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We then need to combine these local rules together to produce a global evo-
lution one for the whole model. For simplicity, we assume that:
ASS-1: Independence of evolutions.
All observable rules are independent.
It means that they do not influent each other. In other words, the probability
that an evolution rule is applied does not affect to that of other rules.
ASS-2: Order of evolutions.
Controllable evolutions are only considered af-
ter observable evolutions.
As discussed, observable rules are analyzed on independent subparts. Prevail-
ing paradigms of software development (
e.g.,
Object-Oriented, Service-Oriented)
encourage encapsulation and loosely coupling. Evolutions applying to subparts,
therefore, are often independent. Nevertheless, if there are two evolution rules
which influent each other, we can combine them into a single one. We assume
that dependent evolutions do happen, but not a common case. Hence manual
combination of these rules is still doable.
The second assumption is the way we deal with controllable rules. If we apply
controllable rules before observable ones, it means we look at design alternatives
before observable evolutions happen. This makes the problem more complex
since under the effect of evolution, some design alternatives are no longer valid,
and some others new are introduced. Here, for simplicity, we look at design
alternatives for evolved requirement models that will be stable at the end of
their evolution process.
After all local evolutions at subparts are identified, we then combine these
rules into a global evolution rule that applies to the whole model. The rationale of
this combination is the effort to reuse the notion of Max Belief and Residual Risk
(
4) without any extra treatment. In the following we discuss how to combine
two independent observable evolution rules.
Given two observable rules:
§
RM
1
p
1
i
and
r
o
2
=
RM
2
p
2
j
n
m
r
o
1
=
−−→
RM
1
i
−−→
RM
2
j
i
=1
j
=1
Let
r
o
is combined rule from
r
o
1
and
r
o
2
,wehave:
RM
1
RM
2
p
1
i
∗p
2
j
r
o
=
∪
−−−−−→
RM
1
i
∪
RM
2
j
1
≤i≤n
1
≤j≤m
Fig. 4 illustrates an example of combining two observable rules into a single
one. In this example, there are two subparts of SWIM Security Service: ISS-ENT
and ISS-BP. The left hand side of the figure displays two rules for these parts,
and in the right hand side, it is the combined rule.
In general case, we have multiple steps of evolution
i.e.
evolution happens for
many times. For the ease of reading,
step 0
will be the first step of evolution,
where no evolution is applied. We use
RM
i
to denote the i-
th
model in step
d
,
and
r
od,i
to denote the observable evolution rule that applies to
RM
i
,
i.e.
r
od,i
takes
RM
i
as its original model.
