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172 M. Adams and N. Russell
Chapter Notes

Exception Patterns

The application of a pattern-based approach to defining exception handling strate-
gies and their use for assessing the exception handling capabilities of PAIS was first
described in [222]. This paper also proposed the notion of a graphical exception han-
dling language for defining exception handling strategies, a notion that ultimately
led to the development of YAWLeX, the language on which the exlet strategies
in YAWL were realized. A comprehensive assessment of the exception handling
capabilities of a number of leading workflow and case handling systems, business
process modeling notations, and business process execution languages is contained
in [220].

Worklets/Exlets

The use of the worklets paradigm for exception handling was first discussed in [26],
and a full description of the conceptual framework of worklets, exlets, and dynamic
exception handling was further detailed in [25]. A full exploration of the worklet
approach, including a complete formalization and exemplary studies, can be found
in [23].

Other Approaches

The need for reliable, resilient, and consistent workflow operation has long been rec-
ognized [101]. Early work in the area [90,268] was essentially a logical continuation
of database transaction theory and focussed on developing extensions to the classic
ACID transaction model that would be applicable in application areas requiring the
use of long duration and more flexible transactions. As the field of workflow tech-
nology matured, the applicability of exceptions to this problem was also recognized
[226, 244]. In [87], Eder and Liebhart presented the first significant discussion on
workflow recovery that incorporated exceptions, and gave the now classic catego-
rization of workflow exceptions into four groups: basic failures, application failures,
expected exceptions, and unexpected exceptions. Subsequent research efforts into
workflow exceptions have mainly concentrated on the last two of these classes, and
on this basis, the field has essentially bifurcated into two research areas. Inves-
tigations into expected exceptions have focussed previous work on transactional
workflow into mechanisms for introducing exception handling frameworks into
workflow systems. Research into unexpected exceptions has established the areas
of adaptive workflow and workflow evolution [212].
Although it is not possible to comprehensively survey these research areas in the
confines of this chapter, it is worthwhile identifying some of the major contributions
in these areas that have influenced subsequent research efforts and have a bearing on