petri-net based analysis of workflows : verification, validation, and performance analysis
TRANSCRIPT
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Petri-net based Analysis of workflows:
Verification, validation, and performance analysis
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Techniques to analyze workflows (design-time)
Validation is concerned with the relation between the model and reality.
Verification is typically used to answer qualitative questions Is there a deadlock possible? It is possible to successfully handle a specific case? Will all cases terminate eventually? It is possible to execute two tasks in any order?
Performance analysis is typically used to answer quantitative questions How many cases can be handled in one hour? What is the average flow time? How many extra resources are required? How many cases are handled within 2 days?
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Three Analysis Techniques
Reachability graph Place & transition invariants Simulation
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Reachability Graph
rg1
go1
or1
r1
g1
o1
rg2
go2
or2
r2
g2
o2
x
(1,0,0,1,0,0,1)
(0,1,0,1,0,0,0) (1,0,0,0,1,0,0)
(1,0,0,0,0,1,0)(0,0,1,1,0,0,0)
Five reachable states.Traffic lights are safe!
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Alternative Notation
rg1
go1
or1
r1
g1
o1
rg2
go2
or2
r2
g2
o2
x
r1+r2+x
g1+r2 r1+g2
r1+o2o1+r2
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Reachability Graph
Graph containing a node for each reachable state.
Constructed by starting in the initial state, calculate all directly reachable states, etc.
Expensive technique. Only feasible if finitely many states Difficult to generate diagnostic information.
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Infinite Reachability Graph
rg1
go1
or1
r1
g1
o1
rg2
go2
or2
r2
g2
o2
x
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Exercise: Construct Reachability Graph
wait enter before make_picture after leave gone
free
occupied
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Structural Analysis Techniques
To avoid state-explosion problem and bad diagnostics.
Properties independent of initial state. We only consider place and transition
invariants. Proof the PN is live (transition invariants) and
bounded (place invariants)
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Place Invariant Assigns a weight to
each place. The weight of a
token depends on the weight of the place.
The weighted token sum is invariant, i.e., no transition can change it
couple
man
woman
marriage divorce
1 man + 1 woman + 2 couple
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Other invariants 1 man + 0 woman + 1 couple
(Also denoted as: man + couple) 2 man + 3 woman + 5 couple -2 man + 3 woman + couple man – woman woman – man
(Any linear combination of invariants is an invariant.)
couple
man
woman
marriage divorce
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Example: traffic light r1 + g1 + o1 r2 + g2 + o2 r1 + r2 + g1 + g2 + o1 +
o2 x + g1 + o1 + g2 + o2 r1 + r2 - x
rg1
go1
or1
r1
g1
o1
rg2
go2
or2
r2
g2
o2
x
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Exercise: Give place invariants
free producer
start_production
end_production
wait consumer
start_consumption
end_consumption
product
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Transition invariant Assigns a weight to
each transition. If each transition
fires the number of times indicated, the system is back in the initial state.
i.e. transition invariants indicate potential firing sets without any net effect.
couple
man
woman
marriage divorce
2 marriage + 2 divorce
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Other invariants 1 marriage + 1 divorce
(Also denoted as: marriage + divorce)
20 marriage + 20 divorce
Any linear combination of invariants is an invariant, but transition invariants with negative weights have no obvious meaning.
couple
man
woman
marriage divorce
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Example: traffic light rg1 + go1 + or1 rg2 + go2 + or2 rg1 + rg2 + go1 + go2 +
or1 + or2 4 rg1 + 3 rg2 + 4 go1 +
3 go2 + 4 or1 + 3 or2
rg1
go1
or1
r1
g1
o1
rg2
go2
or2
r2
g2
o2
x
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Incidence matrix of a Petri net Each row corresponds to
a place. Each column corresponds
to a transition. Recall that a Petri net is d
escribed by (P,T,I,O). N(p,t)=O(t,p)-I(p,t) wher
e p is a place and t a transition.
couple
man
woman
marriage divorce
11
11
11
N
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Example
couple
man
woman
marriage divorce
11
11
11
N
manwoman
couple
marriage
divorce
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Place invariant
Let N be the incidence matrix of a net with n places and m transitions
Any solution of the equation X.N = 0 is a transition invariant X is a row vector (i.e., 1 x n matrix)O is a row vector (i.e., 1 x m matrix)
Note that (0,0,... 0) is always a place invariant.
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Example
Solutions: (0,0,0) (1,0,1) (0,1,1) (1,1,2) (1,-1,0)
couple
man
woman
marriage divorce
)0,0(
11
11
11
X
)0,0(
11
11
11
),,(
couplewomanman
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Transition invariant
Let N be the incidence matrix of a net with n places and m transitions
Any solution of the equation N.X = 0 is a place invariant X is a column vector (i.e., m x 1 matrix)0 is a column vector (i.e., n x 1 matrix)
Note that (0,0,... 0)T is always a place invariant. Basis can be calculated in polynomial time.
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Example
Solutions: (0,0)T
(1,1)T
(32,32)T
couple
man
woman
marriage divorce
0
0
0
11
11
11
X
0
0
0
11
11
11
divorce
marriage
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Performance Analysis
Questions: throughput, waiting and service times service levels
Techniques: simulation