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Direct Connections
in this model. The right side
of the model in Figure 2 shows that it is also
possible to mediate between event sources and
sinks of different types by using filter components
implementing operations well known from the
theory of relational algebra. For example, cell-
sinks are connectable to tuple-sources by means
of a
projection
operation (Projected Connection),
and tuple-sinks can connect to relation-sinks
through a selection operation (Selected Connec-
tion). Obviously, these operations can be nested
to build more complex mediated connections that
could be added to Figure 2. However, for reasons
of simplicity, we do not consider such complex
mediated connections in this chapter.
domains (such as health care), portal users might
frequently customise the information components
and entry forms in their portal pages, for example,
with respect to different patient treatment situa-
tions. In such cases, the cognitive load implied in
the described WPC composition paradigm might
become unacceptable.
Another problem is that the current WPC
interface composition model is a
directed
one
;
that is, the user (who is typically not a software
programmer) has to think in terms of event
sinks
and
sources
instead of just being able to decide
that certain data entries in different components
“should hold the same information.” Besides be-
ing unnatural from a nonprogrammer's point of
view, this directed approach to WPC composi-
tion also restricts the way users can interact with
portal pages. For example, take a physician who
composes a WPC-based page for examining and
diagnosing a patient with particular symptoms.
If the physician composes a patient encounter
report based on three different examination forms
(provided by a WPC component each), she should
have to enter common data, such as the patient ID
and symptoms, just once. Event connections can
be used to propagate such contextual data among
WPC components. However, the directed nature
of the composition model described requires the
physician to follow an implicit order when entering
this information; that is, form components that act
as data sources have to be filled out first. Having to
think of this implicit order puts additional cogni-
tive load on Web portal developers and users. We
will now present an approach to overcoming the
limitations addressed in this discussion.
user-driven wpC Composition:
Criticism of the Current paradigm
WPC platforms enable users to create connections
between WPCs at runtime in an ad-hoc fashion us-
ing a Web browser. For this purpose, the browser
would present to its user all possible (direct or
mediated) connections for a given WPC. This
set of possible connections can quickly become
confusing for complex applications, for example,
when composing a portal to enter electronic medi-
cal records. This is because event interfaces are
not semantically typed with concepts from the
application domain. Currently, semantics can be
associated with event interfaces only informally,
namely by giving human-understandable names
to event sinks and sources (
labels
)
.
This approach
puts the cognitive load on the end user to interpret
and match these labels and to select the desired
connections.
We criticise this composition paradigm be-
cause it is error-prone and might require users
to spend significant effort in configuring portal
pages. Of course, the actual severity with which
these drawbacks are perceived depends on the
evolution frequency of portal pages. Users will be
much more likely to accept the current composi-
tion paradigm if changes to the portal pages they
compose are relatively rare. However, in more agile
semantiC Context
Composition
overview
Figure 3 gives an overview of the approach pro-
posed in order to lighten the cognitive load during
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