Semantics & Verification Research Group Department of Computer Science University of Malta FLACOS 2008 Detection of Conflicts in Electronic Contracts Stephen Fenech Gordon J. Pace University of Malta Gerardo Schneider University of Oslo TexPoint fonts used in EMF. Read the TexPoint manual before you delete this box.: A A A A

Semantics & Verification Research Group Department of Computer Science University of Malta 2 November 2008 Motivation  Are different services compatible together?  (is this (almost) just the beauty criterion we saw yesterday?)  Different views of contracts:  Contracts as properties  A meta-level view of contracts  Composition of services/systems means composition of contracts

Semantics & Verification Research Group Department of Computer Science University of Malta 3 November 2008 The CL View of Contracts  An action-based deontic logic  Enables specification of obligations, prohibitions and permissions  Reparations as (possibly nested) CTDs, CTPs

Semantics & Verification Research Group Department of Computer Science University of Malta 4 November 2008 What are Conflicts?  Conflicts arise when the contract enforces contradictory actions by one or more signatories.  Obliged and forbidden from doing an action  Permitted and forbidden from performing an action  Being obliged to perform two conflicting actions  Being obliged and permitted to perform two conflicting actions

Semantics & Verification Research Group Department of Computer Science University of Malta 5 November 2008 Semantic Detection of Conflicts  A contract is conflict-free if for any sequence of non-violating actions, a contract monitor will not end up in a state where the contract enforces a conflict.  This requires a trace semantics of CL  on finite traces; and  which preserves deontic information

Semantics & Verification Research Group Department of Computer Science University of Malta 6 November 2008  Original trace semantics: ¾ ² 1 c  Example: [{ a, b }, { b }, …] ² 1 [ a ]O( b ) Æ [ b ]P( c ) Deontic Trace Semantics of CL

Semantics & Verification Research Group Department of Computer Science University of Malta 7 November 2008  Three problems:  Infinite traces are not always constructible: 8 ¾ ¢ ¾ 2 1 O( a ) Æ F( a )  Permission has no role in the semantics: [ { b }, … ] ² 1 F( a ) Æ P( a )  No deontic information is used in the semantics Deontic Trace Semantics of CL

Semantics & Verification Research Group Department of Computer Science University of Malta 8 November 2008  New trace semantics: ¾, ¾ d ² f c  Correctness: ¾ ² 1 c, 9 ¾ d ¢ 8 n ¢ ¾ (0.. n ), ¾ d (0.. n ) ² f c Deontic Trace Semantics of CL

Semantics & Verification Research Group Department of Computer Science University of Malta 9 November 2008 Automata with Deontic Information  Given a CL contract c,  ( c ) = h S, A &, s 0, T, V, I, ± i is an automaton:  S is the set of states, s 0 the initial state  A & is the set of concurrent actions  T= S £ A & £ S are the labelled transitions  V is the violation state  I : S ! CL tags states with CL clauses  ± labels states with deontic information  The language of such an automaton Accept(  ( c )) is the set of traces accepted by the automaton, not passing through state V.

Semantics & Verification Research Group Department of Computer Science University of Malta 10 November 2008 Correctness Result Theorem: Given a CL contract c : ¾, ¾ d ² f c if and only if ¾ 2 Accept(  ( c )) A contract is conflict-free if and only if its automaton representation is conflict-free.

Semantics & Verification Research Group Department of Computer Science University of Malta 11 November 2008 CLAN: An Implementation [c]O(b)^[a]F(b)

Semantics & Verification Research Group Department of Computer Science University of Malta 12 November 2008 Other Analysis using the Automaton  Superfluous Clauses  State is labelled with a deontic notion multiple times  Contract Query  What does contract enforce after a sequence of actions  What actions would lead to a specific obligation

Semantics & Verification Research Group Department of Computer Science University of Malta 13 November 2008 Other Analysis using the Automaton  Unreachable clauses  Clauses in the contract which are superfluous  Overlapping clauses  Clauses repeating similar or identical deontic properties

Semantics & Verification Research Group Department of Computer Science University of Malta 14 November 2008 Conclusions  Sound and complete decision algorithm for conflict detection of CL contracts:  Based on a trace semantics of CL  Prototype implementation  Used on a case study involving an airline company check-in desk.  Currently looking into combining this with runtime verification.