Flacos'08 Workshop Proceedings Flacos'08 Second Workshop on Formal Languages and Analysis of Contract-oriented Software Acknowledgments a Framework for Contract-based Reasoning: Motivation and Application 77 Cc-pi: a Constraint-based Language for Contracts with Service Level Agreements

Service Level Agreements are a key issue in Service Oriented Computing. SLA contracts specify client requirements and service guarantees, with emphasis on Quality of Service (cost, performance, availability, etc.). We overview a simple model of contracts for QoS and SLAs that combines two basic programming paradigms: name-passing calculi and concurrent constraint programming. In the resulting calculus, called cc-pi calculus, SLA requirements are constraints that can be generated either by a single party or by the synchronisation of two agents. We rely on a system of named constraints that equip classical constraints with a suitable algebraic structure providing a richer mechanism of constraint combination. Besides small examples, cc-pi has been applied to a Telco case study. The model allows to specify, negotiate, and enforce policies in complex scenarios where policy negotiations and validations can be arbitrarily nested.