Information Technology Reference
In-Depth Information
THE LIMITS OF IMS-LD FOR
SEQUENCING IN LLL
the paradigm. SG follow an imperative paradigm:
if some condition applies, then follow this edge;
after that, if you have followed an exit edge,
then look for more conditions, etc; after that, if
you arrive to a container node, then go down one
level, etc. On the other hand, the conditions in
IMS-LD are more related to a functional paradigm
(Mauny & Cousineau, 2003). All conditions are
evaluated at the same time; if a <imsld:condition>
fires some action that produces some change in a
<imsld:property> of the system, all conditions are
evaluated again. There is no logical or chronologi-
cal order or precedence between all instances of
<imsld:condition>.
Conditions in IMS-LD must capture all the
semantics of those in the SG, plus that of the
spatial distribution of activities. To achieve this,
some special properties had to be defined. These
properties are local (refer to the current UoL) and
personal (refer to the current student), and carry
information regarding the spatial position in the
graph, the traversal of edges, etc. An additional
property holding the value 0 had to be introduced,
because of limitation of IMS-LD when defining
arithmetical operations in which both operands
are constants. The interested reader is referred
to (Gutierrez-Santos et al., 2008) for additional
details and relevant XML excerpts.
As it has been explained above, the process
of exporting SG to IMS-LD involves flattening
a hierarchical structure. Thus, two SG variables
with the same name (e.g. time, qualification, etc)
at different levels of the hierarchy, or even on two
siblings of the same level, collide once they are
put at the same unique level on the imsmanifest.
xml. In order to avoid this problem, a prefix is
added to the name of each IMS-LD property.
The prefix is the name of the node in which the
original value was declared (i.e. the node of its
<sg:init> section) as well as the name of every
ascendant node up to the root. The root is not
included because it is common for all variables
in a graph; thus, the variables declared at the root
node wear no prefix when exported.
The design of the exporting process described
above, its implementation, and the subsequent
tests have drawn some interesting conclusions
about the limits of the IMS-LD specification. First,
the functional paradigm of the condition model
in IMS-LD does not scale well. If the number of
conditions grows, it quickly becomes very diffi-
cult to keep track of all conditions; therefore, the
probability of side effects increases dramatically.
Every condition must be checked for every pos-
sible case, leading to a debugging process that is
both tedious and error-prone. Although this may
be adequate for small control tasks, the creation
of a set of conditions and actions large enough to
control a big number of activities becomes infea-
sible unless automatic tools are used to help. This
clearly shows the value of higher level tools that
produce an IMS-LD output, reducing this com-
plexity. In (Gutierrez Santos, 2007), it is shown
how the introduction of a simple loop in a small
graph results in a sequence of conditions that is
several pages long. Complex sequencings like
those involved in any lifelong learning process
are totally impossible to create and/or maintain
using IMS-LD conditions directly.
Second, IMS-LD does not provide a good
support for cycles when accessing the material.
This is a substantial limitation, because learning
is improved if a system grants the possibility of
going over some already covered material for revi-
sion and reflection. This is especially true when
long periods of time are considered, like it is the
case in lifelong learning scenarios. IMS-LD, on
the contrary, is oriented towards a sequence of
activities that is strongly linear. This comes from
the metaphor of the theatre play, and produces
consequences as critical as the fact that an activ-
ity cannot be “uncompleted” once it has been set
as “completed” by the student. This chapter has
shown that it is possible to express loops in IMS-
LD, but this comes at a cost: resulting imsmanifest.
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