A Nonmonotonic Soft Concurrent Constraint Language for SLA Negotiation

We present an extension of the Soft Concurrent Constraint language that allows the nonmonotonic evolution of the constraint store. To accomplish this, we introduce some new operations: the retract(c) reduces the current store by c, the update(X)(c) transactionally relaxes all the constraints of the store that deal with the variables in the set X, and then adds a constraint c; the nask(c) tests if c is not entailed by the store. We present this framework as a possible solution to the management of resources (e. g. web services and network resource allocation) that need a given Quality of Service (QoS). The QoS requirements of all the parties should converge, through a negotiation process, on a formal agreement defined as the Service Level Agreement, which specifies the contract that must be enforced. c-semirings are the algebraic structures that we use to model QoS metrics.