Contract-Oriented Software Development for Internet Services

47 As recently as several years ago, technology gurus predicted that the next big trend in software system development would be service-oriented architecture: that is, a successful integration of loosely coupled services belonging to different organisations, sometimes competing but on specific tasks collaborating , would storm the world. This would create a myriad of new business opportunities, enabling the formation of virtual organisations in which small and medium-sized enterprises would join forces to thrive in an increasingly competitive global market. The dream lives on, and the industry is pouring resources into developing and deploying Web services. Yet the degree of integration achieved between different organisations remains low. Collaboration presumes mutual trust, and wherever trust is not considered sufficient, business people turn to contracts as a mechanism to reduce risk. In other words, for the SOA to deliver its promised advantages, developers need cost-effective contract management solutions. The main problems and open issues identified for supporting Web services development are the following: 1. Formal definition of generic contracts: currently, no unified system of syntax and semantics exists for contracts (in particular for quality of service (QoS) and security). 2. Negotiable and monitorable contracts: a contract must be negotiated until both parties agree on its final form, and it must be monitorable in the sense that there must be a way to detect violations. Current programming languages provide little support for negotiable and monitorable contracts. 3. Combination of object-orientation and concurrency models based on asynchronous message-passing. The shared-state concurrency model is not suitable for Web service development. 4. Integration of XML into a host language, reducing the distance between XML and object data models. 5. Harmonious coexistence at the language level of real-time and inheritance mechanisms. 6. Verification of contract properties: the integration of contracts in a programming language should be accompanied by the ability to guarantee essential properties. Guaranteeing the non-violation of contracts can be done in (at least) four different ways: (a) with runtime enforcement (eg through monitors); (b) by construction (eg through low-level language mechanisms); (c) with static program analysis techniques; or (d) through model checking. None of the above can be used as a universal tool; they must be combined. In addressing these issues and problems , we need to develop a model of contracts in an SOA that is broad enough to cater for at least QoS and security contracts. A basic requirement is the ability to …