Features and Requirements for an XML View Definition Language:
Lessons from XML Information Mediation
XML indicates a move towards viewing the Web as a big semistructured database,
consisting of multiple autonomous sites which will be modeled around (guess
what?) XML and sibling standards for
In such an environment a query language will serve for more than "a souped-up
version of X-Pointer" [Maier].
Our position focuses on the use of an XML query language as a view definition
language that drives XML mediator systems. A mediator selects, restructures
and merges information from multiple autonomous sources/sites. It exports
an integrated XML view document. Possible applications of such mediator
systems are numerous (e.g., virtual shopping malls, virtual agencies, ...).
structure and ontology definitions (XML-Data, RDF, DCD, namespaces),
APIs (most probably some extension of DOM),
source query capability specifications (see below),
other standards that may describe the transactional abilities of the sites,
and so on.
Section 2 gives a brief presentation of the under development MIX
(Mediation of Information using XML) mediator
system in order to illustrate the architecture and the functionality of
a typical mediator system. Section 3 presents a list of issues and challenges
that we have found in the course of working on the MIX project and the
XMAS (XML Matching And Structuring)
view definition language.
2. The Architecture and Functionality of the MIX Mediator System
The figure below illustrates the architecture of the MIX mediator system
. The system
accesses a set of XML sources, which are currently information systems
wrapped to provide (1) an XML view of their data and (2) a set of XMAS
queries that they can answer. (It is likely that in the future there will
be XML databases that require no wrapping.) Conceptually, MIX exports an
integrated view of the source data. The view definition, provided by the
view developer, specifies how the source data will be integrated. Notice
that the MIX does not materialize the integrated view in advance.
During runtime, an application (which may be the BBQ GUI )
issues a XMAS query against the integrated view. The mediator decomposes
the client query into queries that are routed to the XML sources. The sources
generate and send to the mediator the query results, which are XML documents.
The mediator integrates them into the client query result document.
Note that the client accesses the query result using our homegrown version
of DOM, called DOM-VXD (DOM for Virtual XML
Documents) . DOM-VXD does not require a complete copy of the
query result to be materialized at the client site. In this way we can
pipeline the computation and delivery of the query result document and
even navigate into it.
3. May a Santa "QL Committee" Claus bring the following for XMAS!
There are numerous desiderata for a general purpose XML QL; see [Maier]
for an excellent overview. Here, we focus on first lessons learned from
designing and implementing the XMAS view definition language for information
A recurring theme of the following discussion is that expressive power
comes at a price which may be:
XMAS currently leans towards a conservative database approach thereby delivering
the benefits described in thesections below. The cost is some limitations
in the expressive power.
inefficient query processing,
complex query processors,
inability to perform type inference, etc.
Undoubtedly, more powerful features (e.g., see Section 3.6) will
also have to be encompassed in XMAS (and, in general, any XML view definition
languages). The language should allow for "graceful" scaling of the
expressive power -- a task that will not always be easy.
3.1 XML Faithfulness
An XML view definition should be faithful to XML, i.e., it should
always produce XML output. The immediate benefit for mediation is that
the view is no different from any other XML document.
An ideal solution would be to have a language that can never produce
non-XML output. However, this approach may compromise the expressive power
of the language. A more viable solution is to develop static analysis tools
that can notify the view designer of potential errors, i.e., cases where
non-XML output may be produced. Note that by controling the complexity
of the language we will be able to develop effective static analysis tools.
XMAS takes a middle-of-the-road approach: Its ability to move data
from a source's content to element names or attribute names creates the
possibility of non-XML output. This instance of potential unfaithfulness
can be caught using a trivial static safety check.
On the other hand, XMAS avoids many sources of potential unfaithfulness.
For example XMAS has avoided (in so far!) the use of Skolem functions,
which can be a major source of unfaithfulness. For example, they make it
difficult to check whether attributes of elements are unique. Nevertheless,
Skolem functions can deliver expressive power that cannot always be substituted
by the SQL-like group-by mechanism of XMAS.
3.2 Type Inference
Given descriptions of the source documents (such as DTDs, XML-Data specifications,
or DCDs), the mediator should be able to compute a correct and precise
description of the view. We have developed algorithms [1,2]
that compute precise view DTDs from the source DTDs and the view definition.
Again, the expressive power versus complexity tradeoff is encountered:
DTD inference is not possible for views that move data from a source's
content to element names or attribute names; such views are allowed in
XMAS. On the other hand, the explicit group-by operator allows us
to derive precise types.
A closely related property is type conformance of a view definition
to a predefined DTD. Similar challenges and tradeoffs with type inference
appear in this case as well.
3.3 Closure Under Composition
A typical task for query processors (and not only XML ones) is the following:
Given (1) a client query q that refers to a view V and (2) the query which
defines V in terms of some source database(s) S, derive a query q' that
is equivalent to q and refers directly to S. It is important for the simplicity
and efficiency of query processing that q' is expressed in the same language
as q and V. That is, the language should be closed under query composition.
It is important to note that closure under composition requires careful
design of the query language. For example, one can show that a query language
that allows Skolem object-ids and regular path expressions but does not
allow any other form of recursion, is not closed under composition.
In XMAS, closure under composition is achieved by employing a group-by
construct instead of Skolem functions. Once again, a compromise in expressiveness
leads to a desired property.
3.4 Order: Manipulating, Preserving, and Never-Minding it
XML has order. Elements are organized in lists, as opposed to sets. However
applications, in general, have different requirements on the order issue.
An XML query language should address the following issues under a single
The above policies deal with specifying an order for the view. Similar
considerations apply to checking the order of the input elements. In this
case XMAS' default is "don't care" about the order of elements in the input.
Conversely, if the view/query has to put a condition on the order of input
elements an horizontal navigation regular expression can be used
Order Preservation: By default the element order of the input is
Order Manipulation: The view designer is able to define a new
order of elements. A typical task is the ordering of elements using a ranking
function (e.g., in the style of SQL's order-by). Once again notice that
extremely powerful order manipulation mechanisms will compromise type inference
and closure under composition.
Order Nondeterminism: Finally the view designer may want to specify
that order in the view is unimportant. In this case he can specify that
the order is nondeterministic thereby leaving room for optimization.
3.5 Query Processing: Put an Algebra in Your System
In order to apply similar optimization techniques as those developed for
relational databases, an XML query language should have an equivalent algebra.
An algebra will also facilitate the implementation of pipelining/browsing
as described in Section 2.
3.6 All Processing in One Language: The Challenge of Structural Recursion
It is desirable to have a language that tackles all tasks required in the
mediation process. In this way we can avoid the impedance mismatch caused
by using multiple languages and we create extra opportunities for optimization.
It seems that two language paradigms have emerged in the XML querying
context and they will have to be reconciled:
shows a promising direction in bridging the two paradigms.
The database paradigm has an underlying logic semantics and is convenient
for selecting and integrating objects. It has recently been extended to
allow navigational recursion in a clean way. However it does not support
(without the use of hard-to-process general recursion) restructuring of
The functional paradigm, exemplified by XSL and now XQL, is particularly
suitable for "deep" transformations of XML documents.
3.7 Describing the Query Capabilities of Sources
XML sources will only be able to support a subset of the queries over the
"schema" they export. For example, consider a site that exports a flight
information schema (say, DTD) but it only accepts queries that specify
a destination, an arrival, and a optional price range. It is clear that
a mediator system or, in general, any agent that queries this source must
conform to the set of supported queries exported by this source.
So it is important that mechanisms are developed for describing the
set of queries that are supported by a site .
In communities such as, decision support, OLAP, and scientific databases,
the term "metadata" refers to auxiliary information associated with defined
data sets. For example, this may include attributes that provide further
information about the data, annotations/explications associated with the
data, and/or computed/derived information related to the data. In general,
whether a piece of information is data or metadata depends on one's
point of view. In defining a virtual site, the view specification mechanism
must provide the capability to specify the data as well as the metadata
for that site. Once this distinction between data and metadata has been
made at the site level, this information can be used in a variety of ways
in the system. For example, it can be used at the interface level to support
different query and presentation modes for data vs. metadata, and it can
be used at the data transport level to encode data differently than metadata.
Information mediation poses a challenging set of requirements to an XML
query and view definition language. We discuss above several important
issues: XML faithfulness, type inference, order, closure under composition,
etc. A recurring theme is the search for the right balance between expressive
power, on the one side, and efficiency and viability on the other.