J. Software Engineering & Applications, 2010, 3: 312-330
doi:10.4236/jsea.2010.34037 Published Online April 2010 (http://www.SciRP.org/journal/jsea)
Copyright © 2010 SciRes JSEA
Specifying the Global Execution Context of
Computer-Mediated Tasks: A Visual Notation
and a Supporting Tool
Demosthenes Akoumianakis
Department of Applied Informatics & Multimedia, School of Applied Technologies, Advanced Technological Education Institution
of Crete, Crete, Greece.
Email: da@epp.teicrete.gr
Received August 4th, 2009; revised September 2nd, 2009; accepted September 14th, 2009.
This paper presents the notion of the global execution context of a task as a representational construct for analysing
complexity in softwa re evolution. Based on this notion a visual nota tion and a supporting tool are presen ted to support
specification of a system’s global execution context. A system’s global execution context is conceived as an evolving
network of use scenarios depicted by nodes and links designating semantic relationships between scenarios. A node
represents either a base or a growth scenario. Directed links characterize the transition from one node to another by
means of semantic scenario relationships. Each growth scenario is generated following a critique (or screening) of one
or more base or reference scenarios. Subsequently, representative growth scenarios are compiled and consolidated in
the global execution con text graph. The paper describes the stages of this process, p resents the tool designed to facili-
tate the construction of the global execution context graph and elaborates on recent practice and experience.
Keywords: Non-Functional Requirements, Software Evolution Artifacts, Global Execution Context, Tools
1. Introduction
Over the years a plethora of techniques have been devel-
oped to manage functional requirements of a software
system. Some of these techniques focus on modeling and
implementing functional requirements using constructs
such as goals, scenarios, use cases, or notations such as
UML diagrams and dedicated UML profiles. More recent
efforts shift the focus to packaging and deploying func-
tional requirements as reusable components and Web
services for program-to-program interactions. In particu-
lar, service-oriented architectures (SOAs) appropriate the
benefits of Web services to make it easier to exploit soft-
ware assets from many types of components in sophisti-
cated new solutions, without complex integration projects.
Nevertheless, in all cases current thinking is dominated by
concerns focusing on the lower levels of an enterprise
infrastructure—how to create, manage and combine
business services providing data and logic. These efforts
and the supporting techniques to managing functional
requirements are characteristic of the prevailing paradigm
in software development, which can be broadly qualified
as construction-oriented. At the core of this paradigm is
the goal of designing what a software system is expected
to do, and to this end, the software design community has
faced a variety of challenges in an effort to provide in-
sights to the process of constructing reliable, robust and
useful interactive systems and services.
The advent and wide proliferation of the Internet and
the WWW have expanded an already over-populated
software design research agenda, bringing to the surface
the compelling need to account for a variety of
non-functional requirements such as portability, accessi-
bility, adaptability/adaptivity, security, scalability, ubiq-
uity etc. Some of them are well known to the software
engineering community, while others challenge estab-
lished engineering methods and work practices. For in-
stance, a long-standing premise of user-centered devel-
opment is that of ‘understanding users’; but users are no
longer sharply identifiable, homogeneous or easily stud-
ied [1]. Furthermore, the tasks users carry out keep
changing both in type and scope [2], with every new
generation of technology, from desktop systems to mo-
bile and wearable devices and the emergence of ubiqui-
tous environments. The radical pace of these technical
changes and the proliferation of myriad of network at-
tachable devices introduce novel contexts of use, requir-
ing insights, which are frequently beyond the grasp of
Specifying the Global Execution Context of Computer-Mediated Tasks: A Visual Notation and a Supporting Tool313
software designers.
Recognition of these challenges has motivated recent
calls for departing from construction-specific software
design techniques towards evolution-oriented methods
and tools. In this vein there have been proposals aiming
to provide creative interpretation of best practices (e.g.,
by devising new modeling constructs [3,4], building
dedicated UML profiles [5], specifying architectural pat-
tern languages [6], etc.) in an effort to establish mecha-
nisms for analyzing and/or abstracting from salient fea-
tures of software artifacts. Despite recent progress, de-
signing systems to cope with change and evolution re-
mains a challenge and poses serious questions regarding
the design processes needed, the appropriate methodol-
ogy and the respective instruments. One research path
aiming to establish the ground for such informed design
practices concentrates on non-functional requirements
(NFRs) as a means to shift the focus away from what a
software system is expected to do towards how it should
behave under specified conditions. NFRs or quality at-
tributes represent global constraints that must be satisfied
by the software. Such constraints include performance,
fault-tolerance, availability, portability, scalability, ab-
straction, security and so on. Despite their recognition by
the software engineering community, it is only recently
(i.e., in the early 90s) that researchers have embarked in
efforts aiming to assess their relevance to and implica-
tions for software development [3,4,7]. Nevertheless, in
contrast to functional requirements their non-functional
counterparts have proven hard to cope with for a variety
of reasons [8]. Firstly, most of them lack a standard con-
notation as they are being treated differently across en-
gineering communities and software development disci-
plines (i.e . , the same or similar NFRs hold different
meaning for say, platform developers and usability ex-
perts). Secondly, they are abstract, stated only informally
and requiring substantial context-specific refinement to
be accounted for. Thirdly, their frequently conflicting
nature (e.g., scale of availability may conflict with per-
formance) makes step-by-step implementation or verifi-
cation of whether or not a specific NFR is satisfied by the
final product, extremely difficult. These are some of the
reasons why NFRs are not easily incorporated into stan-
dard software engineering tools and practice.
SOA provide a new context for revisiting several
NFRs and their management during software develop-
ment. Nevertheless, current efforts are almost exclusively
concentrated on qualities such as abstraction, messaging,
service discovery, data integration, security, service or-
chestration/composition, etc, to facilitate two key princi-
ples: 1) creation of business services with defined inter-
faces so that functionality can be built once and then
consumed as required and 2) separation of the provision
of the services from their consumption. In this endeavor,
the software design community continues to devise ab-
stractions (i.e., components, visual notation, models and
tools) which make construction-oriented artifacts first-
class objects, dismissing or undermining software evolu-
tion and the special value NFRs have in this context. On
the other hand it is increasingly recognized that software
evolution is steadily overtaking in importance software
construction. Indeed, Finger [9] argues that ‘…the ability
to change is now more important than the ability to cre-
ate systems in the first place’.
An alternative approach to address this challenge may
be grounded on establishing the appropriate level of ab-
stractions to make evolution (rather than construction)
artifacts explicit, traceable and manageable in the course
of software development. This implies devising abstrac-
tions that allow us to expose how change is brought
about, what it entails, how it is put into effect and how it
may be traced and managed. To meet this goal, there are
key milestones likely to catalyze future developments.
Our understanding of this challenge leads to the conclu-
sion that we need 1) modeling approaches directing
analysis towards early identification of components that
relate to the cause of change, the subject of change and
the effect of change and 2) tools for effecting change in a
compositional fashion, thus relating change to local
components which are assembled without requiring
global reconfiguration of the system. In this vein, the
present work considers change management at a new
level of abstraction by promoting a shift in the unit of
analysis from task- or activity-level to task execution
contexts. It is argued that managing change is synony-
mous to coping with complexity and entails a conscious
effort towards designing for the global execution context
of computer-mediated tasks. Our normative perspective
is that software designers should increasingly be required
to articulate the global execution context of a system’s
tasks, rather than being solely concerned with the devel-
opment of an abstract task model from which incremen-
tally, either through mappings or transformations, a plat-
form-aware version of the system is generated. Moreover,
designing for the global execution context is a goal to be
satisficed rather than fulfilled. To this end, a new method
and a supporting tool is described which allow designers
to reason proactively (i.e., from early concept formation
through to design, implementation and evaluation) about
the global execution context of designated tasks. Both the
method and the tool provide a step in the direction of
making change a first-class design object accounted for
explicitly by articulating the parameters likely to act as
‘drivers’ of change. This is facilitated by an analytical
approach aiming to unfold, identify, represent and im-
plement alternative computational embodiments of tasks
suitable for a range of distinct task execution contexts
considered relevant and appropriate.
The remainder of the paper is structured as follows.
The next section considers change management in in-
Copyright © 2010 SciRes JSEA
Specifying the Global Execution Context of Computer-Mediated Tasks: A Visual Notation and a Supporting Tool
formation systems and frames the problem in two theo-
retical strands relevant to this work, namely change as
evolution and change as intertwining non-functional re-
quirements. This contrast offers useful insight to some of
the research challenges preventing the development of
methods for effectively coping with changes. Then, the
paper elaborates on and defines the notion of a system’s
global execution context, which forms an abstraction for
addressing complexity in software evolution rather than
software construction. The following section describes
the i-GeC tool, which allows incremental specification of
the global execution context by using scenarios. Our ref-
erence example is a light ftp application initially de-
signed for desktop use. The paper is concluded with a
discussion on the contributions of this work and a brief
note on implications and future work.
2. Motivation and Related Work
Change in interactive software is inherently linked with
the context in which a task is executed. Typical context
parameters include the target user, the platform providing
the computational host for the task and/or the physical or
social context in which the task is executed. Each may
give rise to a multitude of potential drivers for change.
Therefore, it stands to argue that change management is
about coping with complexity in construction as well as
in evolution. Managing complexity in construction has
been coined with the handling of functionality. Specifi-
cally, through the history of software design the primary
focus has been on accommodating functional require-
ments so as to develop systems that meet specific user
goals. The resulting systems could cope with minimal
and isolated changes, related primarily to the user, since
no other part of the system’s execution context (i.e.,
platform or context of use) was conceived as viable to
change. On the other hand, complexity in evolution is a
more recent challenge attributed to the adoption of the
Internet and the proliferation of Internet technologies and
protocols. These developments have brought about an
increasing recognition of the catalytic role of NFRs and
have necessitated a paradigm shift in the design of inter-
active software so as to explicitly account for quality
attributes such as abstraction, openness and platform
independence, interoperability, individualization, etc.
Despite the fact that complexity in construction and
complexity in evolution may seem as competing at first
glance (i.e., evolution frequently implies improvements
in the functional requirements), our intention is to argue
that they bring about complementary insights, which may
prove beneficial to the development of systems which are
easier to manage and use.
2.1 Change as Evolution
The term evolution generally refers to progressive change
in the properties or characteristics of the subject of evo-
lution (i.e., software). A common view to conceive soft-
ware evolution is to focus on mechanisms and tools
whereby progressive change in program characteristics
(e.g., functionality) and growth in functional power may
be achieved in systematic, planned and controlled man-
ner [10]. This may be conceived from various perspec-
tives and viewpoints. For instance, it may be viewed
from the perspective of software engineering processes
and thereby explain the proliferation of iterative software
development models such as agile programming [11] and
extreme programming [12], etc. It may also be related to
evolution in requirements and requirements management
[13], giving rise to methods for tracing evolving compo-
nents [14,15], localizing changes to components and de-
veloping component interoperation graphs [16], framing
change to scenarios and supporting scenario evolution
[17,18], etc. Whatever the perspective adopted, it is
widely accepted that the ability to change is now more
important than the ability to create systems in the first
place [9]. Change management becomes a first-class de-
sign goal and requires business and technology architec-
ture whose components can be added, modified, replaced
and reconfigured. The implication is that the complexity
of software has definitely shifted from construction to
evolution. As a result new methods and technologies are
required to address this new level of complexity.
In the past, the software design community addressed
complexity in construction by devising abstractions (i.e.,
components, visual notation, models and tools) to make
construction-oriented artifacts first-class objects of de-
sign. This paradigm has catalyzed developments and
facilitated breakthroughs in areas such as: 1) data man-
agement (leading from the early conception of the rela-
tional model to more recent proposals [19]), 2) software
design (progressively shifting from structured techniques
to object orientation [20], the development of com-
puter-aided software engineering tools [21,22], do-
main-specific design languages [23], architecture de-
scription languages [6] and software factories [24]), 3)
user interface development (facilitating richer interac-
tions with the advent of 2D and 3D graphical toolkits
[25-28]), etc. The common theme in these developments
is that complexity is addressed by establishing levels of
abstraction, allowing software construction artifacts (ex-
pressed as models of some sort, code, or processes) to
become first class objects. It can therefore be argued that
the problem of complexity in software evolution amounts
to establishing the appropriate level of abstraction to
make evolution artifacts explicit, traceable and manage-
able. That is to say, we need to find the abstractions that
allow us to expose how change is brought about, what it
entails, how it is put into effect and how it may be traced
and managed. As already stated, our understanding of
this challenge leads to the conclusion that we need 1)
modeling approaches directing analysis to the identifica-
Copyright © 2010 SciRes JSEA
Specifying the Global Execution Context of Computer-Mediated Tasks: A Visual Notation and a Supporting Tool315
tion of components that relate to the cause of change, the
subject of change and the effect of change and 2) tools
for effecting change in a compositional fashion, thus re-
lating change to local components which are assembled
without requiring global reconfiguration of the system.
2.2 Change and Non-Functional Requirements
Change as evolution of functional requirements is of
course valid but it can only explain partially why modern
information systems need to change. In fact, there is evi-
dence to suggest that most of the changes in modern in-
formation systems do not concern functional components
but their connections and interactions [9]. This explains
recent efforts aiming to frame change in relation to NFRs
[8] and architectural quality attributes [29,30] such as
such as adaptability [31], portability [32], run-time adap-
tive behavior [2,33], etc. Through these efforts, it be-
comes increasingly evident that NFRs concern primarily
environment builders rather than application program-
mers. Nevertheless, there is an equally strong line of re-
search aiming to make NFRs visible and accountable for
as early as possible in the software design lifecycle by
developing conceptual models and dedicated notations.
Specifically, there are techniques aiming to classify
NFRs through taxonomies [34], develop representational
notations for using NFRs [7], advance process-oriented
instruments for working with them [7,29] and study their
relationship to software architecture [30,35]. Although
these techniques have evolved in separate engineering
communities (each with its own point of view) and have
typically been performed in isolation, they share com-
mon ground. For instance, they recognize the important
role to be played by methodological concepts and sup-
porting technology that promote architectural insight
through suitable first-class objects. Phrased differently,
software architecture quality attributes promote a gross
decomposition of systems into components that perform
basic computations and connectors that ensure that they
interact in ways that make required global system prop-
erties to emerge. Thus establishing abstractions at the
level of software architecture may help manage com-
plexity in software evolution.
2.3 Framing Change to the Task’s Execution
The present work focuses on treating change from the
early stages of information systems development where a
variety of critical decisions are taken regarding architec-
ture, platforms, tools to be used, expected and foreseen
behaviors. Our primary concern is to advance a proposal
rooted in the anticipation of change and its incremental
localization in components. To this end, we use the no-
tion of task execution context to define a particular type
of scenarios that are allowed to grow. Then the global
execution context of a task (or a piece of functionality) is
an instance in a continuum of revisions and extensions of
the designated task’s execution context. Consequently,
managing change amounts to a conscious effort towards
designing for the global execution context of com-
puter-mediated tasks.
The execution context of a task is understood in terms
of a triad <Users, Devices, Context>. Users represent the
end (target) users - individuals or communities of users -
who experience an interactive artifact through which the
task is carried out. Devices refer to the technological
platform used to provide the computational embodiment
of the interactive artifact. Finally, context is a reflection
on the (physical and social) context of use in which the
task is executed. It is worth noting that none of these
relate to functional properties of the task. Then, design-
ing for the global execution context of a particular task
with specified functional requirements is directly related
to unfolding the rationale for and the artifacts encoun-
tered in a range of plausible task execution contexts. In-
terpreting the above rather theoretical concept in terms of
practical design guidelines raises several issues with two
standing out very promptly. The first is the commitment
towards exploring and managing complex design spaces,
while the second is the shift of engineering practice to-
wards abstract and specification-based techniques. Al-
though neither is entirely new to the software design
community (e.g., see the works by MacLean [36] on de-
sign space analysis, the work on DRL by [37], etc, as
well as recent advances in device-independent mark-up
languages such as UIML (http://www.uiml.org/) and
model-based development tools such as Teresa [32]),
their meaning and exploitation is slightly different in the
context of the present work.
3. The Global Execution Context of Tasks
The premise of the present work is that software design
lacks a coherent and detailed macro-level method – in the
sense defined in [38] – for the management of change
during the early stages of development where critical
decisions on architecture, tools and platforms to be used,
are taken. Consequently, our interest is in establishing an
integrated frame of reference for identifying and propa-
gating change (i.e., new requirements or evolution in
requirements) across stages in the course of the design
and development processes so as to facilitate designing
for the global execution context (GeC) of tasks.
3.1 Motivating Example & Terminology
It is useful to conceive the global execution context of a
task as a space of transformations depicting possible
and/or desirable mappings of a task’s abstraction to al-
ternative non-functional contexts (i.e., interaction plat-
forms, contexts of use or user profiles). Figure 1 pro-
vides an illustrative example from the field of user inter-
Copyright © 2010 SciRes JSEA
Specifying the Global Execution Context of Computer-Mediated Tasks: A Visual Notation and a Supporting Tool
MyFile: File
Name: String
del myfile
Delete Cancel
Task abstraction
Concrete manifestation
Inte ractiv e ic on sCommand lin e
Interactive directory tree
MyFile: File
Name: String
del myfile
Delete Cancel
Task abstraction
Concrete manifestation
Inte ractiv e ic on sCommand lin e
Interactive directory tree
Figure 1. Possible transformations to derive the global exe-
cution context of a task
face engineering. Specifically, the figure presents sche-
matically possible transformations for an abstract task
‘fileDelete’ of a hypothetical file management applica-
tion to distinct concrete manifestations (potentially)
suitable for different non-functional execution contexts.
It is worth noticing that the example presents a case that
challenges current conceptions of cross-platform or
portable software in the sense that concrete manifesta-
tions need not be bound to native platform-specific ele-
ments; instead, they may use customized facilities, do-
main-specific and/or expanded components.
Being able to explicitly foresee and design a system so
that it can cope with all possible changes in its execution
context is probably a utopia, given the current state of the
art in systems thinking and engineering. Nevertheless, if
we delimit the qualification ‘all possible changes in the
task’s execution context’ to all known, or foreseen and
explicitly modeled changes (within the scope of a ser-
vice-oriented architecture), then it is possible to define a
context-sensitive processing function which under certain
circumstances will deliver the maximally preferred
transformation of an abstract task to a concrete instance
[39]. Consequently, understanding and designing for the
global execution context of a system’s task (i.e., sup-
porting the task’s execution across all designated non-
functional contexts) entails some sort of mechanism or
service for linking to, rather than directly calling, differ-
ent implemented components complying to/supported by
a designated service-oriented architecture. Equally im-
portant is to consider how the service-oriented architec-
ture is to view and link to radically different execution
contexts and platforms. In recent wittings, both in re-
search and development communities, this dimension is
dismissed resulting in proposals for SOA that cannot
cope with radically different non-functional execution
To provide further insight, let us abstract from the de-
tails of Figure 1 to describe a more general situation
where our abstract task T2 (i.e., delete a file) is assigned
to two distinct realizations as shown in Figure 2. The
first, denoted with the solid line, refers to task execution
on a desktop devise, which requires that the selection list
is presented (S1), the user makes a choice (S2) and sub-
sequently the command is issued (S3), followed by a con-
firmation dialogue (S4). The second realization is using a
mobile devise. Once again the selection list is presented
(S1), but this time in order for the user to make a choice
the system augments interaction initiating a scanning
interface S2'. Once the selection is made the command is
issued (S3) followed by a confirmation dialogue (S4).
It is worth pointing out that despite the simplicity of
the example, it poses several challenges. First of all, for
any given task one can easily identify several additional
realizations (execution contexts) depending on the plat-
form or toolkit, the context of use and/or the target user.
Thus, one issue is enumerating requirements and encod-
ing alternatives, but also allowing for incremental up-
dates and evolution to accommodate new realizations.
Secondly, irrespective of the task’s execution context the
functional requirement remains the same (i.e., delete a
file). The cause of change is therefore due to a designated
set of NFRs. It may also be argued that prevalent NFRs
such as portability or platform independence may not
suffice to capture the essence implied by some of these
changes. For instance, if scanning is implemented as a
reusable interaction library, it signifies an augmentation
of the target platform whereby the scanning functionality
is introduced as new interaction technique assigned to
designated interaction elements. This is totally different
from a hard-coded implementation of the scanning inter
face to suit a specific interaction scenario or system.
Similarly, one could envisage alternatives to augmenta-
tion such as platform expansion (i.e., to increase the
range of interaction elements of an existing platform) or
new platform development (i.e. for a designated modality)
and integration (i.e., mixing components from different
platforms). All these represent intertwining (and fre-
quently conflicting) goals to inscribing non-functional
qualities such as usability, portability, individualization,
etc. Moreover, they are not intuitively associated with or
assumed by prevalent NFRs. It stands to argue therefore
that designing for the global execution context of a task
Figure 2. Tasks and execution contexts
Copyright © 2010 SciRes JSEA
Specifying the Global Execution Context of Computer-Mediated Tasks: A Visual Notation and a Supporting Tool317
entails an account of ‘hidden’ quality goals such as plat-
form augmentation, expansion and integration, which are
not so established in the software engineering literature.
Furthermore, in many cases it is these ‘hidden’ quality
goals that determine the type and range of non-functional
contexts to be assigned to a task’s global execution con-
3.2 Modelling the Global Execution Context
Conceptually, the GeC of an abstract task T can be con-
ceived as a five tuple relation <T, g, S, f, C> where g is
the task’s goal to be achieved by alternative scenarios si
S, and a context-sensitive processing function f(si)
which defines the maximally preferred instance of S,
given a designated set of constraints C. Such a definition,
allows us to model change in interactive software in
terms of certain conditions or constraints which propa-
gate alternative interactive behaviours to achieve task-
oriented goals. Three types of constraints are relevant to
the present work, namely user constraints, platform con-
straints and context constraints. User constraints are
user-specific parameters designating alternative interac-
tion and use patterns. Platform constraints relate to prop-
erties of a target device-specific execution environment.
Context constraints designate external attributes of po-
tential relevance to the task’s execution. Then, change
in the execution context of a software system occurs if
and only if there is at least one constraint in C whose
parameter value has been modified so as to justify system
transformation. The result of recognizing
and putting it
into effect causes the deactivation of si S, which was
the status prior to recognizing
and the activation of a
new sj S, which becomes the new status.
3.2.1 Basic Vocabulary and Notation
In our recent work, we have been developing a sce-
nario-based approach in an attempt to formalize elements
of the global execution context of computer-mediated
tasks. In terms of basic vocabulary, the approach makes
use of three constructs namely base (or reference) sce-
narios, growth scenarios and scenario relationships. Base
scenarios depict situations in an existing system or a
prototype, which are defined in terms of functionality.
Growth scenarios extend reference scenarios in the sense
that they describe new execution contexts for the func-
tionality associated to the reference scenario.
Scenario relationships are used to capture semantic
properties of the reference and growth scenarios. Two
categories of relevant scenario relationships have been
identified, namely those describing internal structure of
scenarios in terms of components as well as those de-
scribing scenario realization. The former type of rela-
tionships is well documented in the literature (see [17])
and may be applied to any scenario independent of type
(reference or growth). For the purposes of the present
work we have found two such relationships as being
useful, namely subset-of and preference/indifference.
Subset-of is the relationship defining containment be-
tween two scenarios. It declares that the functions of a
scenario Si are physically or logically part of another
scenario Sj. Si is termed the subordinate scenario, and Sj
is termed the superior scenario. The subordinate scenario
always encapsulates part of the action in the superior
scenario. It should be noted that the subset-of relation-
ship does not entail inclusion in the sense that execution
of the superior scenario is suspended until the execution
of the subordinate scenario is complete. Instead, it im-
plies the set-theoretic notion of subset where the actions
of the subordinate scenario are contained within or are
the same as the set of actions of the superior scenario.
Preference designates the existence of a preference order
for two subordinate scenarios Si and Sj of a superior sce-
nario. Preference is specified by a preference condition
or rule. When executed, the preference condition should
place candidate subordinate scenarios in a preference
ranking (indifference classes), while the most preferred
scenario (first indifference class) is the one to be acti-
vated. The preference relationship is useful for specify-
ing the context-sensitive processing function which acti-
vates/deactivates scenarios at run-time.
Scenario realization relationships provide details of the
mapping (or transformation) between reference and
growth scenarios and are intended to capture evolution of
a reference scenario into growth scenarios. In general,
two properties dictate the evolution of a base scenario
into a growth scenario. The first relates to temporal as-
pects of growth scenario execution, while the second
depicts the resources demanded for realizing the growth
scenario. In terms of temporal aspects of execution these
can be modeled either by alternative or parallel execution.
The resources demanded can be modeled by relationships
such as (platform) augmentation, (platform) expansion
and (platform) integration. As the latter two are special
cases of the alternative relationship, platform augmenta-
tion is the third scenario realization relationship used to
complete the global execution context graph in the con-
text of the present work. The alternative relationship
links two scenarios when each serves exactly the same
goals and one and only one can be active at any time.
Alternative is the main operator for specifying adaptabil-
ity of a system with regards to a designated quality at-
tribute (e.g., platform independence). As already stated,
two scenarios designated as alternative may be realized
either by platform integration (typical case of multi-
platform capability) or by platform expansion which as-
sumes interoperability between the platform and another
platform or third-party libraries used to expand the initial
vocabulary of the platform. Moreover, two alternative
scenarios are considered as indifferent with regards to all
other quality attributes except the ones designated in the
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Specifying the Global Execution Context of Computer-Mediated Tasks: A Visual Notation and a Supporting Tool
Copyright © 2010 SciRes JSEA
alternative declaration (see preference). Augmentation
captures the situation where one scenario in an indiffer-
ence class is used to support or facilitate the mostly pre-
ferred (active) scenario within the same indifference
class. For instance the scanning interface scenario in
Figure 2 augments file management when executed us-
ing a mobile devise. In general, two scenarios related
with an augmentation relationship serve precisely the
same goal through different (but complementary) inter-
action means. Finally, parallelism refers to concurrent
activation of scenarios serving the same goal. At any
time two parallel scenarios preserve full temporal overlap.
Parallelism may have two versions. The simplest version
is when the scenarios utilize resources of the same plat-
form. In this case the relationship is synonymous to con-
current execution of the scenarios (i.e. deleting a file us-
ing command line or an interactive directory tree) with
full temporal overlap. The second version of parallelism
is relevant when the scenarios utilize resources of differ-
ent interaction platforms (toolkits). In this case, it is as-
sumed that the two platforms are concurrently utilized
and an abstract user interface can link with each one to
make use of the respective interaction elements. This
type of parallelism does not require interoperability be-
tween the platforms, as platform-specific interaction
elements are not mixed. A typical example of this type of
parallelism is when two users (i.e. a blind and a sighted
user) are engaged in a collaborative application (i.e., file
management session) and the concrete user interface in
each case utilizes interaction resources of different tool-
kits (one graphical toolkit for the sighted user’s interface
and one non-visual toolkit realizing the blind user’s in-
terface). This type of parallelism is not common in inter-
active applications, but when properly supported, it can
serve a number of desirable features such as adaptivity to
suit individualized requirements, concurrent modal-
ity-specific interaction as well as multimodality.
It should be noticed that the relationships discussed
above are intended to serve the analysis of the global
execution context as described earlier. All of them except
the subset-of relationship are intended to address primar-
ily non-functional qualities of scenarios. Consequently,
these relationships are complementary to others proposed
in the relevant literature (see for example [17]) for cap-
turing semantic properties such as scenario complements,
specialization, temporal suspension of a scenario until
another scenario is completed (i.e. ‘includes’ relationship)
or exceptional scenario execution paths (i.e. ‘extends’
3.2.2 The Global Execution Context Graph
Collectively, the notational constructs described earlier
are presented in Table 1 and constitute the basic vo-
cabulary of the global execution context notation (GeCn).
Using this notation, designers can specify the require-
ments of the global execution context of a task as a graph.
This graph is typically a visual construction consisting of
nodes representing scenarios and directed links repre-
senting scenario relationships. Figure 3 illustrates an
example of the global execution context graph of a task,
namely select files, of a simple ftp application. This ex-
ample will be further elaborated in the following section.
The figure depicts, one reference scenario, namely ‘Se-
lect files with desktop style’ which through the contain-
ment operator links to ‘Single file selection’ and ‘Multi-
ple file selection’. Single and multiple file selection are
parallel (i.e. weak notion of concurrency presented ear-
lier, making use of resources of the same platform). The
reference scenario as a whole (including the contain-
ments) is augmented with ‘Select with scan on’ which is
a growth scenario containing two alternative options,
namely ‘One button/auto’ and ‘One button/manual’
scanning. It is important to note that the designated
growth scenarios do not represent change or evolution of
the functional requirements of the application (either at
the client or the server side). Instead, they designate a
platform-specific non-functional requirement for sup-
porting augmentation of interaction through scanning of
interaction elements. On the other hand there is no
pre-requisite as to how this augmentation is supported
(i.e., through programming or by augmenting toolkit li-
braries to facilitate scanning). This simple example suf-
fices to make two claims regarding the global execution
context graph of a task. Firstly, the technique is intended
to represent ‘hidden’ requirements not commonly col-
lected using conventional requirements engineering
methods – thus it is complementary to rather than com-
peting against such methods. Secondly, the technique is
biased towards platform-oriented requirements leading to
an improved insight on existing NFRs such as platform
independence, portability, etc.
Figure 3. Example of a global execution context graph
Specifying the Global Execution Context of Computer-Mediated Tasks: A Visual Notation and a Supporting Tool 319
Table 1. Basic notation
Symbol Interpretation
Reference or base scenarios depict situations in an existing system or a prototype, which are defined in terms of functionality. They
comprise at least one actor and one explicitly stated goal
Growth scenarios are always linked to a base scenario which they extend in the sense that they describe new execution contexts for
the functionality associated to the reference scenario
‘Preference’ relationship designates the existence of a preference ranking between two or more scenarios; the preference ranking is
conditional upon the preference rule (or condition)
‘Indifference’ relationship designates indifferent execution of two or more scenarios realized in the same design vocabulary (i.e.
development platform)
‘Alternative’ relationship designates alternative realizations / embodiments of a scenario across distinct design vocabularies; at any
time one and only one of the scenarios can be active
‘Augmentation’ relationship designates that a scenario is used to support or facilitate the mostly preferred (active) scenario within
the same indifference class
‘Parallel’ relationship designates the concurrent activation of scenarios of a designated design vocabulary; at any time parallel sce-
narios preserve full temporal overlap
‘Interchangeable’ execution designates parallel execution of scenarios in distinct design vocabularies; no requirement for interopera-
bility as scenarios are not mixed
‘Subset_of’ defines containment of actions of one (subordinate) scenario into actions of another (superior) scenario; the subordinate
scenario appears on the left hand side of the relationship
3.3 Stages in the Construction of Global
Execution Context Graphs
Building the global execution context graph entails an
iterative process of continuous refinement. It is both
useful and important to be able to verify refinements of a
task’s global execution context graph so as to ensure
consistency and correctness. To facilitate these tasks, a
micro method and a supporting tool have been developed
to provide guidelines for building the global execution
context graph. The method and the tool serve two main
goals, namely 1) encoding reference and growth scenar-
ios in alternative representation forms and 2) incremental
and evolutionary construction of the system’s global
execution context graph so as to allow incorporation of
new requirements and requirements evolution (i.e ., ver-
sioning). Figure 4 illustrates the conceptual stages in-
volved in compiling the global execution context graph
Figure 4. Process stages for building
using growth scenarios. As in the case of other sce-
nario-based methods, it involves interplay between re-
flection (analysis and screening) and envisioning. The
first step is usually a preparatory activity carried out by
the analyst in collaboration with domain experts, and
entails the formulation of reference scenarios. Once the
scenario is formulated, typically in a narrative form, the
screening process begins in an effort to compile the ra-
tionale for growth scenarios. To this end the choice of
screening filters is important. One option is to screen the
reference scenarios using designated NFRs so as to un-
fold breakdowns or deficiencies related to global system
qualities (i.e. system architecture, platform commitment,
interaction metaphor). In Table 2 we provide an example
of such NFRs-based screening of our reference ftp ap-
Alternatively, screening may focus on other aspects of
interactive software such as choice of interaction ele-
ments, dialogue styles, presentation, etc. In all cases,
scenario screening assumes the availability of artifacts
(e.g., narratives, pictures, user interface mock-ups, high
fidelity prototypes, etc) and it entails a structured process
whereby implicit or explicit assumptions embedded in an
artifact (and related to the intended users, the platform
used to implement the artifact and the context of use) are
identified and documented. The essence of screening is
in defining appropriate filters or adopting alternative
perspectives to critique a base scenario. It is therefore a
scenario inspection technique in the sense described in
[40], which can be realized through different instruments.
Whatever choice of the screening instrument, it is im-
perative that screening should motivate growth scenarios
so that the latter do not exist in vacuum. Instead, they
should be related to the design breakdowns identified
through screening and the designated new or evolving
requirements. Consequently, the ultimate goal of growth
scenarios is to capture evolution of requirements codified
in base scenarios. Such evolution should depict new
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Specifying the Global Execution Context of Computer-Mediated Tasks: A Visual Notation and a Supporting Tool
Table 2. Screening using NFRs and corresponding design breakdowns
NFR Example of design breakdown New or evolving requirement
Platform independence “… file transfer is not available as WWW or WAP appli-
cation or service …”
Allow choice of delivery medium or interaction style
(i.e. HTML, WAP, Windows style)
Scalability “… the system does not exhibit scalability to platform or
access terminal capabilities … “
Detect context of use and allow operation in text-only
style through a kiosk
Adaptability “…the system cannot be customized to diverse require-
Support manual or automatic customization of interac-
tion style (e.g., scanning)
Adaptivity “…when in operation the system does not monitor user’s
interactive behavior to adjust aspects of interaction…”
Provide auditory feedback upon completion of critical
tasks to inform users on task completion state
Context awareness ‘…the system takes no account of the context of use to
modify its interactive behavior…”
Allow context monitoring and switching between des-
ignated interaction styles
Localization “…the system can not be localized…” Allow choice of language
Accessibility “…the system is not accessible by certain target user
Interview user to determine motor, visual, cognitive
capabilities and define adaptation
execution contexts for the tasks in the base scenario. In
practice, growth scenarios result from relaxing the as-
sumptions identified in the course of screening. Once
agreed, growth scenarios may become more concrete
through prototypes, which specify details of the new task
execution contexts. It is important to note that our inten-
tion is to consider growth scenario management as an
engineering activity [41] rather than a craft, and to con-
tribute towards effective engineering practices for guid-
ing the creation and refinement of scenarios. Conse-
quently, our work links with recent proposals in sce-
nario-based requirements engineering aiming to offer
systematic scenario process guidance (see for example
[42,43]) as well as key concepts and techniques of the
Non-Functional Requirements Framework [8].
4. Designing for the Global Execution
To support designers in gaining insights and analysing the
global execution context, a tool has been developed,
namely interactive Global execution Context (i-GeC).
i-GeC covers all three stages namely scenario recording,
screening and growth scenario compilation. It does not
however, embark into detailed design, which is beyond
the scope of the present work. Figure 5 depicts the logical
view of i-GeC, summarising our notion of reference (or
base) and growth scenarios as well as the scenario rela-
tionships relevant to this work. It should also be noted that
the class model of Figure 5 provides a scheme for inter-
preting the main components of the five tuple relation <T,
g, S, f, C> used to conceptualise the GeC of a task. The
only element not explicitly modelled is the context sensi-
tive processing function f. However, this relates to the
system’s implementation and architectural model for
processing (i.e. enabling/disabling) scenarios. As for the
constraints they are assumed to be parameters of the class
‘Artifact’. Another important consideration regarding the
scheme of Figure 5 is that, although there is a provision
for goals, this should not be confused with functional
requirements. In fact, this work is not concerned with this
type of requirement. Instead, our interest is on non-fun-
ctional requirements and how they are translated into
quality goals. As already mentioned earlier, some
non-functional requirements (i.e. adaptability, portability
and individualization) are well established in the relevant
literature both in terms of scope and techniques used to
cope with them (i.e. [3,8]. Others however are not so
well established (i.e. toolkit augmentation, expansion,
integration) but are considered very important to model-
ling the global execution context of a task. The latter type
of non-functional requirements, partly motivate the work
presented in this paper.
To illustrate the above, we will continue to make use of
our ftp application allowing authorised users to connect
to a server and subsequently manipulate local files (i.e.
transfer, delete). For the purposes of our discussion, we
will consider both the incorporation of new requirements
and requirements evolution. A new requirement is to
support ftp portability to a new platform (i.e., from desk-
top to PDA). As an example of requirements evolution
we will consider various enhancements of the file selec-
tion task so as to support multiple selection by file cate-
gory (i.e. select all files with an extension ‘.ppt’) and
selection through scanning on a PDA. Scanning is an
interaction technique, which entails automatic manipula-
tion of PDA interaction elements in a hierarchical fash-
ion to reduce keystroke level actions. It is therefore con-
ceived as a usability enhancement. Thus, in the remain-
ing of this section, our aim is to show how from a given
set of functional requirements we can progressively
compile a specification of the system’s new execution
context supporting a PDA client with the enhanced file
selection facilities.
4.1 Encoding/Recording Scenarios Using i-Gec
Encoding scenarios using a variety of media and repre-
sentational tools is important, as it allows the designer to
start with a high-level narrative description of a situation
of use (see Figure 6, left hand side dialogue) and pro-
gressively transform it into a bulleted sequence, state
diagram, use case model, etc., reflecting the designers’
incremental improvement of understanding of the situa-
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Specifying the Global Execution Context of Computer-Mediated Tasks: A Visual Notation and a Supporting Tool 321
Figure 5. Class model of the global execution context of a task
Figure 6. Encoding a reference scenario as use cases
tion. This transformation is user-driven in the sense that
the user can employee simple cut & paste techniques or
menu-driven dialogues to map textual elements in the
narrative description to graphical elements in a specific
visual notation (see for example Figure 6 for a transfor-
mation of a narrative to a use case model). Each refer-
ence scenario can be incrementally refined. Reference
scenario refinement involves detailed description of the
scenario and compilation of more analytic views of the
scenario codified as numbered sequence of activities,
partitioned narrative, exception steps, state transitions,
etc., as shown in Figure 7.
4.2 Scenario Screening with i-Gec
Following reference scenario recording, the screening
stage seeks to provide a structured critique of the re-
corded scenario so as to designate issues (in anticipation
of change) or shortcomings. These shortcomings provide
the rationale and the motives for subsequent compilation
of growth scenarios (see next section). In the current ver-
sion, screening a scenario follows the tradition of design
space analysis using Questions Options & Criteria [36].
The analyst can designate both issues and options (poten-
tial solutions) as shown in Figure 8. All designated is-
sues and options are codified per scenario and can be
explored through the memory tool. This type of screen-
ing is intended only to record and make persistent the
results of analysis.
4.3 Compiling Growth Scenarios and Building
Global Execution Context Graphs with i-GeC
In i-GeC, the compilation of growth scenarios entails
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Specifying the Global Execution Context of Computer-Mediated Tasks: A Visual Notation and a Supporting Tool
Figure 7. Manipulation of reference scenarios. (a) Describing the reference scenario; (b) Expanding the reference scenario
reformulation of a use case type representation of the
base scenarios. Specifically, to define a growth scenario,
the user should first declare the growth case and then
assign the appropriate relationships between the growth
case and the base or other growth scenarios. Figure 9.
depicts an example where a growth scenario is intro-
duced (Figure 9(a)) and subsequently elaborated (Figure
9(b)). In the example, ‘iPAQ connection’ is introduced
as ‘alternative to’ the reference scenario ‘connect to a
remote server’ which represents functionality already
supported by the ftp application. The growth scenario is
motivated by the screening criterion of ‘user adaptability’
and the issue ‘how does the user type IP address’ (see
Figure 9(b)). For the same growth scenario there may be
more issues assigned. As shown, reference and growth
scenarios are distinct elements represented as single-line
and double-line ellipses respectively. The semantic sce-
nario relationship is represented as an annotated link. The
Copyright © 2010 SciRes JSEA
Specifying the Global Execution Context of Computer-Mediated Tasks: A Visual Notation and a Supporting Tool 323
Figure 8. The screening stage
Figure 9. Populating a reference scenario. (a) Introducing a growth scenario; (b) Populating a growth scenario
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Specifying the Global Execution Context of Computer-Mediated Tasks: A Visual Notation and a Supporting Tool
growth scenario elaboration dialogue groups the proper-
ties of a growth scenario into four categories. The gen-
eral properties of the growth scenario declare its name,
description and relationship with other scenarios. In a
similar fashion the user can assign the issues relevant to
(or addressed by) the growth scenario, the pre- and
post-conditions and supporting analysis (i.e. a state tran-
sition diagram, numbered sequence, partitioned narrative,
etc). This provides a kind of verification for each growth
scenario, since it ensures the minimum qualities required
(i.e., each scenario is assigned to a goal, each scenario is
realized through a set of actions, etc). This issue is fur-
ther elaborated later on in this paper.
Building the global execution context graph entails
three steps: 1) devising growth scenarios; 2) assigning
quality attributes to justify the derived growth diagram
and 3) commenting on the pseudo verification applied to
check the global execution context graph. Table 3 pro-
vides a summary of the growth scenarios highlighting
both the case of supporting ftp through PDA and the en-
hancement of the file selection task. These extensions are
typically expressed as new/evolving requirements to be
accommodated as growth cases of the initial base sce-
nario. From the descriptions in Table 3, we can deduce
that the global execution context of the new ftp applica-
tion should include one additional growth scenario
namely ‘Select with scanning’ and two parallel compo-
nents designating that selection is augmented by two
growth scenarios namely ‘One button/Auto’ or ‘One
button/Manual’. This is depicted in Figure 10. The rela-
tionships between the various growth scenarios define
the scale and scope of the system’s adaptable and/or
adaptive behaviour. This offers useful insight to the
range of anticipated changes and their implication on
architectural abstraction, the choice of interaction tech-
niques, as well as the conditions under which alternative
styles of interaction are to be initiated.
At any time, designers can justify their decisions by
rationalizing growth scenarios using non-functional qual-
ity models. Such models may be built in advance so as to
establish global constraints on software design or in the
course of building and rationalizing a task’s global exe-
cution context. Figure 11 presents an example decompo-
sition of the ‘accessibility’ quality in terms of alterna-
tives or claims softgoals in the vocabulary of the NFR
Framework [8]. Specifically, the model in Figure 11
details that accessibility can be satisficied either by aug-
menting interaction through scanning, or by expanding a
toolkit library with new interaction object classes or by
integrating another toolkit class library. The relationships
qualify the degree of satisficing a goal. Thus, augmenta-
tion and expansion support (i.e., have a positive influence)
on accessibility, while toolkit integration is indifferent.
Figure 11 on the left hand side represents the link be-
tween the non-functional quality model and the global
execution context graph. The rationale behind the com-
bined model is intended to convey the following meaning:
The iPAQ version of the ftp application should support a
scanning interface which should allow selection in two
alternative modes – one button with automatic scanning
or one button with manual scanning of the highlighter.
Figure 12 depicts an interactive instance of the aug-
mented ftp application with the scanning interface on and
the multiple file selection facility (see Table 3 and
Figure 10 for the rationale of the growth scenarios). As
shown, scanning is activated through explicit function
activation by pressing the button in the left bottom corner
(see Figure 12(a)). Once activated the scanner gives fo-
cus in round-robin fashion to each control in a hierarchi-
cal fashion. It is worth noting the difference in the inter-
active behaviour for each object of focus. Thus, when
scanning is activated and the object of focus is a text en-
try field the fill colour is changed (see Figure 12(b))
while when the object of focus is a button then the label
is underlined (see Figure 12(c)). In Figure 12(d) the
focus is on the Download Button (note the square around
it). By pressing a hardware button on the device the
scanner moves from one level to another (i.e. from scan-
ning selection sets to scanning items within a selection
set and vice versa). Figure 12(e) demonstrates the multi-
ple file selection by checking files in a category. For
example, when the “Remote Files” list has the focus,
pressing the PowerPoint icon on the taskbar, all files with
ppt extension in that list are selected. This multi-selection
task adds checkboxes to the left of all items in the list,
and files with ‘ppt’ extension are automatically checked.
It should be noted that this type of selection is very use-
ful as it reduces keystroke level interactions (i.e. avoids
using the slider to locate files and multiple file checking),
without changing the initial application in any other way.
4.4 Pseudo Verification of the Global Execution
Context Graph
In its current version, the tool consolidates the global
execution context graph in an XML document by im-
plementing a pseudo-verification to ensure that the global
execution context graph satisfies to some degree the cri-
teria of completeness and redundancy. The tests per-
formed aim to satisfy the following:
Each scenario Si in the global execution context
graph GeCg(S) where S denotes the reference system
should be either a base scenario or a growth scenario
Given a system S, for each base scenario SBi
GeCg(S) there is at least one growth scenario SGj
GeCg(S) related with SBi
Given a system S, then a growth scenario SGi
GeCg(S) can be related with a base scenario SBi
GeCG(S) or another growth scenario SGj GeCg(S);
All scenarios are assigned to goals – informally, this
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Specifying the Global Execution Context of Computer-Mediated Tasks: A Visual Notation and a Supporting Tool 325
Table 3. Elaboration of growth scenarios
Base scenario (Desktop) Growth Scenario (PDA) Growth Scenario (PDA & scanning)
Initiator Professional user Professional user User at home
of use The user outside the office
The user enters the classroom and
wishes to ftp the file containing his
The user is at home and wishes to review
his slideshow
User System User System User System
Flow of
o The user
connects to
the server and
o The user
carries out
file selection
o The user
issues a
command (by
button press)
o The system
responds to no-
tify user’s login
and presents the
desktop em-
bodiment of the
user interface
o The system
notifies the user
of current se-
o The system
executes the
o The system
updates the
o The user
connects to
the server and
o The user
makes a se-
lection from
the list of pa-
o The user
issues a
(using the
light pen)
o The system
responds to no-
tify user’s login
and presents the
PDA embodi-
ment of the
user interface
o The system list
the currently
selected file
o The system
executes the
o The user
connects to
the server and
o The user
declares se-
lection mode
o The user
selects all
files with .ppt
o The system re-
sponds to notify
user’s login and
presents the aug-
mented PDA em-
bodiment of the
user interface
o The system lists of
files in a default
o The system initiates
suitable style & in-
forms the user
o The system exe-
cutes the command
Network problems
Error in login procedure
Network problems
Error in login procedure
Network problems
Error in login procedure
User is authorized
Desktop interface is available
System has inferred the task’s
execution context resulting in
HTML style being automatically
User is authorized
User is in possession of the desig-
nated terminal
System has inferred the task’s
execution context resulting in PDA
style being automatically initiated
User is authorized
User is in possession of the designated
terminal User is familiar with scanning
System has inferred the task’s execu-
tion context resulting in PDA style be-
ing automatically initiated
conditions Designated files are successfully
Designated files are successfully
Designated files are successfully trans-
Alternative to base scenario Alternative to base scenario
Augments PDA style
Parallel selection as separate growth
ensures that a scenario is devised to facilitate a desig-
nated goal of the system. Thus, there are no scenarios
beyond the scope of the envisioned system;
Each scenario can be satisfied by at least one goal –
informally, the proposition aims to assert that each sce-
nario is linked to at least one goal.
The above propositions are checked before the global
execution context graph is transformed into XML. This
allows a pseudo verification of the completeness, redun-
dancy and understandability of a global execution context
graph. Specifically, the propositions can be considered as
necessary but not sufficient conditions for ensuring com-
pleteness. Clearly, as the global execution context graph is
subject to refinement, no sufficient condition for com-
pleteness can hold. As for redundancy, the propositions
aim to support a weak notion of redundancy, which asserts
that no scenarios are included that would not be desig-
nated to goals. Obviously, the global execution context
graph could incorporate redundancy both at the level of
growth scenarios (i.e. alternative growth scenarios may
exist which satisfy the same goal) and at the level of ac-
tions (i.e. alternative action sets may be employed to sat-
isfy a user goal). Finally, regarding understandability, the
propositions aim to ensure that all scenarios included in
the global execution context graph are understandable by
tracing their designated goals, which are considered valid.
On the other hand, the propositions offer no guarantee that
the global execution context graph can be understood.
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Specifying the Global Execution Context of Computer-Mediated Tasks: A Visual Notation and a Supporting Tool
At any instance, the GeCg can be traversed by select-
ing and following a particular path from start to end and
understanding the system’s behaviour under certain con-
ditions. Path differentiation is always associated with a
scenario relationship of type alternative or augments.
Referring to our example, we can define possible tra-
versals of the global execution context graph, differenti-
ated by colour. Activating, the reference scenario results
in an iPAQ embodiment of the designated task, with sin-
gle and multiple selection. Activation of scanning would
augment the file selection process with scanning. As for
the type of scanning, two alternative manifestations are
available with only one being active at any point in time.
In terms of system implementation requirements, it is
important to note that the underlying intention is that
both paths should co-exist, while through context-sensi-
tive processing the system should decide on the choice of
optimal path.
Figure 10. Designating an augmentation of a growth scenario
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Specifying the Global Execution Context of Computer-Mediated Tasks: A Visual Notation and a Supporting Tool327
Figure 11. Quality models and the global execution context graph
(d) Hierarchical scanning(e) Multiple selection
Textbox has scan
focus Button has scan
(a) Initial screen(b) Scanning activated(c) Scanning activated (cont.)
Icon pressed results
in selection of files
with .pptextension
Figure 12. Examples of the scanning interface
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Specifying the Global Execution Context of Computer-Mediated Tasks: A Visual Notation and a Supporting Tool
5. Discussion
The work presented in this paper differs from recent re-
lated efforts both in terms of orientation and underlying
perspective. In terms of orientation, our interest is to
frame the problem of software execution across different
non-functional contexts as an issue of software evolution.
To this end, change in functional requirements is of course
valid but it can only explain in part why modern infor-
mation systems need to change. In fact, there is evidence
to suggest that most of the changes in modern information
systems do not concern functional components but their
connections and interactions. This explains recent efforts
aiming to frame change in the context of non-functional
requirements and architectural quality attributes.
In terms of underlying perspective, the present work
pursues a line of research, which is motivated by the fact
that complexity of software is increasingly shifting from
construction to evolution. In the past, the software design
community addressed complexity in construction by de-
vising abstractions (i.e., components, visual notations,
models and tools), which make construction-oriented
artefacts first-class objects. In a similar vein, an approach
to addressing complexity in software evolution could be
focused on making the software evolution artefacts ex-
plicit through modelling them as first class objects. This is
especially relevant for service-oriented architectures
(SOA), aiming to appropriate the benefits of reusability
and maintainability to foster the design of applications in
an implementation independent manner using network
services and connections between network services.
The NfRn provides insights towards this end by pro-
moting a shift in the unit of analysis from task- or activ-
ity-level to task execution contexts. Then, designing
software systems for execution across different non-
functional contexts is conceived as specifying the sys-
tem’s global execution context. This requires an explicit
account of platform-oriented non-functional requirements
such as augmentation, expansion, integration and ab-
straction, which are considered as quality goals inscribed
in a SOA. Moreover as software designers will increas-
ingly be required to articulate the global execution context
of a system’s tasks, there is a compelling need for tools
supporting the management of designated software evo-
lution artefacts. In our work, this is facilitated by ex-
tending the use case notation widely employed for
documenting functional requirements in a manner facili-
tating the construction and refinement of the tasks’ global
execution context graph.
The global execution context notation and the sup-
porting tool have now been applied in a number of case
studies and applications (see [44-46]) in addition to the
initial validation in the Health Telematics domain [47],
providing useful insight to managing change in interactive
software. These experiences provide evidence to support
the claim that the basic vocabulary of the GeC and the
method presented in this paper offer useful insight to
modelling software design evolution necessitated either
by new requirements or evolving requirements. The pri-
mary benefit of the method results from the fact that
change becomes a first class design object modelled
through designated growth scenarios that evolve from
previously codified reference scenarios. Moreover, the
GeCg as an artefact provides designers with useful in-
formation regarding:
The range of alternative execution contexts consid-
ered appropriate at a point in time.
The conditions which characterize activation/deac-
tivation of growth scenarios; this entails an elaboration
and justification of each of the relationships appearing in
the graph.
Guidance in the choice of what paths to traverse or
walk through under specific conditions.
Choice of suitable system architecture; for example
relationships of the type alternative and augments desig-
nate the systems adaptable components, while the rela-
tionship type parallel points out adaptive features of the
target implementation.
Consequently, the main contributions of the presented
work are threefold. Firstly, we described a method for
modelling change early in the development lifecycle.
This is done by introducing a notation, which is simple
and intuitive while resembling the vocabulary used by
other popular notations such as UML. It is argued that
using this notation to specify the current and anticipated
contexts of use constitutes an improvement upon current
practices. Specifically, the burden of using textual de-
scriptions to codify goals (as in the case of RUP) is re-
moved. Instead, visual constructs are used to codify de-
sign logic and rationale in a manner similar to other re-
search proposals for visual goal-oriented requirements
modelling [3-5]. Secondly, the method offers a frame of
reference for considering scenarios as drivers for system
evolution. This departs from contemporary views of sce-
nario-based requirements engineering where scenarios
are considered as static resources appearing at the begin-
ning of a project and lasting until specifications or re-
quirements are documented. In our work, scenarios re-
main ‘live’ and persistent resources driving future system
evolution. Moreover, this is achieved in a systematic
manner and it is documented using appropriate com-
puter-based tools. Another contribution of the present
work is that it is particularly suited to dealing with non
functional requirements – such as adaptability, adaptivity,
scalability and portability – which in contrast to func-
tional requirements, are known to be hard to model and
account for. This offers a perspective on scenario evolu-
tion, which is complementary to existing conceptions
proposed in the relevant literature (e.g. [17]).
Copyright © 2010 SciRes JSEA
Specifying the Global Execution Context of Computer-Mediated Tasks: A Visual Notation and a Supporting Tool329
6. Summary and Future Work
In this paper, we have presented a method and a sup-
porting tool for specifying the global execution context of
computer-mediated tasks. Our motivation has been to
make explicit the artefacts of evolution. Thus, our method
considers evolution as a transformation from the current
situation (codified through reference scenarios) to an
envisioned situation (represented by semantically related
growth scenarios). The links characterizing such trans-
formations are a small set of scenario relationships such as
alternate execution, concurrency, ordering, and set-oriented
relationships between two scenarios, devised to encapsu-
late evolution as change of functional requirements as well
as evolution as change in non-functional qualities. A sys-
tem’s global execution context can then be depicted as a
visual construction, referred to as the global execution
context graph, and can be populated by a supporting tool
suite and transformed to XML.
Future work seeks to address several extensions both in
the method and the i-GeC tool. In terms of methodological
extensions, we are studying the development of a scenario
specification language to formalize the description of
scenarios. On the other hand several refinements of the
tool suite are currently under development. Specifically,
an on going activity seeks to expand the (currently primi-
tive) user interface prototyping features supported by the
tool so as to establish a link between scenarios (either
reference or growth), their underlying rationale and their
(possible) interactive embodiments. In this context, we are
also exploring the possibility of linking the tool’s outcome
with existing task-based notations and model-based user
interface engineering methods such as Teresa [32].
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