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76 N. Russell and A. ter Hofstede
As an aid to understanding the distinctions between the various interactions
described in Table 2.7, it is possible to illustrate them quite effectively using UML
Sequence Diagrams as depicted in Fig. 2.35. These show the range of interactions
between the system and resources that can occur when distributing work items.
An arrow from one object to another indicates that the ﬁrst party sends a request
to the second, for example, in the RRR interaction strategy, the ﬁrst request is a
manual offer from the system to the workﬂow administrator.
Figure 2.36 illustrates how the interaction strategy is conﬁgured for a task in
YAWL. In this case, the Book Accommodation task in the Process Travel Request
process is set to an SRR strategy. This means any associated work items will be
automatically offered to prospective users by the system when the task is enabled;
however, users may choose to commit to undertaking the task (i.e., allocation) and
starting it at a time of their own discretion.

2.6.4 Routing Strategies

The second component of the work distribution process concerns the routing strat-
egy employed for a given task. This speciﬁes the potential user or a group of users
from which the actual user will be selected, who will ultimately execute a work
item associated with the task. There are a variety of means by which the task rout-
ing may be speciﬁed as well a series of additional constraints that may be brought
into use at runtime. These are summarized below. Combinations of these strategies
and constraints are also permissible.

Task Routing Strategies

There are a variety of routing strategies that can be deﬁned at the level of an
individual task as described below.

Direct user distribution: This approach involves routing to a speciﬁed user or
group of users.
Role-based distribution: This approach involves routing to one or more roles.
A role is a “handle” for a group of users who allows the group population to
be changed without the necessity to change all the task routing directives. The
population of the role is determined at runtime at the time of the routing activ-
ity. Figure 2.37 continues the conﬁguration of the Book Accommodation task
and illustrates the use of a role-based distribution strategy for it. In this case,
combined with the interaction approach speciﬁed in Fig. 2.36, the task would
be offered to members of the Travel Consultant role. The same conﬁguration
screen also allows for the speciﬁcation of direct user and deferred distribution
approaches, though they are not utilized in this example.
Deferred distribution: This approach allows a net variable to be speciﬁed that is
accessed at runtime to determine the user or role that the work item associated
with the task should be routed to.

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