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6.2.2 Constraint Templates

Procedural workflow languages such as YAWL provide constructs such as AND-
split, AND-join, cancelation region, etc. These constructs aim at supporting fre-
quently needed workflow patterns. For our declarative approach, we also aim at
supporting frequently needed patterns to model relationships (constraints) between
tasks. However, we aim at different types of patterns, that is, declarative constructs
aiming at more flexibility. In principle, LTL offers everything needed to support
such patterns. However, because LTL formulas can be difficult to understand by
nonexperts, we provide a graphical representation of constraints that hides the asso-
ciated LTL formulas from users of declarative workflow, in the form of constraint
templates. Each template has (1) a name, (2) an LTL formula, and (3) a graphi-
cal representation. A constraint inherits the name, graphical representation, and the
LTL formula from its template. Figure 6.5 depicts some example constraint tem-
plates. The template parameters are depicted by boxes. Templates can be used to
create actual constraints for a specific process. In actual constraints, tasks replace
template parameters, both in the graphical representation and the associated LTL
formula.
The existence template is graphically represented with the annotation “1:: ”
above the task. This indicates that A is executed at least once in each instance. The
template init.A/ can be used to specify that task A must be the first executed task in
an instance. Templates response, precedence,and succession consider the ordering
of tasks. Template response requires that every time task A executes, task B has to be
executed afterwards. Note that this is a very relaxed interpretation of the notion of
response, because B does not have to execute straight after A, and another A can be

1..* init
existence(A) A init(A) A alternate precedence(A, B) A B

response(A, B) A B not co-existence(A, B) A B

precedence(A, B) A B A 1 of 4 D
1 of 4 (A,B,C,D)
succession(A, B) A B B C
Where
existence.A/ ♦A
init.A/ A
response.A; B/ .A ) ♦B/
precedence.A; B/ .:B/WA
succession.A; B/ response.A; B/ ^ precedence.A; B/
not co-existence.A; B/ :.♦A ^ ♦B/
1of 4.A; B; C; D/ ♦A _ ♦B _ ♦C _ ♦D
Fig. 6.5 Constraint templates