While the privacy of personal data has captured great attention in the public debate, resulting, e.g., in the European GDPR guideline, the sovereignty of knowledge-intensive small and medium enterprises concerning the usage of their own data in the presence of dominant data-hungry players in the Internet needs more investigation. In Europe, even the legal concept of data ownership is unclear. We reflect on requirements analyses, reference architectures and solution concepts pursued by the International Data Spaces Initiative to address these issues. The second part will more deeply explore our current interdisciplinary research in a visionary ""Internet of Production"" with 27 research groups from production and materials engineering, computer science, business and social sciences. In this setting, massive amounts of heterogeneous data must be exchanged and analyzed across organizational and disciplinary boundaries, throughout the lifecycle from (re-)engineering, to production, usage and recycling, under hard resource and time constraints. A shared metaphor, borrowed from Plato's famous Cave Allegory, serves as the core modeling and data management approach from conceptual, logical, physical, and business perspectives.