The Language XChange

The research topic investigated by this thesis is reactivity on the
Web. Reactivity on the Web is an emerging research issue covering:
updating data on the Web, exchanging information about events (such
as executed updates) between Web sites, and reacting to
combinations of such events. Following a declarative approach to
reactivity on the Web, a novel reactive language called XChange is
proposed. Novelties of the language are represented by the proposed
data metaphor intended to ease the language understanding and the
supported reactive features tailored to the characteristics of the
Web. Realising this pressuposed refining, extending, and adapting
to a new medium some of the concepts on which active database
systems are built upon. Reactivity is specified in XChange by means
of reactive rules (or event-condition-action rules) having the
following components: the event part is a query against events that
occurred on the Web, the condition part is a query against Web
resources (expressed in the Web query language Xcerpt), and the
action part is a transaction specification (specifying updates to
be executed and events to be raised in an all-or-nothing manner).
Novel in XChange is its ability to detect composite events on the
Web, i.e. possibly time related combinations of events that have
occurred at (same or different) Web sites. XChange introduces a
novel view over the Web data by stressing a clear separation
between persistent data (data of Web resources, such as XML or HTML
documents) and volatile data (event data communicated on the Web
between XChange programs). Based on the differences between these
kinds of data, the data metaphor is that of written text vs.
speech. XChange's language design enforces this clear separation
and entails new characteristics of event processing on the Web.
After motivating the need for a solution to reactivity on the Web,
this thesis introduces the design principles and syntax of the
language XChange accompanied by use cases for demonstrating the
practical applicability of its constructs. Important contributions
of the thesis are the specification of the language semantics and
the description of an algortihm for evaluating XChange programs.