\subsection{Introduction}
\par Developing intelligent tools for the integration of information
extracted from multiple heterogeneous sources is a challenging
issue to effectively exploit the numerous sources available
on-line, in global Internet-based information systems. The problem
to face is the identification of semantically related information,
that is, information describing the same real-world concepts in
different sources having semantic heterogeneity. In fact,
information sources to be integrated are usually preexisting and
have been developed independently. Consequently, semantic
heterogeneity can arise for the aspects related to terminology,
structure, and context of the information, and has to be properly
dealt with during integration in order to effectively and
correctly exploit the information available at the sources.

MOMIS\footnote{http://www.dbgroup.unimo.it/Momis/} stands for
``Mediator envirOnment for Multiple Information Sources''. Momis
is a mediator-based system for information extraction and
integration that works with structured and semistructured data
sources \cite{bergamaschi:vldb2000}. MOMIS was developed as a
joint collaboration among the University of Modena and Reggio
Emilia, the University of Milano and the University of Brescia,
within the INTERDATA national research project, under the
direction of Professor S. Bergamaschi. Now the research activity
continues within the \emph{D2I: Integration, Warehousing, and
Mining of Heterogeneous Data Sources} national research project.


\subsection{The MOMIS project}
\par The goal of information extraction and integration techniques
developed in MOMIS is to construct synthesized, integrated
descriptions (i.e., a global virtual view) of the information
coming from multiple heterogeneous sources, to provide the user
with a uniform query interface against the sources independent
from their location and the level of heterogeneity of their data.
Moreover, to meet the requirements of global, Internet-based
information systems with a possibly high number of sources to be
integrated, it is important to develop tool-assisted techniques in
oreder to automate as much as possible information extraction and
integration activities. This goal has been achieved with the MOMIS
project by developing the SI-Designer \cite{bergamaschi:vldb2000}.
MOMIS follows a ``semantic approach'' to information integration,
based on the building of a conceptual schemas of the information
sources, and on a mediator component. MOMIS implements a \iii
\cite{ARPA} architecture for integration and query optimization,
the mediator component relies on several tools, namely ARTEMIS
\cite{KR1998:Calvanese:Lenzerini}, developed by University of
Milano and University of Brescia, and ODB-Tools
\cite{ICDE_97:odbTools}, developed by University of Modena e
Reggio Emilia andWordNet \cite{wordnet1} developed by the
Cognitive Science Laboratory at Princeton University. Most of the
information for integration is provided by the wrappers, using ODL

Most of the information for integration is provided by the
\emph{wrappers}, using \lang from the source description and
stored in a \lang data structure. By exploiting the WordNet
Lexical system \cite{wordnet2} and the OLCD Description Logic
inference capabilities a Common Thesaurus is built. The Common
Thesaurus is composed of relationships between schema elements
extracted from the schemata descriptions, derived from the lexical
relationships between the concepts in WordNet, and explicitly
stated by the integration designer. Based on the relationships in
the Common Thesaurus, affinity coefficients are computed. These
coefficients give a measure of the level of matching between the
elements of the different sources. MOMIS computes a set of
candidates \emph{Global Classes} by applying affinity-based
clustering on the affinity coefficients. For each \emph{Global
Class Mapping rules} are defined that describe which \emph{local
classes} belong to the global class and how local attributes are
mapped into the global attributes.

Part of the project is the development of a prototype which is the
software application based on the MOMIS theory. The prototype is
written in Java, is based on the CORBA architecture and supports
most of the MOMIS features like the Description Logics
capabilities, the ARTEMIS affinity computation and clustering and
the interaction with WordNet.


\subsection{Architecture}
The MOMIS system is designed for schema integration. \mbox{Figure
\ref{MPA}} shows the architecture of the system in terms of
functional modules. The system is composed by several object that
communicates using the CORBA\footnote{Object Management Group.
Object management group. http://www.omg.org/} standard.
\begin{figure}[ht]
  \begin{center}
    \epsfig{figure=momisarchitecture.eps,width=12cm,angle=0}
    \caption{\textbf{MOMIS} prototype architecture Virtual Network}\label{MPA}
  \end{center}
\end{figure}
The object are:
\begin{description}
    \item[Wrappers] Each data source (relational, object, XML, ...) must be
presented to \mbox{MOMIS} in a standard way, and this is what the
wrappers do. They are placed on the top of the information sources
and are responsible for translating the schema of the source into
\lang language. A wrapper performs also the translation of a query
expressed in \lang language into a local request executable by the
query processor of the corresponding source.
    \item[SI-Designer] SI-Designer is the GUI (Graphic
User Interface) for the  Global Schema Builder. Its goal is to
lead designer from the schemata acquisition of sources to the
tuning of the mapping table through the various steps of the
integration. This is the client object that uses other server
objects.
    \item[WordNet] This is the server whose object provides the
access to the WordNet, and that is able to extract lexicon derived
intensional relationships between attributes and classes,
exploiting the lexical relationships contained in the WordNet
lexical system.
    \item[ODB-Tools] This object provides access to the ODB-Tools
prototype. ODB-Tool is a framework for object-oriented database
(OODB) schema validation, preserving taxonomy coherence and
performing taxonomic inference, and semantic query optimization.
    \item[Global Schema] A Global Schema object contains all
information for querying the resulting global schema by a queries
manager object. Such object is produced by the SI-Designer GUI and
contains the sources schemata, the thesaurus, and the mapping
table.
    \item[Query Manager] A Global Schema object uses information
produced during the integration steps to query a global schema. It
is able to divide a query on the global schema in a set of query
for the local sources and to merge results (for further
information see the Silvia Zanni’s thesis \cite{Zanni2000} in
Italian).
\end{description}


Sources integration is based on the individuation of an ontology
shared by each source. The designer specifies the sources to
integrate and performs integration using SI-Designer graphical
interface. SI-Designer performs the integration of the schemas in
a semiautomatic way, integrating the schemas of heterogeneous data
sources, such as relational, object, XML, or semistructured
sources. Starting from local source descriptions, the Global
Schema Builder generates an integrated view of all data sources
and expresses those views using an XML file. Momis lets you use
the infrastructure with other open integration information systems
by simply interchanging XML data files. Momis creates XML global
schema using different stages, first by creating a common
thesaurus of intra and interschema relationships.


\\ Momis extracts the intraschema relationships by using
inference techniques, and then shares these relationships in the
Common Thesaurus. After this initial phase, Momis enriches the
common thesaurus with interschema relationships obtained using the
lexical WordNet system (www.cogsci.princeton.edu/wn), which
identifies the affinities between interschema concepts on the
basis of their lexicon meaning. Momis also enriches the common
thesaurus using the ARTEMIS system \cite{IEEE2002}, which
evaluates structural affinities among interschema concepts. All
information about integration are saved in the Global Schema which
is the main CORBA object to access the MOMIS integration services.
