Applications in Wireless Sensor Networks (WSNs)are numerous and applied in different areas. However, these applications are mostly application-specific which makes communication between sensors from different networks and the interaction with those sensors difficult. This paper presents the architecture design of a composite-based middleware for WSNs used to receive, store, index, manage and transform sensors measurements. Sensors from different networks are supposed to send them data to a specific composite in the middleware in charge of managing, processing and providing data to end users and large communities through a standard web RESTful API and/or alerts. This composite should be set up by instantiating some default or personalized sub-components. The middleware is based on OSGi, XML and no-SQL databases and it is fully flexible.