service as a means for interacting online with a service provider: this usage is
referred to as the “provision of a negotiation protocol or choreography''. Since the
negotiation is not itself a value this usage is different from the usage (i) but, since the
outcome of the negotiation may be a value this usage may be a service in the sense of
(i).



According to the definition given from the European Commission [5], one of the more abstract
and context independent definition of services might be: “Services are labors that, if
accomplished, produce a non tangible useful benefit''.
Still according to the definition given from the European Commission [5], “Software Services
are functionalities provided by software applications that supply computational resources or
informational ones on request performed by an hypothetical service consumer”. The
interpretation that we give to this definition is that an hypothetical service consumer can use
the provided functionalities for exploiting computational resources and for obtaining
informational data, both of them abstracted in software services. In other words functionalities
are abstracted by services.
The potential of service oriented applications has brought to introduce in the software
development domain (beside the Software Architecture view) the definition of SOA. However,
in order to be an architecture model, the service domain needs more than just service
descriptions: the overall application dynamics has to be described by a workflow between
services.
Software Architectures (SA) are meant to abstractly model the overall structure of a system in
terms of communicating subsystem. In a component based setting, this general definition of
SA can be rephrased by substituting the word subsystem with set of software components
interacting through communication channels.
In a service oriented vision, SOA are meant to abstractly model the overall structure of
service-centric software systems in terms of communicating services. They are a special kind
of software architectures that have several unique characteristics according to a set of
principles that we later discuss.
Alternatively to the traditional tightly-coupled object-oriented models that have emerged in the
past decades, SOA is an alternative loosely-coupled model. Moreover, the most significant
aspect of service-oriented architecture is that it separates the service implementation and the
service provider from its contractual description. In this way, it separates the “WHAT” from the
“HOW”. The service description is given in neutral manner, independently from any platform
and implementation detail. Service consumers view a service simply as an endpoint that
supports a particular request format or contract. Service consumers do not need to know how
the service will act for satisfying their requests; they expect only that it will.
Due to the rapid evolution of software service technologies, the development of service-based
software applications can lay nowadays on a solid support and is adopted in ever wider
application domains. Applications are built as assemblies of existing services, and more
mature tools for discovering (over the web) and adapting services are being developed.
According to the technical protocols and business model used, the European Commission [5]
classifies services saying that Web Services, Grid Services, P2P Services and SOA are
classes at the same level. In our vision, Service Oriented Architectures are the super class of
all the other classes since it is the most general, business model independent protocol.
SOA is not new. A classic example of a proto-SOA system that has been around for a while is
CORBA, which defines similar concepts to SOA. SOA is different from it in the roles played by
“interfaces'” (description). In fact, SOA relies on a more flexible and neutral language for
describing interface. If we consider Web Services (WS) as a particular implementation based
on SOA, then the description of interfaces in an XML-based language (called WSDL [8]) has
moved WS to a more dynamic and flexible interface system than the older IDL of CORBA.
Web services are not the only way to implement SOA. Message-Oriented Middleware
systems, such as the IBM MQ Series [11], represent an alternative and, as just mentioned
earlier, CORBA was a first attempt. The difference is on “how well” they meet the “directives”
of the SOA philosophy.



4.3 Component Orientation VS Service Orientation
In this section we describe the main peculiarities of component and service oriented
approaches starting from the work about service oriented component model in [10]. Then, we
describe how these approaches can be combined in order to obtain a two-layer framework that
provides infrastructures for building service-centric and component-based applications while
keeping composability in both domains. We also discuss to which extent such an approach is
suitable for the PLASTIC development platform.

4.3.1 A classical view
Starting from an assembly-oriented view of software engineering, Component Based Software
Engineering (CBSE) [1] promotes the construction of a software application as a composition
of reusable building blocks called components.
In a component based approach, assemblers build applications by wiring ports of connectors
and “ready to use” components. In other words, integration is carried out at assembling time.
As a direct consequence, composition is structural in nature and the component should
provide the external structure of its instances (such as provided and required functional
interfaces). Preconditions, post-conditions, and a behavioral specification of the interaction
with its environment may be also required to allow assemblers to connect and eventually
adapt the various component instances. Hierarchical structural composition is achieved when
the external view of a component is itself implemented by a composition.
Unfortunately, software components are often hardly reused and Component Adaptation (CA)
is emerging as a well-differentiated discipline. CA focuses on the problem of reusing existing
software components when constructing a new application, and mostly promotes the use of
adaptors (specific computational entities) for solving this problem. One of the main goals of
software adaptors should be to guarantee that components will interact in the right way not
only at the signature level, but also at the interaction protocol and semantic levels.
Our experience [14][16] on component adaptation is based on the automatic synthesis of local
adaptors (one for each component) that constitute a decentralized and possibly distributed
adaptor for the component based system.
Note that, the approach acts at the interaction level and the intent of the distributed adaptor is
to moderate the communication among the components in a way that the system complies
only to a specific behavior.
On the other end, resource awareness identifies the capability of being aware of the resources
offered by an execution environment, in order to decide whether that environment is suited to
receive and execute the application. In this concern, adaptation identifies the capability of
changing the application in order to comply with the current resource conditions of the
execution environment. In [9] we propose a framework for the development and deployment of
adaptable component-based software systems.
Sharing the same idea of component orientation, in service orientation software applications
are assembled from reusable building blocks, but in this case the blocks are represented by
services.
Each service has a description of provided functionalities that can be contractually defined as
a mix of syntax, semantics and behavior specifications. The basis for service assembling are
only service descriptions. In this way, differently from the component orientation, the service
integration may be carried out prior or at run time. In such a context, it is clear that service
orientation concentrates on how the service are described in order to support dynamic
discovery and possibly run-time integration.



In order to understand how dynamic discovery may be supported, it might be useful to
describe the Service Discovery Pattern [3] (see Figure 20) and the roles played by the service
registry, service provider and the service consumer entities.

Service Registry

Service Description
Service Description

publish discover

Service Oriented
Interaction Pattern
Service Provider Service Requester
bind

Figure 20 - Service Discovery

Service providers publish their service descriptions into a service registry. According to the
service request format, service requesters query the service registry to discover service
providers for a particular service description. If one or more providers are present in the
service registry at the moment of the request, the service requester can select and bind to any
of them. When a service requester binds to the service provider, the latter returns a reference
to a service “object” that implements the service functionality.
Service orientation promotes the idea that a service may be supplied by different service
providers. In fact whenever other providers comply with the contract imposed by the service
description they can be interchanged. In this way a requester is not coupled with a particular
provider.
Another fundamental characteristic of service orientation is that services exhibit dynamic
availability, since they can be added or removed from the service registry at any time.
Consequently, running applications must be capable of releasing departing services or of
incorporating newly arriving services.

Service Orientation Component Orientation
Emphasis on software service descriptions. Emphasis on component external-view.
Natural separation from description and No evident separation between component
implementation. external-view and implementation.
Only service description is available during Component physically available during
assembly. assembly.
Integration is made prior or during execution. Integration at assembly-time.
Better support for run-time discovery and No native support for run-time discovery and
substitution. difficult substitution.
Native dynamic availability. No native dynamic availability.
Abstract composition. Structural composition.

Table 1 – Main aspects of Service Orientation and Component Orientation.

Since services composition is based only on service description, it can be seen as an abstract
composition that becomes concrete only at run-time.


Table 1 summarizes in a point-to-point view the previous discussion about service orientation
and component orientation.
The growing request of dynamic and mobile software systems that are able to exploit available
resources and to adapt to them in order to achieve a certain level of QoS has pointed out
several limits of the component orientation approach for this type of software applications.
Component orientation does not provide mechanisms crucial for mobile and adaptable
applications that need to evolve the system at run-time. As reported in the table, component
orientation (i) has no native support for dynamic discovery and availability and (ii) it allows
easy integration only at the assembly-time that makes dynamic component substitution very
difficult to implement. These limitations are strictly related to the lack of clear separation
between the component description and its implementations.
Service orientation instead overcomes these limits by providing useful mechanisms for
dynamic evolution. However, the service orientation, differently from the component
orientation, is more abstract. In fact, it does not describe or give indications on how services
should be implemented, leaving to developers the freedom to choose the way to implement
the software services. On the other hand, component orientation allows excellent structuring of
the implementation code easing the integration and maintenance of available components.
We believe that component and the service orientation can be combined to overcome the
limits and the lacks of both approaches. We discuss in the following section how we propose
to realize such combination.

4.3.2 Our view: a two-layer approach combining services and components
In this section we present our vision of software architectures in the B3G domain [23]. The
vision has been driven from the characteristics of this domain that force a tight integration of
component and service concepts. In B3G applications services should always guarantee a
certain level of QoS to the consumer despite the heterogeneity and richness of the underlying
networks. At the same time, the components implementing the services should have the
capability to adapt to different contexts. In few words, QoS and adaptation are the keywords
that characterize the vision presented in this section.
Figure 21 shows our two-layer vision of a software application. The layers are: service layer
and component layer.
The component layer represents the computing environment for networked services that is
able to manage the life cycle of the software components while delivering functionalities for a
device from anywhere in the network. Software components can be deployed, updated, or
removed on the fly without interrupting the operativity of the device.



Service Service Description

S8
Service
S4 Layer
S9
Service
S5
S3 Service Composition
request S7
S10

Service
Consumer
C7 C9
Software
C4 Developer
C2
C1
Component Assembling
C3 C6
C5 Component
C8 Layer

Component Component Description

Figure 21 Two-layer approach.

Components developers bind to services by only using service descriptions. In other words,
components contractually implement service descriptions but the selection of the specific
component to instantiate is deferred at run-time by taking into account the service requester
needs and the environment/context of service access.
The service layer manages two aspects: (i) the description of services and their composition,
(ii) the mapping between services and set of components implementing them.
Two main actors are represented in Figure 21: Service Consumer and Software Developer.
The former can only act at the service layer by formulating a service request. The request may
refer to an existing service or may bring to assemble several services to provide a new service
as specified from the Service Consumer. Software Developer may act either at the service
layer by composing services, or at the component layer by implementing new services through
component assembling.

4.4 The PLASTIC conceptual model
In domains such as the one of PLASTIC, where the focus is on service design and
development, it is important for service designers and developers to agree on concepts and
principles of SOA, so that they can have the best benefits, in terms of integration and
adaptation, when using related technologies.
Services in PLASTIC are meant to be adaptable to the context whether related to the user
setting (e.g. type of device, user mobility) or the available network resources (e.g. network
bandwidth). The goal is to provide services that achieve the best tradeoff between the user
needs (in terms of Quality of Service) and the current context characteristics (in terms of
available resources).
In order to achieve a high degree of adaptability, services must be aware of the context where
they run. Thus, we make the context characteristics emerging at the service level: a service
description will be not limited to its functional characteristics (i.e. signature and behaviour), but
it will bring additional attributes that model its QoS characteristics (e.g. service reliability,



latency time). Obviously, these attributes are parametric in that they may depend on the
context.
Our approach is somehow opposite to a typical layered architectural model, where differences
in a lower layer (in terms of functional and non-functional characteristics) disappear at the
upper layer. Our approach is in line with [24] and [25] where the authors stress the importance
of application-awareness as opposed to distributed system transparency.
We believe that some features/views of current heterogeneous networks need to be exploited
at the service level instead of being systematically standardized through middleware that aim
at hiding differences among devices and connections. QoS should become a tool in the hands
of service architects and developers that can exploit this additional information to achieve the
maximum user satisfaction.
In order to achieve this goal two main supports have to be provided: (i) non-functional models
that link QoS at the platform/network level with QoS at the service level, (ii) Service Level
Agreement strategies to achieve the best tradeoff.
Based on the considerations made above, in Figure 22 we present the PLASTIC conceptual
model as a Class Diagram, where two different graphical notations have been used to
differentiate the roles: rectangles to represent entities and humans to represent actors. The
actors are entities that interact with the PLASTIC platform in several ways.
In Figure 22, the entities and actors that have been explicitly introduced and were not part of
the SeCSE conceptual model appear in bold
The types of actors devised are: Service Developer, Service Provider, Service Consumer,
Service Integrator, Service Registry and Component Assembler.
The first class entities in the conceptual model are: Software Service, Assembled Component
and Context. The software functionalities that a system provides correspond to software
services that are implemented by means of component based software systems that we name
assembled component. The software services might be combined to obtain complex services.
The components and services compositions take into account the characteristics of the
context where the software services will be hosted.
More in details, a software service is developed by a Service Developer and it is a Provided
Functionality. Whenever a software service is implemented, the Service Provider (that can
coincide with the service developer) provides it for future usage. The service provider is the
network-addressable entity that accepts and executes requests from Service consumers. To
make services accessible from the consumers it publishes them in a Service Registry.
A service registry is a network-based directory, possibly distributed, that contains available
services, building upon service discovery technology [27]. It is an entity that accepts and
stores service descriptions from service providers and provides those descriptions to
interested service consumers.
Software services can be simple or can result from composition of other software services,
and in this case we name them as Composed Software Services. The Service Integrator is
responsible of such composition to obtain more complex services as required by service
consumers. To deal with service composition, the integrator has to retrieve the software
services and related information by involving the Service Registry.
A Service Description1 is associated to each service and it is composed by: (i) a Service
Specification defined by the service provider which describes characteristics of the provided

1
Note that all the descriptions that we define in the model (i.e. component, service and context descriptions) are
not explicitly modeled in details. However, we can assume here that all of them contain functional and non-
functional characteristics of the described entity.



service; (ii) optionally a Customer-side Service Additional Information that represents the
feedback from the service customers that used the service.
According to the SecSE conceptual model, Service specification contains information about
the service: the behaviour of the service, the service signature, other services required for its
function, optionally its operational semantics and the service invariant, the specification of the
exception thrown by the service. Again it contains non functional information: its price, the
service policy, QoS such as security, dependability and performance, its compatibility with
existing standards, and so on.
Customer-side Service Additional Information contains user supplied information of the
service. Such information is collected and stored by the user during and after the execution of
the service. It contains usage history, the satisfaction degree of the service, information
related to the execution environments, the profile of the typical user of the service and the
typical access devices used.

The service consumer asks for a service by expressing a Service Request. The service
request hence specifies several Service Request Requirements that cover functional and non
functional aspects. When a suitable service is found in the registry or a new one is
implemented, the corresponding service description matches the request. The service request
might also bring information on the Available Resources at the consumer side such as the
available memory, the size and the characteristics of the screen, the type of network
connectivity the device used to connect to the Plastic platform. Such information is a piece of
information of the Context where the service will run and will be used by the Plastic platform to
adapt the required service to the device features. According to the SecSE conceptual model,
the service request as well as the service description are representations of an Abstract
Service that can be concretized or not by a software service.

Each provided functionality, that is each service, is implemented by an Assembled Component
that is composed by a set of Software Components assembled by the Component Assembler.
The components assembler identifies the software components to use. In particular, such
components can be COTS, newly implemented ones or other Assembled Components. The
software component has associated a Component Description specifying functional and non
functional aspects of the component itself. The non functional aspects, in particular QoS, of a
component may be parameterized with respect to the context where the service will be
provided. Since service characteristics, especially QoS aspects, are affected by the
component quality, to represent this dependency in the model there is a relation between
service description and component description.

There are two main positions in the software architecture field about connectors: (i) one claims
that in software architectures components are distinct from connectors, where the former ones
are software entities implementing the logics of the system, while the latter ones are software
entities that enable the communication among components; (ii) the other assesses that a
software architecture is made by only components, that is components are software entities
providing different functionalities that span from system logics through communication
capabilities. At the moment, the proposed conceptual model contains the concepts related to
software components without explicitly considering connectors, thus complying to the second
position. However, a discussion about the role of connectors in the B3G domain is ongoing
within the PLASTIC Consortium.

The context is a relevant concept in our vision and it represents the logical and physical
resources available in the service provision. The context has a Context Description that
contains the description of the resources in terms of hardware devices, network connectivity
and software services available in the execution environment. We can use context description
in the adaptation of software components, in software services composition and in the SLA
procedure when a new service request is formulated.


In a context-aware and adaptable system domain where services have to respect the QoS
requests made by the consumer, the context drives the component adaptation. Adaptation is
made at the software component level and combine information of the context, SLA and
component description. In the conceptual model, adaptation specs is the entity that contains
the specification of the adaptation rules. The definition of such rules is driven by the execution
context.

Since we have also to model mobility, we must identify the different contexts from/to which it is
possible to move. Therefore, the context description contains an explicit entity that specifies a
location. In other words a location is a label that can be associated, if needed, to whatever
context the components will be moving across. In order to track the mobility aspect, each
component brings a reference to its (current) location. Mobility will be simply modelled by
changing the location value.

Finally the SLA is an entity modelling the conditions on the QoS accepted by both the service
consumer and the service provider. In [20] a language to precisely specify SLA has been
proposed.

SLA represents a kind of contract that is influenced by the service request requirements, the
service description and the context where the service has to be provided. When a new service
request is formulated, the PLASTIC platform has to negotiate the QoS of the service on the
basis of the service request, the context the service has to be provided and the service
descriptions of similar services already available by some providers. The contractual
procedure may terminate with an agreement about QoS of the service from the consumer and
the provider, or with no agreement. In case of agreement, an SLA is reached.

The conceptual model in Figure 22 represents the core of the conceptual model for PLASTIC
platform. This means that it contains only the first class entities for the PLASTIC domain and
the relations among them. Such first class entities will be defined in more details by means of
new sub conceptual models, one for each first class entity. as soon as these concepts are
clarified during the project development


June, 2006___ _____________________________ PLASTIC Consortium

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adapts has
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provides defines
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Composed 0..* 0..* Service Registry
Software Service serves concretizes 1..*
1..* 1..*
1..* 1..*
1 expresses
Service Request publishes
Abstract Service
Is composed by
0..* 0..* represents 1..*1..*
1..* 0..*
represents
0..*
Service Consumer matches
1..* 1..* Service Description
1..*
Service Integrator 0..1
Available Resources Specs Ser vice Request
Requir ements
Customer-side
influences 1..* 1..* influences Service Additional
SLA Information
defines
searches
0..*
influences 1..*

depends

Figure 22 - Conceptual Model of PLASTIC mobile context-aware services



4.5 Detailed Description of the original entities introduced in the
conceptual model

This section gives a detailed description of all the new entities introduced in the conceptual
model of SeCSE by PLASTIC vision and that are identified in bold in Figure 22. It is worth
noting that for all such new entities we will provide a sub-conceptual model that specifies the
content and structure of it.

Context:
This entity represents the (physical/logical) context in which the service will be executed. It is
used by the PLASTIC platform to provide/adapt services in order to better meet the user
requirements on the service. The context is a first class entity in the PLASTIC conceptual
model since it influences the Service Level Agreement procedure, the composition of software
services and the adaptation of software components.
Context Description:
This entity represents the information the PLASTIC platform is able to retrieve from the
environment, suitably structured. In general this information is partial since it is too heavy (and
useless) to gather the complete information on context. It contains descriptions of the (logical
and physical) available resources.

Available Resources Specs:
This entity is part of the context and it contains the description of the resources available at the
service consumer side. In other words it contains specifications about the device the service
consumer uses. This is a piece of information about the context the service will run on and it
is contained in the service request expressed by the consumer.

Location:
Location is an identifier of a context. It can be related both to physical and logical context. This
entity is useful to model mobility in PLASTIC platform. The context is hence identified by a
location contained in the context description. Each component implementing a service resides
in a single location

Component Description:
Component Description contains functional and non functional specification of the software
component it corresponds. Among non functional aspects the PLASTIC platform requires that
QoS attributes of the component are explicitly defined and that it contains the location where
the component will be deployed and executed. The information specified in this entity is used
in the software component assembling and in the component adaptation. There is a strict
relation between the description of the software service and the one of the software
component implementing the service.

Adaptation Specs:
This entity models the rules used to adapt a software component to a specific context. The
specified rules make use of the component description in order to suitably adapt the involved
software component(s) to the context where the software service it (they) implements will run.



Service Level Agreement:
This entity models the agreement reached between the service consumer and the service
provider. In practice, SLA is composed from multiple different clauses and each clause
addresses a particular service request requirement [20]. It represents the contracts between
them. Context, Service Request and Service Description influences the procedure that finishes
with the agreement.

4.6 An example scenario: video-streaming service for e-learning
In this section we depict a simple example of use of e-learning service presented in Section
2.2, in particular the example is a simplified instantiation of Trusted Content Repository
scenario (see Section 2.2.2 for a detailed description of the scenario). The example is firstly
described from the service consumer side and, secondly, from the PLASTIC platform side.
The example is aimed at illustrating the usage of concepts defined in the PLASTIC conceptual
model, thus showing the support that the conceptual model provides to the application
developers. Other concepts and actors may be involved in this scenario, but for sake of
illustration we have kept this example as simple as possible.

Satellite

Internet

Wlan
Access
Point

List of found solutions

Service Description:
GSM Base Station
e-learning video: marketing
strategies, cost: 35 euros, high
video quality and a high audio
quality, to pay by credit card,
Connection: GSM, reliability:
high

Service Description:
PocketPC
e-learning video: marketing
Service Consumer: Alice strategies, cost: 23 euros, high
video quality and a medium
Service Request audio quality, to pay by credit
Context card, Connection: WLAN,
Service Request reliability: medium
Requirements: Context Description:
GSM connection, Satellite
e-learning video session connection, WLAN access Service Description:
on marketing strategies,
cost: 25 euros, high video points
quality and a medium e-learning video: marketing
audio quality, to pay by strategies, cost: 20 euros,
credit card medium video quality and a
medium audio quality, to pay by
credit card, Connection: WLAN
Available Resources
Specs:
Connections: GSM, Service Description:
WLAN, Bluetooth, S.O. :
windows ME, screen size: SLA e-learning video: marketing
200x400 pixels, memory: strategies, cost: 30 euros, high
1 Mb... video quality and a high audio
quality, to pay by credit card,
Connection: GSM, reliability:
medium

Figure 23 – Example of e-learning service usage



Figure 23 illustrates an example of usage of the e-learning service emphasizing the entities of
the PLASTIC conceptual model involved in the service acquisition phase. In the following
sections we highlight those entities in italic while shortly describing the scenario.

4.6.1 E-learning service: the consumer side

Alice has subscribed to the PLASTIC platform and she has deployed on her pocket PC an
application that allows her for accessing PLASTIC services. The pocket PC is equipped with:
Wireless LAN, Bluetooth and GSM network connectivity type.

Alice wants to access a short e-learning video session on marketing strategies from her mobile
phone while she is travelling to get to the place where she has an interview for a job. While in
the bus Alice’s pocket PC is connected via GSM to the PLASTIC platform. She wants to spend
at most 25 euros to have a high video quality and a medium audio quality, and to pay by credit
card.

After having processed Alice's service request, the PLASTIC platform provides a list of
solutions whose characteristics almost satisfy the requirements Alice expressed in her
request. Note that in searching for solutions the PLASTIC platform takes into account the
service request requirements that include additional requirements defined by Alice which might
affect the way the service will actually be delivered. For each solution it is also displayed a
table reporting some metrics of the trade-off level reached between service specifications and
Alice's request requirements about cost, video and audio quality, and payment method. Alice
chooses a particular solution since the reached compromise is suitable for her even though
the video quality is not the desired one.

To initiate the streaming, Alice has to provide the credit card number, company, and expiration
date. Within few seconds, a message acknowledges that the credit card transaction has been
successfully completed and the e-learning session starts.

At the end of the video, Alice decides to send a positive feedback (i.e. customer-side service
additional information).

4.6.2 E-learning service: the PLASTIC side

In response to the Alice's service request reception, the PLASTIC platform searches the
available service registry to find service providers for particular service descriptions that match
Alice's request.

After having identified a set of available providers and their locations (as part of the provider
side context), the request handler takes into account (i) the available resources specifications
of the Alice's mobile device (as part of the consumer side execution context in terms of
memory size, screen size, operating system, and network connectivity type that is being used),
(ii) the service request requirements expressed by Alice (i.e., desired cost, video quality, and
payment method) and the (possibly different) characteristics of the context in terms of the
heterogeneous “infrastructural paths” connecting each provider’s location to the Alice’s
location (and hence to the Alice’s device).

Since more than one provider has published her e-learning video service descriptions into the
service registry, the search activity successfully returns the reference list of the available
services and relative providers. All the providers are able to supply the e-learning video
session respecting the available resources of the Alice's pocket PC, thus adaptation to the
execution context can be fulfilled. Unfortunately, no provider is able to fully satisfy the
requirements of Alice so negotiation is necessary. The negotiation phase starts by showing the


table reporting the reached trade-off between service characteristics (for each provider) and
service request requirements. In our case this phase quickly finishes. In fact, even though
Alice's request requirements are not completely satisfied and she would appreciate a better
service, she chooses a solution (and hence a provider) and there is no need to reiterate the
negotiation. In other words, the SLA representing the contract between the service consumer
and the service provider has been established.

After that the provider has been selected, the e-learning functionality is activated according to
the request requirements and the consumer side context. Actually, the service required by
Alice represents a composed software service capable of combining and dynamically
discovering other services in order to accomplish the task.

From the implementation point of view, the overall required service is implemented as the
composition of at least the component implementing the payment service and the component
offering the e-learning video session.



5 Conclusions
This deliverable collects and structures the work carried out in the first three months of project
life. It consists of three parts: (i) identification of the scenarios of interest in the Plastic context,
(ii) definition of a first list of requirements, and (iii) description of an initial version of the
conceptual model, respectively. This document corresponds to one of the milestone of the
project and represents the initial input to the work of WP2, WP3 and WP4. According to the
project workplan, the work carried on in the previously mentioned workpackages will be based
on interactions and feedbacks to and from WP1. Along the project life we anticipate
interactions that will lead to revise, in several iterations, the content of WP1 mainly concerning
the evolution of requirements and the conceptual model. These characteristics make D1.1 and
the following related deliverables as dynamic artefacts that will continuously evolve and
change according to the work progressing in WP2, WP3 and WP4.
The scenario phase involved the industrial partners and led to the codification of the set of
scenarios that build upon existing e-services developed by the industrial partners. During this
phase the main effort was devoted to identify the key issues and challenges that the
applications described in the scenarios will have to face in order to evolve towards meeting
the huge population of mobile and/or nomadic users equipped with wireless devices. This
phase was conducted in parallel, but not independently, with the requirements phase. The set
of scenario appearing in this document are structured according to a common format and
clearly identifies key requirements and challenges.
The requirements phase involved all partners to contribute scenarios and specific
requirements for PLASTIC in a semi-structured way. Requirements were gathered directly
from all stakeholders and used to establish the baseline requirements. The requirements were
entered into the Bugzilla issues-tracking system in order to maintain a traceable (live)
requirements document that can be used to track the evolution of the requirements as the
project develops.
The conceptual model phase resulted in the definition of the initial conceptual model. The
model builds on the service model proposed in the SeCSE project suitably extending it
towards the B3G domain. In order to build the model, interactions with the scenarios and
requirements tasks were carried on to properly validate the proposed extensions. One
difficulty faced during the development of the conceptual model has been related to the ability
to keep the model not too concrete thus anticipating or constraining design choices that should
be taken in the following WPs. On the other hand the model should also not be too abstract
thus loosing effectiveness. This problem has been relieved by deciding to rely on the SecSe
model that has been already successfully validated in a wider context.



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