## 2 Resource Model
### Analysis and Discussion of DRSM  
A number of resource models and related systems have been proposed. Some examples are typical operating systems [4], Resource Description Framework (RDF) [5], Representational State Transfer (REST) [6], the resource space model (RSM) [7], entity-relationship model [17] and the Resource Description Template (RDT) [10]. In all models, the designers try to express all attributes by describing the resources in different views. In addition, most of them also provide mechanisms to manipulate the resources. However, cyber-physical systems involve a heterogeneous collection of resources and have different requirements about their specifications and management. The DRSM is designed for dealing with such complex resources types. A detailed comparison of DRSM with the other approaches is given below.  
(1) In an operating system, the CPU, cache and memory resources are extremely fine-tuned to support higher level software. However, the operating system is a special middleware program that just needs to consider limited types of resources, like CPU and memory. Therefore, it does not support a generic resource description and related operational approach.  
(2) RDF [5] is a framework to represent resources in the web using the Uniform Resource Identifier (URI). This resource model only describes the information resources and is therefore not suitable for describing physical resources or a more generic resource model.  
(3) REST [6] is an architecture style for resource description, describing the sources of specific information on the web. Compared with RDF, it is extended to manipulate the resources by transferring the representational states of the applications. However, this model still aims at the web environment and is only suited for virtual resources.  
(4) RSM [7] is a special resource description method that treats any resource as an entity located in a multi-dimensional space, where each dimension is one resource attribute. This resource model is at the logical layer to describe the resources and suitable for applications, like search engines. However, our resource model supports not only static description, but also dynamic manipulation.  
(5) The resource-explicit service model (RESM) proposed in [17] can be treated as an example of the entity-relationship model. In their model, the physical devices are considered as resources to support services. The resources and service contexts are modelled as a bundle in a single ER diagram. The resource in this model is not a concept independent from services, but just an important property of services. The resource and service models are coupled too tightly.  
(6) The Resource Description Template (RDT) separates the concerns between resources and services [10,11]. However, this resource model is still part of the resource-centric service in CPS, which means that the RDT is tightly coupled with the service model. In our resource model, the RDT is extended and separated from services.  
Resource specifications in DRSM can adapt to different types of resources. In complex CPS, there are usually many thousands of resources and related dynamically changing data. The idea of central control, as we have designed in RODM, is obviously not enough to manage all resources. Therefore, we are currently working to make this model work in a distributed environment. The basic idea is to design an appropriate mechanism to build a multi-agent system. Then, any RODM in this whole system can make decisions from their own knowledge and requests.