Applications in Wireless Sensor Networks are numerous and in different areas. However, these applications are mostly application-specific which makes communication between different sensors and the interaction with those sensors difficult. This paper aims at providing a generic back end middleware used to receive, store, index, manage and transform sensors measurements. Sensors from different networks and devices are supposed to send them data to a specific composite in the middleware. This composite should be set up by instantiating some default or personalized sub-components. The middleware is fully flexible and it is based on OSGi, XML and no-SQL databases.