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

5.3.1 Calculating Weighted Joint Distribution
The first step is to calculate weighted joint distribution. The statistic information, i.e.
activity duration distribution and activity weight, can be obtained from system logs
by statistical analysis [2, 57]. Afterwards, given the input process model for the
scientific workflow, the weighted joint distribution of activity durations for the
entire scientific workflow and workflow segments can be efficiently obtained by the
composition of the four basic building blocks as illustrated in Section 5.2.1.
5.3.2 Setting Coarse-grained Temporal Constraints
The second step is to set coarse-grained upper bound temporal constraints at build
time. Based on the four basic building blocks, the weighted joint distribution of an
entire workflow or workflow segment can be obtained efficiently to facilitate the
negotiation process for setting coarse-grained temporal constraints. Here, we denote
the obtained weighted joint distribution of the target scientific workflow (or
n
2
workflow segment) SW as N(
µ sw,
σ sw)
where µ sw = ∑ w i µ i and
i=
1
n
σ sw = ∑ w 2 i σi
2 . Meanwhile, we assume that the minimum threshold for the
i=
1
probability consistency is β % which implies user’s acceptable bottom-line
probability, namely the confidence for timely completion of the workflow instance;
and the maximum threshold for the upper bound constraint is max(SW )
which
denotes user’s acceptable latest completion time. The actual negotiation process can
be conducted in two alternative ways, i.e. time oriented way and probability oriented
way.
The time oriented negotiation process starts with the user’s initial suggestion of
an upper bound temporal constraint of U (SW ) and the evaluation of the
corresponding temporal consistency state by the service provider. If
U ( SW ) = µ sw + σ sw with λ as the α % percentile, and α % is below the threshold
of β % , then the upper bound temporal constraint needs to be adjusted, otherwise
the negotiation process terminates. The subsequent process is the iteration that the
user proposes a new upper bound temporal constraint which is less constrained as
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