A theory of contracts for web services

Abstract

Contracts are behavioral descriptions of Web services. We devise a theory of contracts that formalizes the compatibility of a client to a service, and the safe replacement of a service with another service. The use of contracts statically ensures the successful completion of every possible interaction between compatible clients and services.The technical device that underlies the theory is the filter, which is an explicit coercion preventing some possible behaviors of services and, in doing so, make services compatible with different usage scenarios. We show that filters can be seen as proofs of a sound and complete subcontracting deduction system which simultaneously refines and extends Hennessy's classical axiomatization of the must testing preorder. The relation is decidable and the decision algorithm is obtained via a cut-elimination process that proves the coherence of subcontracting as a logical system.Despite the richness of the technical development, the resulting approach is based on simple ideas and basic intuitions. Remarkably, its application is mostly independent of the language used to program the services or the clients. We outline the practical aspects of our theory by studying two different concrete syntaxes for contracts and applying each of them to Web services languages. We also explore implementation issues of filters and discuss the perspectives of future research this work opens.

Planning and verifying service composition

“…In [20,25], a contract defines the possible flows of invocations that are received and emitted by the service. Subcontract preorders [25] formally characterise the ability of replacing one service with another, without breaking the correctness of the composition. Instead, [20] considers multi-party compositions where services can be replaced with sub-services.…”

“…Similarly to these works, our effects represent a multi-party contract among services. Differently, we do not feature synchronization, and only have an internal choice, while [25] also has an external one to model client choices. A peculiar point of our proposal is the ability of constraining effects to obey given safety and liveness policies.…”

A theory of contracts for strong service compliance

“…We now consider related work in the theory of contracts: first that due to Fournet et al (Fournet et al 2004) and then the theory proposed by Carpineti et al (Carpineti et al 2006) and extended in Laneve and Padovani (2007) and Castagna et al (2008). We now consider related work in the theory of contracts: first that due to Fournet et al (Fournet et al 2004) and then the theory proposed by Carpineti et al (Carpineti et al 2006) and extended in Laneve and Padovani (2007) and Castagna et al (2008).…”

“…The explicit information about the input/output alphabet used in Laneve and Padovani (2007) allows the corresponding theory of contracts to be applied in multiparty compositions also, but the independent refinement that we advocate can only be partially achieved. The dynamic filters of Castagna et al (2008) allow for independent refinement, but at the cost of synthesising a specific filter for each service that eliminates the additional behaviours introduced by services that also expose a subcontract. In other words, the selection of one subcontract with a larger input/output alphabet can influence the choice of the possible other subcontracts in the system.…”

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We investigate, in a process algebraic setting, a new notion of correctness for service compositions, which we call strong service compliance: composed services are strong compliant if their composition is both deadlock and livelock free (this is the traditional notion of compliance), and whenever a message can be sent to invoke a service, it is guranteed to be ready to serve the invocation. Several papers have already investigated contracts (Carpineti et al 2006;Bravetti and Zavattaro 2007a;Laneve and Padovani 2007;Castagna et al 2008), exploiting them for checking service compliance. Finally, we present a characterisation of the strong subcontract pre-order by resorting to the theory of a (should) testing pre-order.

M.

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