Interoperability for Searching Learning Object Repositories:

The "ProLearn Query Language" is a query language developed for
repositories of learning objects. The most relevant standards for
describing learning objects are LOM, Dublin Core, and MPEG-7. The
IEEE Learning Object Metadata (LOM) is a hierarchical metadata
standard usually encoded in XML, published by the IEEE in 2002. Its
purpose is to enable the description of learning objects through
attributes that include the type of object, author, owner, terms of
distribution, and format, as well as pedagogical attributes, such as
typical learning time or interaction style. LOM is based on early
work in ARIADNE and IMS. Dublin Core (DC) is a standard for generic
resource descriptions. The simple DC metadata element set consists
of 15 elements, including title, creator, subject, description,
publisher, contributor, date, type, format, identifier, source,
language, relation, coverage, and rights. MPEG-7 is an ISO/IEC
standard for describing multimedia content. MPEG-7 Multimedia
Description Schemes (DSs) are metadata structures in XML that
facilitate searching, indexing, filtering, and access. PLQL is
primarily a query interchange format, used by source applications
(or PLQL clients) for querying repositories (or PLQL servers). In
this article, we give a precise description of the semantics of PLQL,
concerning both kinds of clauses and their mutual relationship and
describe two experimentation efforts around PLQL: one involving the
ARIADNE repository and the other the EUN Learning Resource Exchange
initiative. We present PLQL as an emerging standard for querying
worldwide, heterogeneous learning object repositories. This work has
followed on the heels of previous achievements in defining SQI
(the Simple Query Interface), a unified access point to distributed
and heterogeneous repositories. PLQL is a "query interchange format"
accommodating the great diversity of the different repositories and
their capabilities. PLQL combines exact and approximate search, and
supports queries on XML-based hierarchical metadata. PLQL was designed
in a way that allows for easy mapping to various paradigms for
metadata management. So far, we have mapped PLQL to Lucene, XML Query
(XQuery), and SQL. Apache Lucene is an important deployment context
for PLQL as this open source toolkit for text indexing and searching
is widespread and easy to use. XQuery is the W3C recommendation for
querying XML data; with this mapping, all PLQL levels can be mapped
and executed on XML database systems.