Opportunistic Reasoning for the Semantic Web: Adapting Reasoning to the Environment.
Opportunistic Reasoning for the Semantic Web:
Adapting Reasoning to the Environment
Carlos Pedrinaci1, Tim Smithers2, and Amaia Bernaras3
1Knowledge Media Institute, The Open University, Milton Keynes, UK.
2Fatronik, San Sebasti´ an, Spain. firstname.lastname@example.org
3Idom, San Sebasti´ an, Spain. email@example.com
Abstract. Despite the efforts devoted so far, the Semantic Web vision
appears to be an eluding target. We propose a paradigm shift for the
Semantic Web centred around the pragmatics of developing Semantic
Web applications in order to overcome the bootstrapping problem it
suffers from. This paradigm is based on the vision of the Semantic Web as
the result emerging from the integration and collaboration of a plethora
of Semantic Web applications, rather that as a global entity. On the basis
of this assumption we describe and propose Opportunistic Reasoning
as a general purpose reasoning model suitable for the development of
reasonably scalable Semantic Web applications.
The Semantic Web aims to extend the current Web with well-defined seman-
tics, represented in a machine interpretable form, to support better and greater
automation of tasks over the Web. Prototypes have been developed aiming at ex-
ploring the capabilities of ideas and technologies the Semantic Web builds upon
and latest results exhibit a certain degree of maturity. Still, achieving the results
initially envisioned in  seems to be a somehow eluding target. The Seman-
tic Web suffers from a bootstrapping problem that is limiting its success despite
the amount of effort and the number of tools that have been produced so far.
This had led researchers to seek other forms of reasoning that could eventually
circumvent the current limitations focussing on scalable reasoning as the major
barrier to surmount .
We propose a paradigm shift for the Semantic Web centred around the prag-
matics of developing Semantic Web applications. This paradigm shift is based on
a subtle distinction on how we foresee the eventual emergence of the Semantic
Web with respect to what appears to be the underlying and implicit assumption
within the Semantic Web research community. Typically, the Semantic Web is
considered as a whole, as an entity composed of semantic annotations over the
existing Web. Thus, most efforts have been devoted to creating systems that
generate annotations (semi)automatically and semantic search engines that can
use these annotations to increase search accuracy. As a consequence, scalability
is regarded as a major issue and it is limiting to some extent the uptake of Se-
mantic Web technologies. These applications are necessary elements to obtain a
Web with annotations but we are somehow missing the intelligent applications
that make use of existing annotations to better support or to enable the au-
tomation of (new) tasks over the Web. Semantic search engines and information
aggregators fall into this kind of intelligent applications but these are certainly
not the unique or the most appealing usage we can make of annotations.
We consider the Semantic Web rather as the result emerging from having
intelligent agents that make use of semantic data on the Web, to perform new
tasks and eventually contribute to the overall Semantic Web with more semantic
data. Indeed, this view of the Semantic Web is perfectly compatible with current
Semantic Web research. It requires and makes use of the languages, and tools
developed within the Semantic Web community, but it shifts the current focus
towards the creation of Semantic Web applications. The main fundamental dis-
tinction this vision implies is the fact that we don’t see the Semantic Web as a
Knowledge Base but instead as a source of knowledge for the intelligent applica-
tions. This carries an important set of consequences with respect to reasoning in
the Semantic Web which we review in detail in Section 2 taking  as a starting
A straightforward implication is that the uptake of the Semantic Web tech-
nologies and ideas does not depend so heavily on the availability of Web scale
reasoners. After all, the Web was there much before we had access to high-speed
networks, ultra efficient search engines or online stores. Instead the evolution
of the Semantic Web becomes an incremental process–much like the Web itself–
where the industry may progressively find commercial interest. Motivated by this
subtle change of viewpoint we propose in Section 3 Opportunistic Reasoning as a
general purpose reasoning model suitable for the development of reasonably scal-
able Semantic Web applications. Finally in Section 4 we conclude and introduce
some future considerations.
2The Semantic Web: So far, So good
After approximately ten years of research on integrating Artificial Intelligence
techniques with the Web, we are still seeking for the ”killer application”. So far,
research on the Semantic Web has focussed on defining ontology languages, on
creating tools that manipulate them, on automating the generation of metadata,
on integrating search engines with ontology reasoners, etc. There are indeed
many interesting and promising results which have even been applied in the
industry. However, the Semantic Web vision as it was presented in  is yet to
become a reality. The Semantic Web is certainly an appealing goal and it clearly
requires applying Artificial Intelligence techniques. What is not so clear is why
it is still facing this bootstrapping problem.
In  the authors identify some of the reasons for this, focussing mainly on the
lack of approaches that can scale to reasoning on the Semantic Web. They revisit
the underlying assumptions in state-of-the-art solutions and argue for the fusion
of reasoning with search and the application of the principle of limited rationality
as a solution for overcoming these limitations. Scalability is indeed a key aspect
when it comes to the Web, and the ideas exposed in  or in  appear to
be promising approaches to overcoming or at least minimizing scalability issues.
However, scalability is not all there needs to be. In the remainder of this section
we revisit these very same assumptions  and expand on them trying to identify
the main features Semantic Web applications should provide.
The first assumptions which are questioned are the number of axioms and
facts contemplated which are too low to be realistic. Indeed, it is perfectly plausi-
ble to expect a huge set of facts and axioms in an environment like the Semantic
Web. This has crucial implications in terms of scalability but we would like to
put more emphasis here on the consequences from a Knowledge Engineering
perspective. Classical Knowledge Based Systems were based on a rather na¨ ıve
view that complex systems could be built upon a more or less extensive set of
rules. The numerous systems developed under such an assumption proved that
it had some important weaknesses , like the complexity of the Knowledge
Bases or the fact that the control of the application of the knowledge was im-
plicit in the ordering of the rules. As a consequence the development of complex
Knowledge-Based Systems was a hard and tedious task, requiring the creation of
complete systems mostly from scratch with very little possibility for knowledge
or software reuse (other than the inference engines themselves). Similarly, main-
tenance of these systems was often an even harder task because of the inherent
inter-dependencies existing between the many rules.
Ontologies provide for some modularity and reusability but, ontology lan-
guages defined for the Web have (purposely) limited expressivity in order to
maintain computationally attractive characteristics. And the development of
Knowledge-Based Systems that can reason with the information on the Web
does not, indeed cannot, uniquely rely on the manipulation of ontologies . In
fact for problem-solving beyond classification we need dynamic knowledge ac-
companying the static knowledge encapsulated in the ontologies . This need
is recognised within the Semantic Web research community as can be distilled
for example from the research in Semantic Web Services [9,6] and ongoing stan-
dardization efforts as pursued by the Rule Interchange Format within the W3C.
Unfortunately, adding further reasoning capabilities carries computational draw-
backs which are to be added to the previously mentioned scalability issues .
Leaving aside the well-known tradeoff between expressivity and tractability, the
bottom-line conclusion one can draw is the need for embracing Knowledge Engi-
neering principles if we want Semantic Web applications to scale. Among these
principles the main one is perhaps modularity and in this sense research in
Problem-Solving Methods is probably of the biggest relevance [17,15,7].
The third assumption which is questioned is the completeness of the inference
rules, which given the characteristics of the Web, does not seem to be a reason-
able requirement. Completeness is indeed a desirable feature but it is often not
realistic (especially in the Semantic Web) and most importantly, it is not even
necessary in many occasions. Humans exhibit a remarkable ability with respect
to reasoning and we definitely do not take every single fact, and possible im-
plication into account before taking decisions. Many knowledge intensive tasks,
such as Design, are carried with a characteristic opportunism with respect to
the exploration of the problem to be solved . For instance when an architect
is designing a building the fact that a room does not have an entrance is not
particularly relevant at early stages of the design where designing the spaces is
definitely more important. Later on, the designer will detect the situation and
will perform the appropriate adaptations. The lack of awareness with respect to
this fact does not prevent the architect from working on the design. Instead an
overload of information could perfectly be cumbersome and distracting. In other
words, many decisions can be, and usually are, adopted without knowing every
single fact that could be derived from the existing facts. Actually, this simpli-
fication is an essential feature of Knowledge-Based Systems, for ”the very act
of preparing knowledge to support reasoning requires that we leave many facts
unknown, unsaid, or crudely summarized” . Therefore rather than complete
reasoning, what it is required is an appropriate means for dynamically guid-
ing the reasoning process in an opportunistic and incremental manner, deriving
knowledge or solving problems as the need arises.
A fourth aspect which is challenged in  is the need for consistency and
the trustworthiness and correctness of inference rules. The authors rightly point
out that it does not make sense to assume or enforce this since the Web is
populated with contradictory information. Dropping this assumption allows to
apply probabilistic approaches to reasoning, whereby decisions can be performed
over incomplete knowledge. This is a common approach to dealing with realistic
domains that are characterised by their complexity . This approach opens
up interesting opportunities towards scalable solutions as required for the Se-
mantic Web. However, a possible consequence is that, reasoning over incomplete
knowledge leads to a plausible yet unfortunately incorrect decision. For exam-
ple it could be possible, although highly unlikely that throwing a dice 1000
times we never obtain a 6. With respect to reasoning this implies that future
facts observed could invalidate previously assumed ones, which requires applying
non-monotonic reasoning. Making use of probabilistic reasoning is perhaps an
appropriate way for reasoning over huge knowledge bases like the Semantic Web.
However, it is worth noting that using this technique within some knowledge in-
tensive task will eventually require the integration of some truth maintenance
A final issue raised by the authors is that the Web is essentially dynamic and
as a consequence the previous arguments are simply reinforced. The number
of facts and axioms not only will be huge but it will also tend to increase. And
enforcing or assuming the completeness and correctness of the inferences does not
seem to be justified since new facts are continuously added, modified or retracted.
This reinforces the fact that the Semantic Web as a whole should be considered as
an open world. However taking into account the previous discussions, it also leads
towards the fact that applications that want to reason about the information on
the Semantic Web should rather consider their workspace as a closed world where
new facts can be continuously added based on changes on the Web which could
possibly invalidate previous inferences or assumptions. Again, it turns out that
opportunism is a desirable behaviour, if not a required one for Semantic Web
3Opportunistic Reasoning for the Semantic Web
In any kind of reasoning, a central question is: what to do next? When it comes
to automated reasoning this involves determining which pieces of knowledge to
apply and when to do it. This is what is usually referred to as the “control
problem”. The Oxford English Dictionary defines opportunism as ”the practice
or policy of exploiting circumstances or opportunities to gain immediate ad-
vantage, rather than following a predetermined plan”. Opportunistic Reasoning
was defined in  as “the ability of a system to exploit its best data and most
promising methods”. Opportunistic Reasoning establishes that reasoning should
respond opportunistically to changes in the current state of the problem solv-
ing process. The Blackboard Model is the Opportunistic Reasoning Model per
excellence and we shall use it in this paper to illustrate its main characteristics
taking into previous use cases and analysis performed. Finally, we review the
general applicability of this reasoning model and contrast it with the essential
features of the Web.
3.1 The Blackboard Model
During the 1970’s and 1980’s the “Blackboard Model” was proposed, developed,
and applied in an important number of applications as a way to surmount the
inconveniences of the classic Knowledge Based Systems approach . The Black-
board Model is often presented using the analogy of a group of persons in front
of a blackboard trying to put together a jigsaw puzzle. Each of them looks at his
or her pieces and sees if any of them fit, in which case they update the solutions.
New updates cause other pieces to fall into place and the whole puzzle can be
solved in complete silence. The solution is built incrementally and opportunis-
tically as opposed to, say, starting systematically from the top left corner and
trying each piece.
In the blackboard literature, the group of people are referred to as Knowl-
edge Sources (KSs) to better account for the role they play . The fundamental
philosophy of this problem-solving model establishes that KSs do not communi-
cate with each other directly, instead all the interactions strictly happen through
modifications on a shared workspace, i.e., the blackboard. Experts of particu-
lar aspects of the problem contribute to the overall problem-solving activity in
an incremental and opportunistic way. The Blackboard Model as described by
the metaphor, is a conceptual definition of a reasoning behaviour and does not
prescribe any particular implementation detail. It is therefore important not to
take the Blackboard Model as a computational specification, but rather as a
conceptual guideline about how to perform problem-solving reasoning . Even
though the Blackboard Model is relatively easy to understand, it encapsulates an
important number of characteristics which are not that obvious at first glance.
We shall review them main ones briefly in the remainder of this section, the
reader is referred to  and  for further details.
The Blackboard Model stems from a direct application of the “divide and
conquer” principle. The rationale underlying such a principle is to decompose
complex problems into more manageable and presumably simpler ones. Solutions
to a problem can be obtained by bringing together solutions to sub-problems.
This is a well-known, widely applied, and often successful approach to dealing
with complex or large problems which makes it particularly useful for an en-
vironment like the Web. In addition to simplifying problem-solving, the divide
and conquer approach leads to partitioning the expertise required for the rea-
soning activity. The divide and conquer approach underpining the Blackboard
Model thus, relies in partitioning the problem-solving expertise that propagates
over the whole application development cycle. The modularity brought by the
independence of expertise supports the incremental development, extension and
replacement of Knowledge Sources in a relatively simple way and supports the in-
tegration different reasoning mechanisms to overcome the limitations of a unique
technique. This allows for a rather straightforward application of Knowledge En-
gineering principles as presented in [15,17].
The blackboard metaphor, establishes that all the interactions among the
knowledge sources must take place indirectly via the shared blackboard. As a
consequence, communication among experts does not rely on any agreed inter-
action protocol. Instead, it requires a common representation language for the
information to be understood by various Knowledge Sources; ontologies being a
rather obvious candidate to fulfil this role. A natural requirement, if we want
communication to take place only via the shared blackboard, with no direct
communication among the Knowledge Sources, is that KSs have to be “self-
activating”. Hence, this reasoning model relies on event-based activations of the
Knowledge Sources, who—much like the people solving the jigsaw puzzle—are
watching the blackboard to react to changes made by other KSs. The event-
based activation, enables applying this reasoning model to particularly dynamic
domains like the Web.
As promoted by the blackboard metaphor, the Blackboard Model is an incre-
mental reasoning model where reasoning progresses step-by-step. Consequently,
solutions are generated incrementally guided by the different and collaborating
sources of expertise. This allows for a guided exploration of the possible solu-
tions, potentially reducing the amount of calculations to be performed in order
to find a solution (if any). Moreover, thanks to the application of several sources
of knowledge focussing on different aspects of the problem, this reasoning model
represents a good heuristic that avoids some typical problems suffered by tradi-
tional algorithms whose behaviour is pre-defined, such as Hill-Climbing, to cite
a well-known example. The behaviour exhibited by the Blackboard Model is
characterised by its ever changing focus of attention, or, in Feigenbaum’s words,
the “island-driven exploration of the solution space” , which confers on this
reasoning model the outstanding ability to deal with uncertainty and smoothly
adapt to changing conditions.
The distinctive property of the Blackboard Model, which stems from the pre-
vious characteristics, is what is usually referred to as Opportunistic reasoning.
Opportunistic reasoning is a flexible knowledge application strategy as opposed
to fixed algorithms whose behaviour is pre-established and cannot therefore be
adapted and reoriented facing particular situations, at least, not easily. Hence,
this expression accounts for the prominent ability of the Blackboard Model to
seamlessly integrate a collection of collaborating Knowledge Sources into the
overall reasoning activity, applying their expertise at the most opportune time.
Perhaps, the most prominent example of the opportunistic reasoning capabil-
ities of the Blackboard Model, is illustrated by the BB1 blackboard control
architecture . BB1 was a task-independent implementation of the blackboard
control architecture whose goal was to provide a solution to the control problem
(i.e., deciding which of the potential actions the system should perform next).
Through this work, the authors explored the ability of the Blackboard Model for
meta-reasoning and conscious reasoning when applying the Blackboard Problem-
Solving Model. Their resulting framework, and conclusions regarding the general
application of the Blackboard Model for this particular problem-solving task
(i.e., solving the control problem), accounted for the outstanding capabilities to
seamlessly, flexibly and dynamically coordinate the various collaborating experts
towards achieving a solution.
3.2On the Pragmatics of the Blackboard Model
So far we have focussed on the main characteristics of the Blackboard Model as
the Opportunistic Reasoning Model per excellence. In this section we focus on
the applicability of this reasoning model in general but also in particular to the
Web. Some authors, have already realised the importance of better establishing
the conditions under which the Blackboard Model is appropriate. However, these
are not much more than useful guidelines. For instance,  argues that the
Blackboard approach is generally suitable for solving ill-structured problems
(i.e., a problem with poorly defined goals and where no predefined algorithm
exists that leads to a solution) and complex problems (i.e., one made up of a
large number of parts that interact in a nonsimple way).  also gives some
properties that often characterise the problems that were successfully solved
through the Blackboard Model.
Taking into account the current state-of-the-art in blackboard systems, we
have extended the set of properties to include those we have noticed are recur-
rent . These characteristics have been identified from a large set of applica-
tions for performing diverse knowledge-intensive tasks within diverse domains.
These include voice recognition, military situations monitoring and assessment,
signal processing, drug design support, events design support, military plan-
ning, process scheduling systems, etc [3,14,13,11]. In general the occurrence of
some combination of the following problem characteristics can serve as a good
indication of the appropriateness of the blackboard approach1:
1. A large solution space;
2. Noisy or unreliable problem data;
3. A continuous data flow;
4. The need to integrate diverse and heterogeneous data;
5. The need to integrate different sources of knowledge;
6. The need to apply several reasoning methods;
7. The need to develop various lines of reasoning;
8. The need for incremental reasoning;
9. The need for an opportunistic control of the reasoning process;
10. The need for an event-based activation of the reasoning;
11. High complexity of the task;
12. The need for mixed initiative control. That is, the need for a collaborative
framework where computer and users can interchangeably take the initiative;
13. Meta-reasoning or conscious reasoning;
14. The need for psychologically reasonable implementations. This stands for the
ability to map human problem-solving into the automated implementation
as well as for the capacity to provide explanations of the reasoning steps
As can be distilled from the previous characteristics, the Blackboard Model
is a general and versatile reasoning model, particularly well suited for support-
ing reasoning processes over the Web. It provides an outstanding support for
reasoning in highly dynamic environments. It supports adapting the reasoning
process to the very typical and diverse events of the Web, such as remote ex-
ecution exceptions, a continuous data flow, connectivity problems, etc. In fact,
the event-based activation of the expertise paves the way for the seamless, effec-
tive and Knowledge-Based choreography of remote (knowledge-based) services
distributed over the Web, as shown for instance in [12,13].
This reasoning has been shown to be particularly well suited for dealing with
noisy, unreliable, heterogeneous and massive data. This is, as discussed in 
and in Section 2, of crucial importance for reasoning over the Web, where its
extension, the lack of central control and the principle of tolerance have led
to a highly noisy, often contradictory, unreliable and increasingly expanding
source of information. In fact, the island-driven exploration of the solution space
that characterises Opportunistic Reasoning together with its prominent capacity
for integrating diverse reasoning techniques, provides an excellent support for
reasoning over the Web.
1The existence of some of these characteristics does not make of the Blackboard Model
an appropriate reasoning model. However, the more of these characteristics that
appear in a problem, the more likely the blackboard model would be appropriate.
4Conclusions and Outlook
The Web opens up a plethora of possibilities which have already been demon-
strated in a variety of different scenarios. Reasoning over the Web is part of the
great potentials that can be brought to reality with some efforts and discipline.
To do so, we must however, assume the very nature of the Web and adapt our
techniques and technologies to it. Any kind of application conceived for the Web
must be prepared for this particular environment. This involves being ready to
deal with an extremely large and distributed environment, populated by uncer-
tain, dynamic and to some extent autonomous information, served by machines
which are prone to errors.
We have proposed viewing the Semantic Web as the emerging result from
having intelligent agents consuming and generating semantic data on the Web,
as a means to surmount the so-called bootstrapping problem. The incremental
nature of this approach provides the simplifications required for creating useful
reasoning systems out of the existing semantic data on the Web while it al-
lows the creation of more complex system on the basis of existing ones which
are considered as knowledge services providers. Finally, we have presented Op-
portunistic Reasoning as a general purpose reasoning that can appropriately
be applied to the developing applications that reason over the Web, therefore
contributing to the overall evolution of the Semantic Web.
The approach presented in this paper introduces pragmatic ideas that could
alleviate to an important extent the bootstrapping problem within the Semantic
Web. This research does not however intend to be a solution to the development
of a Web scale semantic search engine. Still, there are many ideas which could
be reused and in fact there are currently other researchers applying similar ap-
proaches in order to develop Web scale reasoning systems . In this sense we
expect that the integration of probabilistic reasoning as proposed in  and 
together with the outstanding qualities of the Opportunistic Reasoning model
could yield promising results.
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