Web services orchestrations and choreographies require establishing Quality of Service (QoS) contracts with the user. This is achieved by performing QoS composition, based on contracts established between the orchestration and the called Web services. Typical QoS parameters include maximum query throughput, response time, security, and validity of the response. Usually, QoS contracts are stated in the form of hard guarantees (e.g., response time always less than 5 msec). However, experiments and measurements from existing Web services show evidence that soft guarantees, not hard ones, should be stated in contracts (e.g., response time less than 5 msec in 95% of the cases).

In this paper we argue for using soft contracts, not hard ones, by taking a probabilistic approach. In this approach, contracts are characterized by means of probability distributions for QoS parameters. We show how to compose such contracts, to yield a global QoS (probabilistic) contract for the orchestration. Our approach is supported by the TOrQuE tool, that performs probabilistic contract composition, automatically, from orchestration specification and QoS characteristics of the Web services called by the orchestration. With this tool, we are able to show that overly pessimistic contracts can be avoided and significant room for safe overbooking exists.

This work is partially supported by RNRT (National Research Network in Telecommunication) through the SWAN  project (Self aWare mANagement).