fabio gadducci university of pisa joint work withchoppy/ifip/udine/... · overview general theme...
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Introduction Models Encoding Technology Transfer Conclusion
On the net encoding of asynchronous interactions
Fabio Gadducci
University of Pisa
Joint work withPaolo Baldan (Univ. Padova), Filippo Bonchi (CWI Amsterdam)
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Introduction Models Encoding Technology Transfer Conclusion
Overview
General Theme
Relating calculi with asynchronous communication and Petri nets
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Introduction Models Encoding Technology Transfer Conclusion
Asynchronous calculi
Asynchronous process calculi
Formal models of distributed and concurrent systems withasynchronous communication [Honda,Tokoro’91], [Boudol’92]:
no handshake between sender and receiver
non-blocking send
the message is sent, it travels to destination and it is(possibly) received
Observations
Only message sending is observable, reception is not
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Introduction Models Encoding Technology Transfer Conclusion
Asynchronous calculi
Asynchronous process calculi
Formal models of distributed and concurrent systems withasynchronous communication [Honda,Tokoro’91], [Boudol’92]:
no handshake between sender and receiver
non-blocking send
the message is sent, it travels to destination and it is(possibly) received
Observations
Only message sending is observable, reception is not
Asynchronous CCS
CCS fragment of asynchronous pi-calculus
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Introduction Models Encoding Technology Transfer Conclusion
Petri nets
Petri Nets
Widely used model of concurrent and distributed systems:
formal semantics
intuitive graphical representation
Asynchrony in Petri nets
Tokens are first generated by some transition and then consumedby others
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Introduction Models Encoding Technology Transfer Conclusion
Relating asynchronous calculi and Petri nets
Can this intuitive correspondence between asynchronous calculiand Petri nets made formal?
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Introduction Models Encoding Technology Transfer Conclusion
Open Petri nets
Open Petri nets
Generalising Petri nets with composition and reactivity formodelling “open” systems
interface / interaction with the environment through somedesignated places
composition between nets (using an interface)
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Introduction Models Encoding Technology Transfer Conclusion
Open Petri nets
Open Petri nets
Generalising Petri nets with composition and reactivity formodelling “open” systems
interface / interaction with the environment through somedesignated places
composition between nets (using an interface)
Related ...
Compositional semantics for Petri nets (SCONE, Petri box calculus,Petri Net algebra)
Petri nets as reactive systems in the sense of Leifer, Milner([Milner], [Sassone,Sobocinski])
Workflows and web-service models (e.g., [van der Aalst])
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Introduction Models Encoding Technology Transfer Conclusion
Results: Encoding asynchronous CCS into open nets
Encoding bounded asyncronous CCS into open nets
it preserves structural congruence
message exchanges as interactions at open places
operational semantics: CCS reductions ! PN firings
it preserves and reflects weak and strong bisimilarity
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Introduction Models Encoding Technology Transfer Conclusion
Results: Technology transfer on Expressiveness
Intimate connection between the two formalisms, useful for sometechnology transfer on expressiveness
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Introduction Models Encoding Technology Transfer Conclusion
Results: Technology transfer on Expressiveness
Intimate connection between the two formalisms, useful for sometechnology transfer on expressiveness
Undecidability of bisimilarity
(Strong/weak) bisimilarity for bounded asynchr. CCS isundecidable
"(Strong/weak) bisimilarity for open nets is undecidable
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Introduction Models Encoding Technology Transfer Conclusion
Results: Technology transfer on Expressiveness
Intimate connection between the two formalisms, useful for sometechnology transfer on expressiveness
Undecidability of bisimilarity
(Strong/weak) bisimilarity for bounded asynchr. CCS isundecidable
"(Strong/weak) bisimilarity for open nets is undecidable
Decidability of convergence
Reachability is decidable for open Petri nets"
Reachability/convergence is decidable for bounded asynchr. CCS
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Introduction Models Encoding Technology Transfer Conclusion
Asynchronous CCS
[Amadio,Castellani,Sangiorgi]
Syntax
P ::= M, a, (!a)P , P1 | P2, !a.P (Processes)
M ::= 0, µ.P , M1 + M2 (Sums)
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Introduction Models Encoding Technology Transfer Conclusion
Asynchronous CCS
[Amadio,Castellani,Sangiorgi]
Syntax
P ::= M, a, (!a)P , P1 | P2, !a.P (Processes)
M ::= 0, µ.P , M1 + M2 (Sums)
Reduction semantics
a.P + M | a # P ".P + M # P !a.P | a # P |!a.P
(+ usual structural axioms)
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Introduction Models Encoding Technology Transfer Conclusion
Asynchronous CCS: behavioral equivalences
Barb
Equivalence based on the notion of barb
P " a if P $ a | Q
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Introduction Models Encoding Technology Transfer Conclusion
Asynchronous CCS: behavioral equivalences
Barb
Equivalence based on the notion of barb
P " a if P $ a | Q
Barbed equivalence
A barbed bisimulation is a symmetric relation R % Proc & Proc s.t.whenever (P ,Q) ' R then
1 if P " a then Q " a,
2 if P # P ! then Q # Q ! and (P !,Q !) ' R .
Barbed bisimilarity ( is the largest barbed bisimulation
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Introduction Models Encoding Technology Transfer Conclusion
Asynchronous CCS: equivalences
Barbed congruence
P (b Q if P | S ( Q | S for all processes S ' Proc
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Introduction Models Encoding Technology Transfer Conclusion
Asynchronous CCS: equivalences
Barbed congruence
P (b Q if P | S ( Q | S for all processes S ' Proc
1-bisimilarity
A 1-bisimulation is a symmetric relation R % Proc & Proc s.t.whenever (P ,Q) ' R then
1 if P # P ! then Q # Q ! and (P !,Q !) ' R ,
2 )a ' N . (P | a,Q | a) ' R ,
3 if P $ P ! | a then Q $ Q ! | a and (P !,Q !) ' R .
Strong 1-bisimilarity (1 is the largest strong 1-bisimulation
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Introduction Models Encoding Technology Transfer Conclusion
Open nets
Interface of the net
open places
the enviroment can put/remove tokenss
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Introduction Models Encoding Technology Transfer Conclusion
Open nets: Behaviour
Interactions at the interfaces / internal firings
Weak and strong bisimilarities are totally standard
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Introduction Models Encoding Technology Transfer Conclusion
Open nets: Behaviour
Interactions at the interfaces / internal firings s
+s*#
Weak and strong bisimilarities are totally standard
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Introduction Models Encoding Technology Transfer Conclusion
Open nets: Behaviour
Interactions at the interfaces / internal firings s
!*#
s
+s*#
Weak and strong bisimilarities are totally standard
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Introduction Models Encoding Technology Transfer Conclusion
Open nets: Behaviour
Interactions at the interfaces / internal firings ss
!*#
s
+s*#!
*#
Weak and strong bisimilarities are totally standard
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Introduction Models Encoding Technology Transfer Conclusion
Open nets: Behaviour
Interactions at the interfaces / internal firings ss s
*s+ ,s
*s
+s*#!
*#!
*#
Weak and strong bisimilarities are totally standard
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Introduction Models Encoding Technology Transfer Conclusion
Encoding asynchronous CCS into open nets
Bounded asynchronous CCS processes
The encoding is restricted to bounded processes: restriction neveroccurs under the scope of replication
!a. (. . . (!b)(. . .) . . .) NO!!
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Introduction Models Encoding Technology Transfer Conclusion
Encoding asynchronous CCS into open nets
Bounded asynchronous CCS processes
The encoding is restricted to bounded processes: restriction neveroccurs under the scope of replication
!a. (. . . (!b)(. . .) . . .) NO!!
Idea
open places represent the free channels of a process
messages represented by tokens in places
transitions encode the control flow
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Introduction Models Encoding Technology Transfer Conclusion
Encoding: Prefix, parallel, restriction
a.0
a.0
a
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Introduction Models Encoding Technology Transfer Conclusion
Encoding: Prefix, parallel, restriction
a | a.0
a
a.0
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Introduction Models Encoding Technology Transfer Conclusion
Encoding: Prefix, parallel, restriction
a | a.0 | b.a.ba.b
a
b
b.a.b
a.0
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Introduction Models Encoding Technology Transfer Conclusion
Encoding: Prefix, parallel, restriction
("a)(a | a.0 | b.a.b)a.b
a
b
b.a.b
a.0
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Introduction Models Encoding Technology Transfer Conclusion
Encoding: Sum
b
!.a.0
a
a.0
b.0
!.a.0 + b.0
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Introduction Models Encoding Technology Transfer Conclusion
Encoding: Sum
b
a
a.0
!.a.0 + b.0!.a.0
b.0
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Introduction Models Encoding Technology Transfer Conclusion
Encoding: Replication
ba
b.0!a.b.0
!a.b.0
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Introduction Models Encoding Technology Transfer Conclusion
In general . . .
Any bounded asynchr. CCS process P encoded as an open net [[P ]]
Any Q such that P #" Q corresponds to a marking m(Q) of [[P ]]
("a)(a | a.b | b.a.0)
("a)(b | b.a.0)
"
("a)(a.0)
"a.b
b
a
b.a.0 a.0
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Introduction Models Encoding Technology Transfer Conclusion
In general . . .
Any bounded asynchr. CCS process P encoded as an open net [[P ]]
Any Q such that P #" Q corresponds to a marking m(Q) of [[P ]]
("a)(b | b.a.0)
"
("a)(a.0)
"
b
a
a.0b.a.0
("a)(a | a.b | b.a.0)a.b
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Introduction Models Encoding Technology Transfer Conclusion
In general . . .
Any bounded asynchr. CCS process P encoded as an open net [[P ]]
Any Q such that P #" Q corresponds to a marking m(Q) of [[P ]]
("a)(b | b.a.0)
"
("a)(a.0)
"
b
a
a.0b.a.0
("a)(a | a.b | b.a.0)a.b
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Introduction Models Encoding Technology Transfer Conclusion
Properties of the encoding
Preservation and reflection of the operational semantics
For any bounded process P
P # Q i! m(P) # m(Q) in the open net [[P ]]
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Introduction Models Encoding Technology Transfer Conclusion
Properties of the encoding
Preservation and reflection of the operational semantics
For any bounded process P
P # Q i! m(P) # m(Q) in the open net [[P ]]
Preservation and reflection of (strong/weak) bisimilarity
For any two bounded processes
P ( Q i! [[P ]] ( [[Q]]
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Introduction Models Encoding Technology Transfer Conclusion
Undecidability of bisimilarity
Undecidability of bisimilarity for bounded asynchronous CCS
2-register machines:two integer registers r , sprogram instructions: increment a register, jump on zero
encoding 2-register machines as bounded aCCS processesregisters are represented as channels and their content asmessages on such channelszero testing can be only “weakly” simulated
for any given machine we can construct two processes P andP ! such that P ( P ! i! machine halts
# bisimilarity on bounded asynchronous CCS is undecidable
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Introduction Models Encoding Technology Transfer Conclusion
Undecidability of bisimilarity
As a consequence of the properties of the encoding . . .
Corollary
Bisimilarity is undecidable for open Petri nets
Note
Outside the known undecidability results for PNs as we onlyobserve interactions with the environment (all “traditional nets”are weakly bisimilar)
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Introduction Models Encoding Technology Transfer Conclusion
Convergence/reachability is decidable
Convergence in process calculi
A process P is called convergent if there is Q such that P - Q .#
Reachability and presence of deadlocks is decidable for (open) nets"
Corollary
Convergence is decidable for bounded asyncronous CCS
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Introduction Models Encoding Technology Transfer Conclusion
Convergence/reachability is decidable
Convergence in process calculi
A process P is called convergent if there is Q such that P - Q .#
Reachability and presence of deadlocks is decidable for (open) nets"
Corollary
Convergence is decidable for bounded asyncronous CCS
More generally ...
For P ,Q bounded processes, the problem
P |R - Q for some R = a1 | . . . | an is decidable
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Introduction Models Encoding Technology Transfer Conclusion
Conclusions
Tight relation between asynchronous CCS and open Petri nets,exploited for a technology transfer in expressiveness
Generalisation to full CCS and pi-calculus
Infiniteness of channels and variable topology. Open dynamic nets?Open GTSs?
Concurrent semantics
well-understood for open Petri nets
few studies for asynchronous calculi
Step equivalences
Weak concurrent equivalences coincide with non-concurrent ones:intriguing connection between concurrency and asynchrony