Flexibility and interoperability make web services well suited for designing highly-customizable reactive service-based applications, that is interactive applications that can be rapidly adapted to new requirements and environmental conditions. This is the case, for example of personal data managers that many users tailor to their needs to meet different usage conditions and requests. In this paper, we propose a model-based approach that provides users with the ability of rapidly developing, adapting and reconfiguring reactive service-based applications to meet new requirements and needs. Users specify their needs by describing sample executions that include interactions with web services through an intuitive interface. Interactions are stored in a visual formalism that integrates live sequence charts with graph transformation systems. Models can be visualized, modified, executed and automatically analyzed to identify inconsistencies. Categories and Subject Descriptors: D.2.1 [Soft...