Reactive systems have been investigated and used for a long time. Due to new methods and new technology development, the reactive systems needs their redefinition. These systems are currently an interesting topic for IT (Information Technology) solution providers. In this paper the authors try to define a new view of the architecture of reactive systems, because reactive systems are evolving and there is no clear definition of them. The starting point of the investigation was the reactive

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... sto which defines reactive systems by four main features (quality attributes): responsiveness, resilience, elasticity and message driven interoperability. The mentioned quality attributes are the basis for developing a system solution. For each of the quality attributes, a set of tactics are proposed to maintain attribute required behavior. The suitability of the proposed tactics was investigated for Reactive Sensor Middleware which is part of a CoS (Cloud of Sensors) in the PaaS (Platform as a Service) layer. A cloud of sensors for pollution monitoring in urban areas was used as an example. Verification of the tactics has confirmed that some of the proposed tactics are suitable for the selected CoS subsystem.