Xiao Liu PhD Thesis.pdf - Faculty of Information and Communication ...

2.5 Temporal Violation Handling
After temporal verification, the current temporal consistency state is verified, and if
a temporal violation is identified, the follow-up step is to handle such a violation, i.e.
temporal violation handling. So far, the study on temporal violation handling is still
in its infancy.
Temporal violation handling is very different from conventional violation
handling which mainly deals with functional failures in workflow systems [42, 83].
Usually, after functional failures are detected, violation handling process such as
rollback and recovery is triggered [9]. Another way to prevent functional failures is
to run duplicate instances of the same workflows, and hence if one instance fails, the
others can still be running. However, duplicate instances will result in huge
management cost which is several times higher than the needed execution cost in
general [28]. As for temporal violations, as a type of non-functional QoS failures,
they cannot be handled after they actually took place since no actions in the real
world can reduce the execution time of workflow activities after they have already
been finished. In fact, in a cloud computing environment, if a workflow application
has exceeded its deadline (i.e. the global temporal constraint), the service provider
can do nothing but most likely to pay the penalty according to service contracts or
negotiate with the service user for endurance, i.e. extra waiting time. Therefore,
rather than passive recovery, temporal violations need to be proactively prevented
by temporal verification and temporal violation handling before actual large
overruns take place [56]. In other words, effective handling of local temporal
violations along workflow execution instead of recovery of global temporal
violations is what needs to be investigated for the delivery of high temporal QoS in
scientific cloud workflow systems.
The work in [82] introduces five types of workflow exceptions where temporal
violation can be classified into deadline expiry. Meanwhile, three alternate courses
of recovery action, i.e. no action (NIL), rollback (RBK) and compensate (COM), are
also presented. NIL which counts on the automatic recovery of the system itself is
normally not acceptable for the ‘risk-free’ purpose. As for RBK, unlike handling
conventional system function failures, it normally causes extra delays and makes
current inconsistency state even worse. However, COM, i.e. time deficit
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