  In this paper, we survey state-of-the-art research outcomes in the burgeoning
field of reconfigurable intelligent surface (RIS) in view of its potential for
significant performance enhancement for next generation wireless communication
networks by means of adapting the propagation environment. Emphasis has been
placed on several aspects gating the commercially viability of a future network
deployment. Comprehensive summaries are provided for practical hardware design
considerations and broad implications of artificial intelligence techniques, so
are in-depth outlooks on salient aspects of system models, use cases, and
physical layer optimization techniques.
