The increasing popularity of Web services for application integration has resulted in a large body of research on Web service composition. However, the major lacuna so far in Web service composition is the lack of a holistic requirements-driven approach for modeling the entire Web service lifecycle, i.e., composition, joint execution, midstream adaptation in response to failures or changing requirements, and finally re-execution until successful completion. In this paper we present such an approach based on our earlier work on context-driven Web service modeling. In particular, we separate requirements into two parts – functional and extra-functional requirements (FRs and EFRs, respectively). We express FRs as commitments made by individual Web services towards the composite Web service, and EFRs as rules that constrain the behavior of the individual Web services while they execute against their FRs. We also show how midstream adaptation in Web service execution – caused either by changes in user requirements or execution failures – can be managed in our approach. We believe that ours is the first such approach towards a comprehensive modeling of requirements for composite Web service executions, and especially during adaptation.