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5 Exception Handling 151
chosen for handling them needs to be as generic as possible to ensure that it has
broadest applicability.

5.2.2 Exception Handling at Work Item Level

The second factor that is associated with any exception handling strategy is the way
in which the work item that detected the exception will be handled. There are a
multitude of ways in which an exception may be dealt with; however, the actual
approach pursued will depend on the current execution state of the detecting work
item and its state after the exception has been dealt with.
To quantify the range of possible options, we first need to consider a general-
ized set of states that are associated with the execution life-cycle for a work item.
Figure 5.1 illustrates as solid arrows the states through which a work item progresses
during normal execution from a resourcing perspective (cf. Chap. 2). It is initially
offered to one or more resources for execution. A resource issues an allocate com-
mand to indicate that it wishes to execute the work item at some future time; the
work item is then allocated to that resource. Typically this involves adding the work
item to the resource’s work queue and removing any references to the work item that
other resources may have received, either on their work queues or via other means.
When the resource wishes to commence the work item, it issues a start command
and the state of the work item changes to started. Finally, once the work item is
finished, the resource issues a complete command and the state of the work item is
changed to completed. Note that there are two possible variations to this course of
events, shown as dotted arcs in Fig. 5.1, (1) where a work item offered to a resource
is selected by another resource, it is withdrawn from the first resource’s worklist and

failed
force−fail−a
reoffer−s
force−fail−o
continue−offer continue−allocation force−fail fail

allocate start
offered allocated started continue−execution
reoffer−a reallocate−s

reoffer reallocate force−complete restart complete
withdraw
force−complete−a
withdrawn force−complete−o completed

Fig. 5.1 Options for handling work items