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Hoare vs. Milner: Comparing Synchronizations in a Graphical Framework With Mobility

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Hoare vs. Milner: Comparing Synchronizationsin a Graphical Framework With Mobility

Ugo Montanari

Università di Pisa

in collaboration with

Ivan Lanese

Università di Pisa

- Graphical Calculi for Distributed Systems
- Synchronized Edge Replacement Systems
- Mobility
- Hoare and Milner Synchronization, with Fusion
- Direct Comparison
- Comparison with Translations
- Conclusions and Future Work

- Graphical Calculi for Distributed Systems
- Synchronized Edge Replacement Systems
- Mobility
- Hoare and Milner Synchronization, with Fusion
- Direct Comparison
- Comparison with Translations
- Conclusions and Future Work

Motivations:

- Intuitive representation of distribution
- Natural concurrent semantics
- No need of structural axioms
- Existing modeling languages, e.g. UML
- Applications to software architectures and ADL’s
- Well-developed foundations

- Terms
- LTS defined via SOS rules
- Reduction rules
- Abstract semantics
- Non-interleaving semantics

- Graphs
- Double-pushout derivations
- Concurrent semantics based on shift equivalence
- Synchronized (hyper)edge replacement

1

4

2

M

3

- Edge: Atomic item with a label from alphabet LE= {LEn}n=0,1,… with as many (ordered) tentacles as the rank of its label.
- Graph: A set of nodes and a set of edges such that each edgeis connected, by its tentacles, to its attachment nodes. A set of external nodes, identified by distinct names, defines the connecting points with the environment.

x

L

L

y

1

M

4

2

3

z

Representation of graphs as syntactic judgements

N set of names

G

G set of edges

fn(G) binds as usual

G ::= L(x) | G|G | x. G | nil

- Edge: Atomic item with a label from alphabet LE= {LEn}n=0,1,… with as many (ordered) tentacles as the rank of its label.
- Graph: A set of nodes and a set of edges such that each edgeis connected, by its tentacles, to its attachment nodes. A set of external nodes, identified by distinct names, defines the connecting points with the environment.

Well formed judgements for graphs

- Structural Axioms

(AG2) G1|G2 = G2|G1

(AG1) (G1|G2)|G3 = G1|(G2|G3)

(AG3) G1| nil = G1

(AG4) x.y.G = y.x.G

(AG6) x.G = y.G {y/x} if y fn(G)

(AG5) x.G = G if x fn(G)

(AG7) x.(G1|G2 ) = (x. G1) | G2 if x fn(G2)

L LEm yi {xj}

G1 G2

Well formed judgements for graphs

- Syntactic Rules

(RG1)

(RG2)

x1,…,xn nil

x1,…,xn L(y1,…,ym)

, x G

(RG3)

(RG4)

G1|G2

x. G

x,y z, w. C(x,w) | C(w,y) | C (y,z) | C(z,x)

w

z

Ring Example

- Graphical Calculi for Distributed Systems
- Synchronized Edge Replacement Systems
- Mobility
- Hoare and Milner Synchronization, with Fusion
- Direct Comparison
- Comparison with Translations
- Conclusions and Future Work

- Productions: A context free production rewrites a single edge labeled by L into an arbitrary graph R. (Notation: L R)

L

R

H

3

3

4

4

2

2

1

1

L

R

H

3

3

4

4

2

2

1

1

1

1

2

2

3

3

R’

L’

- Productions: A context free production rewrites a single edge labeled by L into an arbitrary graph R. (Notation: L R)

Rewritings of different edges can be executed concurrently

- Synchronized rewriting: Actions are associated to nodes in productions. Each rewrite of an edge must match actions with (a number of) its adjacent edges and they have to move simultaneously

How many edges synchronize depends on the synchronization policy

- Synchronized rewriting propagates synchronization
all over the graph

Hoare synchronization

a

a

a

B1

A1

a

3

3

B2

A2

- Hoare Synchronization: All adjacent edges must match the actions on the shared node

- Milner Synchronization: Only two of the adjacent edges synchronize by matching their complementary actions

- Graphical Calculi for Distributed Systems
- Synchronized Edge Replacement Systems
- Mobility
- Hoare and Milner Synchronization, with Fusion
- Direct Comparison
- Comparison with Translations
- Conclusions and Future Work

- Synchronized rewriting with name mobility

- Add to an action in a node a tuple of names that it wants to communicate
- The synchronization step has to match actions and tuples
- The declared names that were matched are used to merge the corresponding nodes

a< x >

a < y >

a<x> a<y>

B1

A1

a<x> = a<y>

( x )

( y )

x= y

B2

A2

- Transitions

G1 , G2

: (A x N* ) (x, a , y) if (x) = (a , y)

o

is the set of new names that are used in synchronization

= {z | x. (x) = (a , y), z , z set(y)}

Formalization of synchronized rewriting as judgements

- Productions

x1,…,xn L(x1,…,xn) x1,…,xn , DG

Free names can: i) be added to productions; and

ii) renaming is possible

- Derivations

2

1

n

0 G0 1 G1 … n Gn

Formalization of synchronized rewriting as judgements

- Transitions
are generated from the productions by applying the transition rules

of the chosen synchronization mechanism

Hoare synchronization

- On each node all edges must have the same action
- Synchronization is possible if there is a most general unifier of the new nodes

For any R x A x N* (not necessarily a partial function)

(R): n(R) is the mgu of equations (a= b) (Y = Z) with (x,a,Y) and (x,b,Z) in R where (as usual) = {z | (x,a,Y) R, z set(Y), z }

Initial Graph

Brother:

Star Reconfiguration:

x

C

r(w)

C

C

C

(w)

S

C

r(w)

C

C

C

S

Brother

Brother

Brother

Star Rec.

x

C

S

S

C

C

C

C

C

C

S

(1)

(2)

(3)

(4)

(5)

b

)

Milner synchronization

- On each node at most two edges must have actions, and in this case they must be complementary
- Synchronization is possible if there is a most general unifier of the new nodes

L,p

G1 , f G2

L: (A x N* ) (x,a,y) L if L(x) = (a, y)

p: idempotent

n(L) = { z | $x. L(x)=(a,y), z Set(y) }

L = n(L) \

f = p + L

o

Synchronized rewriting with mobility and fusion

- Graphical Calculi for Distributed Systems
- Synchronized Edge Replacement Systems
- Mobility
- Hoare and Milner Synchronization, with Fusion
- Direct Comparison
- Comparison with Translations
- Conclusions and Future Work

- Grammars for distributed systems
[Castellani and Montanari, LNCS 1953, 1982], [Degano and Montanari, JACM 1987]

- Graph amalgamation
[Boehm, Fonio and Habel, JCSS, 1987]

- CHARM (R for restriction)
[Corradini, Montanari and Rossi, TCS 1994]

- Mobile version (w. applications to software architectures, only p-I-like mobility, Hoare synchronization)
[Hirsch and Montanari, Coordination 2000]

- Modeling p-calculus (Milner synchronization)
[Hirsch and Montanari, Concur 2001]

- Modeling Ambient calculus [Ferrari, Montanari and Tuosto, ICTCS 2001]
- Modeling Fusion calculus [Lanese and Montanari, to appear in TCS]

- Graphical Calculi for Distributed Systems
- Synchronized Edge Replacement Systems
- Mobility
- Hoare and Milner Synchronization, with Fusion
- Direct Comparison
- Comparison with Translations
- Conclusions and Future Work

C-behavS(P)(G) = reachable graphs

initial graph

1 : one-step computations

max: maximal computations

all: all computations

set of productions

synchronization style: H, M

(S1,C1) ≥ (S2,C2)

(i.e. style S1 is more expressive than style S2)

iff there exists a uniform simulation function f such that for all P and G

C2-behavS2(P)(G) = C1-behavS1(f(P))(G)

(Milner,C1) ≥ (Hoare,C2) for all C1 and C2

i.e. Hoare cannot be uniformely simulated by Milner

The reason is that Milner synchronization style is monotone, i.e. in a Milner computation we can always add to a graph an additional part which stays idle, while Hoare style is not monotone

(Hoare,C1) ≥ (Milner,C2) for all C1 and C2

i.e. Milner cannot be uniformely simulated by Hoare

The reason is that in Hoare synchronization style restriction just hides part of the observation, while in Milner style restriction may forbid computations

- Graphical Calculi for Distributed Systems
- Synchronized Edge Replacement Systems
- Mobility
- Hoare and Milner Synchronization, with Fusion
- Direct Comparison
- Comparison with Translations
- Conclusions and Future Work

- Amoeboids are graphs with suitable edge labels and corresponding productions which simulate the behavior of nodes in a different synchronization style
- Function [[-]] replaces nodes with amoeboids while function [[-]]-1 replaces amoeboids with nodes.
- We always have that [[([[G]])]]-1 = G

- H-amoeboids implement broadcasting. C-amoeboids saturate nodes with less than 3 tentacles. We have rules for every action a (here with arity 2).

We have C-behavH(P)(G) = [[C-behavM(f(P))([[G]])]]-1

- M-amoeboids implement routing. We have rules for every action a

and two analogous productions for synchronizing x with z and y with z.

We have only

C-behavM(P)(G) [[C-behavH(f(P))([[G]])]]-1

since the amoeboids can also synchronize several pairs in parallel.

- Graphical Calculi for Distributed Systems
- Synchronized Edge Replacement Systems
- Mobility
- Hoare and Milner Synchronization, with Fusion
- Direct Comparison
- Comparison with Translations
- Conclusions and Future Work

- Graph models with synchronized hyperedge replacement allow for more general synchronization mechanisms than ordinary process algebras, e.g. processes can synchronize at more than one channel and with more than one other process.
- These extensions are needed for implementing one synchronization style into another.
- Reachability in Hoare/Milner synchronization styles cannot be simulated uniformely
- No countexample uses mobility, and thus the expressivenesses are incomparable even without mobility, and mobility does not bridge the gap
- Distributed simulation via amoeboids of Milner style routers allows only concurrent pairwise synchronization
- Generic synchronization styles and more general notions of implementation and refinement involving atomicity and bisimilarity can be considered: see the forthcoming PhD thesis of Ivan Lanese