  Digital twin (DT) is the recurrent and common feature in discussions about
future technologies, bringing together advanced communication, computation, and
artificial intelligence, to name a few. In the context of Industry 4.0,
industries such as manufacturing, automotive, and healthcare are rapidly
adopting DT-based development. The main challenges to date have been the high
demands on communication and computing resources, as well as privacy and
security concerns, arising from the large volumes of data exchanges. To achieve
low latency and high security services in the emerging DT, multi-tier computing
has been proposed by combining edge/fog computing and cloud computing.
Specifically, low latency data transmission, efficient resource allocation, and
validated security strategies of multi-tier computing systems are used to solve
the operational problems of the DT system. In this paper, we introduce the
architecture and applications of DT using examples from manufacturing, the
Internet-of-Vehicles and healthcare. At the same time, the architecture and
technology of multi-tier computing systems are studied to support DT. This
paper will provide valuable reference and guidance for the theory, algorithms,
and applications in collaborative multi-tier computing and DT.
