Lessons Learned - User-Level Workflow Design: A Bioinformatics Perspective - page 156

to identify the required domain- and problem-specific constraints incremen-
tally. For this experimentation with constraints and obtained solutions, it has
turned out to be advantageous to proceed in two principal phases:
1. define constraints that explicitly exclude inadequate solutions,
2. define constraints that explicitly include adequate solutions.
Often, the exclusion of undesired solutions already constrains the solution
space clearly enough so that a manageable set of solutions remains. If this is
not the case, providing more structure by explicitly describing parts of the
intended solutions very effectively narrows the remaining solutions further.
This strategy for the incremental specification of constraints has also been
demonstrated for the EMBOSS workflow scenario (cf. Section 3.3.4): The
first three of the finally six constraints exclude clearly useless (refinement
step 1) and in the actual setting unwanted services (refinement step 2). The
next three constraints explicitly include particular solutions by enforcing the
use of specific services or service categories (refinement 3). In this particular
example, a constraint set that specifies an assessable set of adequate solutions
is reached at this point.
Note that especially the definition of constraints that include particular
solutions comes with a certain risk of missing adequate solutions due to (un-
intended) over-constraining: It is easily possible to express requirements that
can in their concrete form not be met by the domain model, although an
acceptable solution to the actual synthesis problem principally exists. Hence,
it is indeed advisable to leave as much freedom as possible to the synthesis
algorithm, and to identify the adequate set of constraints incrementally.
Although the constraint identification process is only discussed explicitly
for the EMBOSS workflow scenario, and not for the other three application
scenarios (Chapters 4 - 6), it basically followed the same pattern there. Con-
sequently, the domain constraints for these examples comprise both excluding
and including constraints. The additional problem-specific constraints, which
are provided by the workflow designer for a particular loose branch, are mostly
constraints that include particular solutions, as at this point the user typi-
cally expresses particular intents about the solution that are too specific to
be included in the general domain model.