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2 The Language: Rationale and Fundamentals 79

Fig. 2.38 Work distribution specification (other constraints)

routing decisions. A capability-based distribution filter can be specified for each
task, which allows user capabilities to be used in making work distribution deci-
sions. Figure 2.38 continues the specification of the work distribution approach
for the Book Accommodation task specifying a capability distribution filter,
which requires that the task only be distributed to users with an IATA Certifi-
cate (a standard travel industry qualification). In combination with the previous
configurations, this would result in work items corresponding to the task being
offered to resources in the Travel Consultant role with an IATA Certificate. Thus,
it can be seen that relatively fine-grained work distribution strategies can be spec-
ified for YAWL tasks. This figure also shows some other configuration items that
will be discussed shortly.
Historical distribution: The use of pluggable allocators allows historical data –
essentially the content of the execution log – to be used in making work dis-
tribution decisions. There are a range of standard predefined allocators that can
be utilized to implement history-based task distribution strategies such as dis-
tribute it to the user who completed it least recently, most recently, has the highest
success rate, and completes it the quickest.

Additional Routing Constraints

There are two additional constraints supported by YAWL that can be used to fur-
ther refine the manner in which work items are routed to users. They are used in
conjunction with the routing and interaction strategies described above.