Emergent Semantics in Distributed Knowledge
Management
Carola Aiello2 , Tiziana Catarci2 , Paolo Ceravolo1 , Ernesto Damiani1 , Monica
Scannapieco2 , and Marco Viviani1
1
Dipartimento di Tecnologie dell’Informazione- Università di Milano
via Bramante, 65 - 26013, Crema (CR), Italy
{damiani, ceravolo, viviani}@dti.UNIMI.it
2
Dipartimento di Informatica e Sistemistica - Sapienza Università di Roma,
Via Ariosto 25, - 00185, Roma, Italy
{carola.aiello, tiziana.catarci, monica.scannapieco}@dis.uniroma1.it
Abstract. Organizations and enterprises have developed complex data
and information exchange systems that are now vital for their daily operations. Currently available systems, however, face a major challenge.
On todays global information infrastructure, data semantics is more and
more context- and time-dependent, and cannot be fixed once and for
all at design time. Identifying emerging relationships among previously
unrelated information items (e.g., during data interchange) may dramatically increase their business value. This chapter introduce and discuss
the notion of Emergent Semantics (ES), where both the representation of
semantics and the discovery of the proper interpretation of symbols are
seen as the result of a selforganizing process performed by distributed
agents, exchanging symbols and adaptively developing the proper interpretation via multi-party cooperation and conflict resolution. Emergent
data semantics is dynamically dependent on the collective behaviour of
large communities of agents, which may have different and even conflicting interests and agendas. This is a research paradigm interpreting
semantics from a pragmatic prospective. The chapter introduce this notion providing a discussion on the principles, research area and current
state of the art.
1
Introduction
On todays global information infrastructure, access to information involves interaction with distributed sources. Moreover network agents
want to access information efficiently, protecting their sensible information and preserving their autonomy. One of the most important ways of
improving the effectiveness of information retrieval and service sharing is
by explicitly describing information services semantics. Ontology serves
this purpose: it consists of explicit, partial definitions of the intended
meaning of symbols for a domain of discourse. Unfortunately, building
shared ontology is a complex process and top-down ontology design, even
when done collaboratively, is known not to scale well. Also domains are
rapidly evolving and the semantics of data cannot be fixed once and
for all at design time. This requires to make semantics more and more
context- and time-dependent.
Emergent Semantics has been proposed as a solution to the semantic
interoperability problem. The general idea is that if we renounce to
a centralized control over semantic description, we can improve them
by exploiting implicit information emerging during data exchange. For
instance repeated downloads are a confirmation of data quality, while
the frequency of interactions can define a degree of correlation. This
paradigm can be applied in many environments such as human-computer
interaction, language games for robot-robot and robot-human communication, scientific databases, e.g. biological data, where data is captured
through experiments and subsequently analyzed. In this work, we limited
our analysis to distributed knowledge management. This is an environment characterized by the multiple information agents interacting with
different level of cooperations. In this context data exchange can be taken
as a source of additional information to be used in a process of enrichment of semantic representation. The chapter is organized as follow. In
Section 2 we introduce the notion of Emergent Semantics. In Section 3
we discuss the main principles of this research paradigm. In Section 4
we summarize the principal research areas, such as P2P data integration, service discovery or trust and reputation management. Finally, in
Section 5 we provide some conclusive remarks.
2
The Notion of Emergent Semantics
The notion of semantics has various definitions in different domains.
In the domain of Programming Languages, semantics basically refer
to rules which relate inputs and outputs [74]. In logic, semantics is
the Kripke’s theory of truth, usually expressed by a a set-theoretic
interpretation [62]. On the web, semantics is often intended as the
metadata used for annotating resources, in this case the term “equivalent
semantics” designates for equivalent annotation imposed on different
resources, using a common vocabulary, as for instance in [20]. For some
applications in the field of image retrieval the semantic interpretation
of the image content is available as the result of an automatic or
semi-automatic image analysis process, applied to images belonging to
specific domains and described in advance [63]. Here the semantics of a
resource is a typical pattern related to that resource. As a least common
denominator, we can characterize semantics as a relationship or a
mapping established between an information items syntactic structure
and some external domain.
The Emergent Semantics approach consolidates the local semantics
held by autonomous information agents into a global, population-wide
semantics that results from the continuous interaction of the agents
among themselves. The large-scale structures emerging from these
continuous interactions dynamically provide meaning to the local
symbols. Semantics constructed incrementally in this way is called
Emergent Semantics (ES). This is a research paradigm interpreting
semantics from a pragmatic prospective. More complete presentations
of this paradigm can be found in [22], [2], [71].
3 Principles and main features of the Emergent
Semantics Paradigm
Emergent semantics is the form of semantic interoperability viewed as an
emergent phenomenon [2] constructed incrementally among data sources
in a bottom-up, semi-automatic manner without relying on pre-existing,
global semantic models. In such a dinamic scenario, global information
is highly evolutionary: documents of already existing sources may be
updated, added or deleted; new sources and services may appear and
some may disappear (definitively or not). At any given point in time,
the state of the semantic interoperability depends on the frequency, the
quality and the efficiency with which negotiations can be conducted to
reach agreements on common interpretations within the context of a
given task. The main principles and features of the emergent semantics
paradigm can be summarized as follows:
– Semantic agreement. Two or more peers need to establish a semantic
agreement, that is to share the meaning of a model, like a conceptual
model, or other relevant meta-data information to the task at hand.
– Agreement negotiation. Agreements are negotiated between peers,
and are dynamically built and maintaned. Such agreements constitute the basis for communication between peers which is realized in
terms of message exchanges. Negotiations of semantic agreements
are performed on local basis, that is a peer directly contacts all
the peers it wants to communicate with. In this way the number
of interactions is greater than the one characterizing environments
that involve third parties with the role of mediating the communication. However, though the agreements are built on a local basis, a
global agreement is nevertheless obtained as a result of their propagation. This is, in a sense, the real essence of the emergent semantics
paradigm: it is not necessary to contact all the network, in fact the
communication can be incrementally realized by exploiting semantic
agreements. Agreements are established on the basis of the peer’s
specific goals; hence for each distinct goal a peer could establish
distinct agreements.
– Self organization. The emergent semantics paradigm relies, at the
end, on a self-organization process. Self-organization processes have
been studied in many scientific disciplines such as physics or biology
for a long time and are recently being investigated in computer science as well. Under this perspective, emergent semantics is another
application of this successful principle in the computer science field.
4
Principal Research Areas
Important efforts in many research areas are needed to achieve semantic
interoperability by the emergent semantics paradigm. In this section we
detail the principal involved research areas : (i) P2P Data Integration,
(ii) Service Discovery and (iii) Trust and Reputation.
4.1
P2P Data Integration
Information systems have been characterized by a multitude of autonomous, heterogeneous information repositories. The problem of how
to provide transparent access to heterogeneous information sources while
maintaining their autonomy is a time-honored one. Information integration systems typically provide a uniform query interface to a collection of
distributed and heterogeneous information sources, giving users or other
agents the illusion of querying a centralized and homogeneous information system. As such, they are considered as mediation systems between
users and multiple data sources which can be syntactically or semantically heterogeneous while being related to the same domain. Existing mediator-based information systems can be distinguished according
to: (1) the type of mappings between the mediated schema (on global
schema) and the schemas of the sources: there exist basically two approaches for such mappings, the Global As View (GAV) and the Local As View (LAV). The global-as-view approach describes the global
schema as a view over all local schemas, whereas the local-as-view approach describes each local schema as a view over the global schema;
(2) the languages used for modelling the global schema and the source
descriptions and (3) the expressivity of the global schema. Starting from
this main thread, a recent research line aims to integrate data in a peerto-peer (P2P) environment and is identified as peer data management or
peer-to-peer data integration. A P2P system is characterized by a structure constituted by various autonomous nodes that hold information and
that are linked to other nodes by means of mappings. No global ontology
a peer can refer to is actually available. The goal of P2P data integration
is to provide unified access to this set of heterogeneous data sources. The
lack of any agreed-upon global schema or ontology makes it very difficult
for the participating parties in the system to reach a global consensus on
semantic data.
In the following we describe the principal research issues related to P2P
data integration: (i) Formal Semantics, (ii) Ontology Matching, (iii)
Query Processing and (iv) Data Quality .
Formal Semantics Initial approaches rely on some pre-defined corpus
of terms serving as an initial context for defining new concepts [41] or
make use of gossiping and local translation mappings to incrementally
foster interoperability in the large [1]. However, there is still a fundamental lack of understanding behind the basic issues of data integration
in P2P systems. Indeed, since no single actor is in charge of the whole
system, it is unrealistic to assume restrictions on the overall topology of
the P2P mappings [42, 31]. Hence, one has to take into account the fact
that the mappings may have an arbitrary structure, possibly involving
cycles among various nodes. This needs to be addressed both from the
point of view of modeling the system and characterizing its semantics
(see, e.g., [17, 42] for a first order logic characterization and [13, 14] for
an alternative semantics proposal) and from the point of view of computing answers to queries posed to a peer. Query answering has difficulties
that arise from the necessity of distributing the overall computation to
the single nodes, exploiting their local processing capabilities and the
underlying technological framework. A recent proposal highlighting the
peculiarities of P2P data integration systems is [37].
Ontology matching Dynamic ontology matching techniques can be
used as the basis for driving the negotiation of agreements in order to
discover the mappings between concepts of different peer ontologies and
maintain them.
The general goal of ontology matching techniques is to compare different
ontological descriptions for finding concepts that have a semantic affinity. A survey of ontology matching techniques is provided in [70], where
formal and heuristic approaches are classified. The former are founded on
model theoretic semantics and automated matching techniques [52, 10,
38], while the latter are based on the idea of guessing mappings that may
hold between similar graph structures through a combination of analysis,
matching, and learning techniques [27]. In [16], heuristic techniques for
performing dynamic ontology matching in open, peer-based networked
contexts are proposed. Peculiar features of these techniques are the capability of performing the matching process in a flexible and adaptable
way, by dynamically configuring the matching algorithm with the most
appropriate matching model for the specific matching case. This is done
by taking into account the level of richness of ontological knowledge description as well as the requested degree of mix between the importance
of linguistic and contextual features of concepts in the evaluation of their
matching degree.
Query Processing A P2P data integration system consists of a set
of (physical) peers. Each peer has an associated schema that represents
its domain of interest. Some peers store actual data, and describe which
data they store relative to their schema; the stored data does not necessarily cover all aspects of the peers’ schema. Peers are linked through
peer mappings. A mapping is the the medium for exchanging data and reformulating queries among different schemas; in particular, the mapping
defines the overlapping parts of acquaintances’ schemas. Peer mappings
describe the semantic relationship between the schemas of pairs (or small
sets of) peers. Given a query over a peer Pi the system will use the peer
mappings to reformulate the query over the schemas of the neighboring
peers. Typically, when a peer joins a P2P data integration system, it
will supply mappings, as is most convenient, to a small number of peers.
Semi-automatic schema mapping techniques have been investigated in
[64, 25, 26].
The key step of query processing in a P2P data integration system is
reformulating a peer’s query over other peers on the available semantic
paths. Broadly speaking, the P2P data integration system starts from
the querying peer and reformulates the query over its immediate
neighbors, then over their immediate neighbors, and so on. Whenever
the reformulation reaches a peer that stores data, the appropriate query
is posed on that peer, and additional answers may be found. Since peers
typically do not contain complete information about a domain, any
relevant peer may add new answers.
Among the principal approaches to query processing in P2P systems we
cite Piazza [42], Hyperion [50] and PeerDB [61].
In Piazza two types of mappings, peer and definitional mappings are defined, and used for performing query answering. Peer mappings describe
data within the stored relations (generally with respect to one or more
peer relations); definitional mappings are instead between the schemas of
the peers. This approach is alternative to the one used by the Hyperion
project [3]. Hyperion mappings rely on the usage of mapping tables that
store the correspondence between values. As outlined in [50, 51], mapping tables are often the result of expert knowledge and are manually
created by domain specialists. However, mechanisms to partially support
the task of mapping discovery can be used: new mappings can be inferred
from already existing mappings present in the mapping table.
PeerDB [61] is another P2P-based system for distributed sharing of relational data. Similar to Piazza, PeerDB does not require a global schema,
but it doesn’t use schema mappings for mediating between peers. Instead,
PeerDB employs an information retrieval based approach for query reformulation. In their approach, a peer relation (and each of its columns)
is associated with a set of keywords. Given a query over a peer schema,
PeerDB reformulates the query into other peer schemas by matching the
keywords associated with the two schemas. Therefore, PeerDB does not
have to follow semantic paths to reach a distant peer. The resulting reformulated queries in PeerDB may not be semantically meaningful, and
ultimately the system requires user input to decide which queries are to
be executed.
Besides mappings, also routing indices can be used for propagating
queries in the system [21, 43], that provide aggregated information about
records that are retrieved in the query processing phase. Hence, such indexes can be used for query optimization purposes.
A further approach for indexing data in peer-to-peer systems is based
on communities. A community is built on the basis of the similarity of
the schema (or the schema mappings) hold by peers. Query processing
benefits from the fact that if a query is posed on a certain peer than
members of its community will also have similar data. An example of
such an approach is provided in ESTEEM [30].
Data Quality In peer-to-peer environements, where information is
exchanged between heterogeneous data sources, the quality of data exchanged and provided by different peers is extremely important. A lack
of attention to data quality can imply data of low quality to spread all
over the system.
In [56], the problem of the quality of web-available information has been
faced in order to select data with high quality coming from distinct
sources: every source has to evaluate some pre-defined data quality parameters, and to make their values available through the exposition of
metadata.
When considering the issue of exchanging data and the associated quality, a model to export both data and quality data needs to be defined. Some conceptual models to associate quality information to data
have been proposed that include an extension of the Entity-Relationship
model [73], and a data warehouse conceptual model with quality features
described through the Description Logic formalism [44]. Both models are
thought for a specific purpose: the former to introduce quality elements
in relational database design; the latter to introduce quality elements in
the data warehouse design. In [69], a model for associating quality metadata to data exported in the context of a cooperative and distributed
system is described.
An important step for quality-aware data integration is the assessment
of the quality of the data owned by each peer. For this task, some of the
results already achieved for traditional systems can be borrowed, such
as record linkage techniques or data cleaning tools [5].
Data integration must take into account data sources heterogeneity. As
described in [75], when performing data integration two different types
of conflicts may arise: semantic conflicts, due to heterogeneous source
models, and instance-level conflicts, due to what happens when sources
record inconsistent values on the same objects. The Data Quality Broker described in [30] is a system solving instance-level conflicts. Other
notable examples of data integration systems within the same category
are AURORA [75] and the system described in [68]. AURORA supports
conflict tolerant queries, i.e. it provides a dynamic mechanism to resolve
conflicts by means of defined conflict resolution functions. The system
described in [68] describes how to solve both semantic and instance-level
conflicts. The proposed solution is based on a multidatabase query language, called FraQL, which is an extension of SQL with conflict resolution
mechanisms. Similarly to both such systems, the Data Quality Broker
supports dynamic conflict resolution, but differently from them the Data
Quality Broker relies onto quality metadata for solving instance-level
conflicts. A system that also takes into account metadata for instancelevel conflict resolution is described in [33]. Such a system adopts the
ideas of the Context Interchange framework [11]; therefore, context dependent and independent conflicts are distinguished and accordingly to
this very specific direction, conversion rules are discovered on pairs of
systems.
The ESTEEM architecture [30] is an example of an emergent semantics
system with data quality support. In the Esteem architecture the data
quality profile module involves the computation of data quality metrics
on the peer data that are available to other peers. More specifically, each
peer has the possibility of associating quality metadata to the exported
data (at value level). Such metadata represent data quality measures
corresponding to some specific quality dimensions. Metrics for the most
common quality dimensions (column completeness, format consistency,
accuracy and internal consistency (see [6] for the definition of such metrics) are already implemented and the model is ready to be extended
to other dimensions. Besides the data quality profile module, a quality
manager module is invoked during query processing in order to exploit
quality metadata and to take data inconsistencies into account. More
specifically, it is assumed that data can exhibit key-level conflicts [7].
This implies that a record linkage step must be performed in order to
provide answers to user queries.
4.2
Service Discovery
The emergence of semantics is a key issue to enforce timely discovery
and dynamic composition of distributed services. Recently, many organizations have heavily invested in Web Service technologies and, as a
consequence, a growing number of services is being made available. Service proliferation over the Web has been facilitated by the development
of several standards, like WSDL for service description, UDDI for service registry, SOAP for message exchange and BPEL4WS for service
orchestration.
The discovery of services is the most important functionality in distributed and service-oriented environments. Standards like UDDI or
WSDL support description of services and discovery functionalities from
a syntactic perspective. But the major problem remains: the semantics of
the service description. Often, the same services are described by users
and service providers in different ways. As services and their descriptions are evolving quickly responding to market changes, it is in general
impossible to keep up with all requirements in time. A decentralized
discovery service exploiting emergent semantics approaches to extend
the standards in a controlled way and distribute the changes among the
peers appears as a concrete possible solution. With such an approach, a
peer could ”learn” about new descriptions and mappings incrementally
departing from existing standards used for bootstrapping the process.
Modern approaches for service discovery have to address the treatment
of dynamical aspects both with respect to the continuous addition and
removal of services in a highly variable environment and with respect
to different contexts in which a service could be invoked [15]. Advanced
techniques and tools for enabling semantic service discovery are therefore
highly desired and required. In particular, it is necessary that services
are described in a formal way and service semantics is well captured. In
the literature, ontology-based approaches are being developed to exploit
the benefits of the ontology technology, such as inferencing, in the context of service discovery. In the Semantic Web, the ontology description
languages OWL [24] and OWL-S [19] have been recently proposed. Service description is composed by a service profile (what the service does),
a service model (how the service works) and a service grounding (how to
invoke the service).
The Semantic Web Services Initiative (SWSI, see www.swsi.org) relaxes
the constraint of using a description logic formalism for defining service
workflow, and uses a first-order logic based language. In the Unified
Problem Solving Method Development Language (UPML) framework
[34] logical expressions defined in goals, mediators, ontologies and Web
Services are expressed using frame logic. UPML distinguishes between
domain models, task models, problem solving methods and bridges, and
is also the basis of the Internet Reasoning Service (IRS) [57], a knowledgebased approach to Semantic Web Services. Domain models are effectively
the domain ontology, while task models provide a generic description of
tasks to be solved. Problem solving methods provide implementationindependent descriptions of tasks, while the bridges map between the
various components.
Service matchmaking has been addressed by several approaches in literature: given a request R and a set of advertisements S, the matching
procedure must return the set of advertised services that match better
with R, possibly ranked with respect to their level of matching (if it
can be evaluated). In most approaches the starting point is the UDDI
registry, where service descriptions are published; UDDI registry offers
searching functionalities that use traditional keyword-based techniques,
featured by low precision and recall. To provide semantic matching between service descriptions, some approaches consider concept definitions
within ontologies (concept-based techniques). In [58] a framework for
semi-automatically marking up Web service descriptions with ontologies
is proposed with algorithms to match and annotate WSDL files with relevant ontologies; domain ontologies are used to categorize Web services
into domains. The use of ontologies enables service matchmaking in the
discovery process. In fact, the elements used for service capability description refer to concepts that can be properly defined and semantically
related in domain ontologies. Semantic relationships between concepts
are then exploited to establish the type of matching between advertisements and requests. Dynamic discovery of distributed services is based
on semantic interoperability. In [76] a service ontology specifies domain
concepts with a set of synonyms to allow a flexible search and a set of
service classes to define the properties of services, its attributes and operations. In [12] a new technique for Web service discovery which features
a flexible matchmaking by exploiting DAML-S ontologies is proposed. In
[49] a Web Service Modeling Ontology (WSMO) is expressed by using
the formal F-Logic language to describe various aspects related to Semantic Web Services. They start from the Web Service Modeling Framework (WSMF)[35], that consists of four elements: ontologies that provide
terminology used by other elements (concepts, axioms, relations and instances), goals of Web Services (by means of pre- and post-conditions),
Web Service description (non functional properties, choreography and
orchestration aspects) and mediators which bypass interoperability problems.
4.3
Trust and Reputation
As outlined in previous sections, today’s Web infrastructure is increasingly used for semantics-driven access to services and resources. This
problem is twofold. The first aspect is related to information retrieval,
and can be adressed by intelligent search and selection techniques. The
second is deciding which among many sources is most reliable and it is
usually presented by the notions of trust or reputation.
In human society, trust and reputation are social knowledge allowing to
evaluate which agents can be considered as a reliable sources of information or services. In computer science Trust is not a new research topic in
itself; however even, if there is a rapidly growing literature on the theory
and applications of trust in different kind of settings, there is a considerable confusion around the terminology used to describe them. In fact,
depending on the area where the concept of trust is used – security and
access control in computer networks, reliability in distributed systems,
game theory and agent systems, and policies for decision making under
uncertainty – it varies in these different communities in how it is represented, computed, and used.
The concept of trust is often connected to the mechanism that verify the
identity of a remote source of information; in this context, it is investigated it is in association with signatures and encryption mechanisms,
whose purpose is to provide protection against malicious parties. The
policies used to manage authorizations, allowing to distinguish trusted
and untrusted parties, are defined a-priori by a central authority. In a
community, however, trust in strictly connected to the notion of relationship among parties. In distributed community-oriented scenarios, for
evaluating the reliability of resources we need to deal with the notion of
relationship between a trustor, the subject that trusts a target entity, and
a trustee, the entity that istrusted. The formalization of this notion can
significantly improve the quality of the retrieved resources. As stated
in [53] and [47], trust can be an important factor in decision-making,
because it forms the basis for allowing a trustee to use or manipulate
resources owned by a trustor or may influence a trustor’s decision to use
a service provided by a trustee. According to the Emergent Semantics
approach the subjectivity of knowledge is seen as a potential source of
value and local misalignment is used as a way for improving and evolving
semantic mappings.
Defining Trust Trust is a complex subject relating to belief in the
honesty, truthfulness, competence, reliability, etc., of the trusted person
or service. There is no consensus in the literature on what trust is and on
what constitutes trust management, because the term trust is being used
with a variety of meanings [55]. Many researchers have recognized the
value of modeling and reasoning about trust computationally; however,
here is no entierly shared notion of trust nature as many authors in the
field have noted, the meaning of trust as used by each researcher differs
across the span of existing work.
In [46] two definitions of trust are introduced: reliability trust and decision trust respectively. The first one is introduced by means of the
Gambetta definition of trust [36] as “the subjective probability by which
an individual, A, expects that another individual, B, performs a given
action on which its welfare depends”. This definition includes the concept of dependence on the trusted party, and the reliability (probability)
of the trusted party as seen by the trusting party.
However having high (reliability) trust in a person in general is not necessarily enough to decide to enter into a situation of dependence on that
person [32]. Jøsang et al. introduces the definition inspired by McKnight
& Chervany [55] where “decision trust” is “the extent to which one party
is willing to depend on something or somebody in a given situation with
a feeling of relative security, even though negative consequences are possible”.
An alternative definition given in Mui et al. [59], which refers to past
encounters, and may be thought as reputation-based trust, described as
“a subjective expectation an agent has about another’s future behavior
based on the history of their encounters”.
Another interesting definition affirms that trust is “the firm belief in
the competence of an entity to act dependably, securely, and reliably
within a specified context” (assuming dependability covers reliability and
timeliness) [39].
The close relationship between trust and belief is emphasized by the
definition by Olmedilla et al. [60], which refers to actions and not
competence like the previous one: “Trust of a party A to a party B for
a service X is the measurable belief of A in that B behaves dependably
for a specified period within a specified context (in relation to service
X)”.
Depending on the environment where the notion of trust must be
applied the suitable definition can spotlight different features. In a
distributed environment one of the main features to be considered is
the ”dynamic nature” of trust, as discussed in [72]. As time passes, the
trust one entity has in another might not stay the same. In particular
trust can change depending on the experience that a trustor has about
a trustee, and this experience is always related to a context. Another
important aspect is the difference between trust and reputation. While
the concept of reputation refers to a perception that is generally said or
believed about an agent; trust ultimately is an individual phenomenon
that is based on various factors or evidence, some of which carry more
weight than others. The difference between trust and reputation can be
illustrated by the following statements [46]: (a) “I trust you because of
your good reputation” and (b) “I trust you despite your bad reputation”.
A distributed system relaying on the notion of trust must support two
important tasks: learning reputation and reasoning on trust. An agent
can learn reputations interacting with other agents and aggregating trust
evaluations of other agents. While the action of reasoning on trust describes the process in which an agent integrates the reputations from
other agents, with a trust model (public or private) and its own beliefs,
to update its local trust model.
Modeling Trust In the previous Section we have introduced the definition of trust. Now we focus our discussion on how realizing this notion
can be supported by a model. Models for computing trust can belong to
two categories: (i) policies-based models or (ii) reputation-based models
[8]. The definition of “hard evidence” used in policies opposed to the subjective estimation of trust used in reputation systems, as appear in [4],
reflect the difference between the term hard security used for traditional
mechanisms like authentication and access control, and soft security for
social control mechanisms in general, of which trust and reputations systems are examples. The difference between these two approaches was
first described by Rasmusson & Jansson in 1996 [66].
Policies-based models describe the conditions necessary to obtain trust,
and can also prescribe actions and outcomes if certain conditions are
met. Policies frequently involve the exchange or verification of credentials, which are information issued (and sometimes endorsed using a digital signature) by one entity, and may describe qualities or features of
another entity. In this field the terms authorization and authentication
are often connected to trust. Authorization can be seen as the outcome
of the refinement of a more abstract trust relationship. We define authorization as a policy decision assigning access control rights for a subject
to perform specific actions on a specific target with defined constraints.
Authentication is the verification of an identity of an entity, which may
be performed by means of a password, a trusted authentication service,
or using certificates. There is then an issue of the degree of trust in the
entity that issued the certificate. Note that authorization may not be
necessarily specified in terms of an identity. Anonymous authorization
can be implemented using capabilities or certificates
Reputation-based systems model an assessment on the history of interactions with or observations of an entity, either directly with the evaluator
(personal experience) or as reported by others (recommendations or third
party verification). How these histories are combined can vary, and recursive problems of trust can occur when using information from others
(i.e., can I trust an entity’s recommendation about another entity?).
At a basic level, both credentials and reputation involve the transfer of
trust from one entity to another, but each approach has its own unique
problems which have motivated much of the existing work in trust.
Trust Research Classification Due to the growing interest about
trust and the resulting growing corpus of literature on it, there is no
shared taxonomy of trust research. However some survey literature does
exist approaches have been divided in different areas in literature. In
particular [4] organizes trust research in four major areas:
1. Policy-based trust. Trust is established simply by obtaining a sufficient amount of credentials pertaining to a specific party, and applying the policies to grant that party certain access rights. The
recursive problem of trusting the credentials is frequently solved by
using a trusted third party to serve as an authority for issuing and
verifying credentials.
2. Reputation-based trust. History of an entity’s actions/behavior is
used to compute trust, and may use referral-based trust (information from others) in the absence of (or in addition to) first-hand
knowledge.
3. General models of trust. Trust models are useful for analyzing human
and agentized trust decisions and for computable models of trust
operational. Work in trust modeling describes values or factors that
play a role in computing trust, and leans more on work in psychology
and sociology for a decomposition of what trust comprises. Modeling
research ranges from simple access control polices (which specify
who can be trusted when accessing data or resources) to analyses of
competence, beliefs, risk, importance, utility, etc.
4. Trust in information resources. Trust is an an increasingly common
theme in Web related research regarding whether Web resources and
Web sites are reliable. With the advent of the Semantic Web, new
work in trust is harnessing both the potential gained from machine
understanding, and addressing the problems of reliance on the content available in the web so that agents in the Semantic Web can
ultimately make trust decisions autonomously. Provenance of information is key to support trust decisions, as is automated detection
of opinions as distinct from objective information.
In his short survey Griffiths [40] divides trust researches in three areas:
1. Security-oriented trust. A mechanism for ensuring security, encompassing issues of authentication, authorization, access control, privacy.
2. Service-oriented trust. A mechanism for achieving, maintaining, and
reasoning about quality of services and interactions.
3. Socially-oriented trust. A social notion for modeling and reasoning
about the relationships between agents, influenced by social science,
psychology or philosophy.
In addition to the taxonomies given by [4] and [40], in [65] trust research
is categorized according to the definitions of individual- or system-level
trust; in the former, individual agents model and reason about others,
while in the latter agents are forced to be trustworthy by externally
imposed regulatory protocols and mechanisms (this category includes the
already described area of policy-based trust). Finally, depending on the
fact that agents either trust others directly based on their experiences,
or base their trust on the recommendations of others, models can divided
in direct-trust-based or recommendation-based.
Trust and Emergent Semantics As seen in Section 3, in an ES
setting, the notion of uncertain knowledge is crucial. In [29] authors analyze Knowledge Management Systems describing the shared knowledge
distinguish to two dimensions: (i) the number of domains involved and
(ii) the number of conceptualization used in order to describe these domains. While in a centralized system domains and conceptualizations
are usually in a 1 : 1 relationship, and if multiple domains are taken
into account a single conceptualization is used (n : 1); distributed systems have 1 : n or n : m relationship. This simple remark clearly shows
how the mapping among the representations of different peers are subject to uncertain knowledge. Trust can be one of the ways for modeling
some aspects related to uncertain knowledge. Reconsidering the discussion proposed in Section 3, we can see that trust can have an impact
on many typical actions of peer-to-peer environments. Trust related P2P
actors include:
– Grouping: a group of peers that agree to be considered as a single
entity, at least with respect to a given task, by other peers. To become a member of a peer must provide some credential that can vary
from proving to manage some specific type of resource, to providing
a token.
– Discovery: is a mechanism that allows the user to discover resources
in the peer to peer network.
– Query Propagation: when a provider receives a query, it might propagate that query to another provider it considers expert on what
it believes is the meaning of the request. In order to decide where
to propagate a query a peer has two possibilities: (i) a proximity
criteria, i.e. the query will be sent to known peers (i. e. by using
the discovery functionality) and selection will be done according to a
quantitative criteria (number of peers, number of possible re-routings
hops-, etc.); this way peers or providers that are not directly reachable by the seeker or that have just joined the system, can advertise
their presence and contribute to the resolution of the query; (ii) a
semantic criteria: if the provider computes some matching between
a query and concepts in its own context, the query resolution mechanism might look for addresses of other peers that have been associated to the matching concept. Here propagation is done on the base
of explicit trust since the provider defines other peers as “experts”
on the query topic.
Projects and Applications In this section we discuss the state of
the art of projects and applications relying on ES for managing Trust in
Distributed Knowledge Management. Several projects adopt an approach
that can be associated to the ES principles. For example in [9] authors
state that Knowledge Management systems should be designed in order
to support the interplay between two qualitatively different processes:
the autonomous management of knowledge of individual groups and the
coordination required in order to exchange knowledge among them. The
authors introduces the notion of a proximity criteria, that is related to
semantic closure and trustworthiness among peers. Another project directly focusing on Distributed Knowledge Management is SWAP, [28].
Here the notion of Trust is adopted in order to rate the resources shared
in a semantic-based peer-to-peer system. However this work does not propose a process for managing the resource rating. A first work discussing
architectures and functions form managing aggregation and distribution
of reputations on semantic is [45]. In [67] the same problem is addressed
using path algebra. In [18] the authors provide a algorithm for aggregating rating on trustworthiness. No mature system implemented on these
algorithms exist right now.
Enlarging our attention to the notion of imprecise knowledge another
group of researches is available. Here again we note the lack of implemented solutions. In [48] authors provide a preliminary discussion on
the role of belief and trust in managing emergence in the context of information and service semantics. A taxonomy of belief level is proposed
and a measure of data quality based on belief and trust is scratched. In
[1] the issue of managing semantic interoperability among data sources
in a bottom-up, semi-automatic manner without relying on pre-existing,
global semantic models is addressed. A solution is proposed based on an
heuristic focusing on the cyclic exchange of local mappings. This work
strongly underlines the need for a theoretical progress on the uncertainty
reduction problem in a distribute environment. A contribution on this
line is given in [54] where authors discus how fault tolerance can be
managed in a system where trustworthiness on data is constructed by
computing a reputation chain. Also in [23] a solution is proposed based
on the use of decentralized probabilistic models to reduce uncertainty on
schema mappings.
5
Conclusions and Open Issues
In a distributed environment of information agents such as in the Semantic Web or in peer-to-peer systems, where information is spread over
heterogeneous sources and no global ontology nor centralized control are
available, the aim is to enable agents to interoperate irrespective of the
source of their initial/local semantics. Semantic interoperability is a crucial element for making distributed information systems usable.
In this chapter we have presented the emergent semantics paradigm, its
features and the principal involved research areas. Emergent semantics
consolidates the local semantics of autonomous information agents into a
global, population-wide semantics that results from the continuous interaction of the agents among themselves, for this, emergent semantics has
been proposed as a solution for the semantic interoperability problem.
Of course, as discussed in previous sections, a lot af open issues need
to be addressed by future research. Major challenges to be addressed in
future researches are:
– A robust theoretical foundation to advanced methodologies for evolvable semantics representation.
– Developing proof-of-concept knowledge management systems for
information-bound organisations and communities, capable of extracting actionable meaning from social interaction patterns.
At first glance, the second challenge might seem narrower than the
first one, but it has become clear that emergent semantics applications
are legion, so that extraction and representation must be somewhat
tunable to domain-specific requirements. In particular focusing on
the theoretical foundation researches have to explores the notion of
Incomplete Knowledge. In a large-scale distributed environment of
autonomous agents, information and information needs can no longer
be expressed concisely, as expected by database and semantic web
technologies, but have to deal with numerous sources of knowledge
that can be inconsistent or can express uncertain information. Also
inconsistency can insist on global level but not on local level, and
this two dimensions must be managed by a composition not effacing
disalignment of local sources.
In distributed environments, several qualitatively different sources
of uncertainty have to be dealt with as well. Besides uncertainty
about users information needs, a comprehensive approach must deal
with diverse uncertainty types such as uncertainty on knowledge
conceptualizations, uncertainty on metadata assertions and uncertainty
on trustworthiness of the information providers. Current reasoning
techniques for handling uncertainty have been developed for isolated
problems. It is a well-known fact that complete, probabilistic reasoning
is as computationally intractable as reasoning in full first order logic
is. A number of other formalisms are available for reasoning under
uncertainty, such as Bayesian networks or possibilistic and fuzzy
logic. Trust is another important notion related with uncertainty and
formalized according to different approaches. Also there are various
application domains where no definite semantics can be attached to
facts captured in the real world. This is the case for scientific databases,
e.g. biological data, where data is captured through experiments and
subsequently analyzed to guess what is the phenomenon that may
actually be materialized into that data. Another example is forensic
science, where investigators may try to understand what is the situation
behind the factual evidences that have been collected here and there.
Potential semantics are associated by formulating hypotheses, which are
then tested to see if they can be supported by the collection of available
data. If yes the running hypothesis can be confirmed. If not another
hypothesis will be formulated and tested, till a plausible semantics is
found. Semantics emerges gradually, thanks to a reformulation and
test cycle rather than agreement between different agents. In general it
is important to stress that ES approach needs a common abstraction
framework for reasoning under uncertainty, handling complex conditional relationships between various sources of uncertainty and their
models.
Another way to face the problem is to focus on the notion of data
quality. Data stored electronically is usually affected by a number of
quality problems, ranging from poor accuracy at a syntactic level (data
is wrong, for example as a consequence of errors during data collection or
manipulation), to various forms or inconsistency, both in a single source
or across multiple sources, to problems related to their currency, and so
on. This is true also in classical data integration settings, but the risk
of poor data quality becomes even more critical in emergent semantics
settings. Due to the open nature of these networks, no restriction at all
can be enforced on the quality of the data shared by the peers. Thus, it
is essential that data quality control be enforced both through the trust
and privacy mechanisms, and with proper measures at query processing
time that explicitly take quality into account.
6
Acknowledgments
This work was partly funded by the Italian Ministry of Research under
FIRB contract RBNE05FKZ2_004 (project TEKNE).
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