# Geo-Spatial Context Provision for Digital Twin Generation
## 3 A Digital Twin Architecture Linking CPS Behavior and Produced Operational Data
### Behavior-Centered Digital Twins  
We build upon the behavior-centered conception of digital twins of CPS as it was outlined in [8], with subject-oriented process modeling [23] as an approach for creating behavior specifications. This particular understanding of digital twins is depicted in Figure 2. As outlined in [8], besides the digital model of the CPS, run time support connected to the cyber-physical components is available for model execution. This allows for changes to the digital model during operation that propagate to the components executing the behavior (and thus the model). The authors refer to this aspect as \"design-integrated engineering\", entailing that the models of the CPS are subsequently refined to the point that they can be executed. As can be seen in the figure, data (such as collected sensor data) also flows back from the CPS to its digital representation. Therefore, this digital twin conception also provides an infrastructure for the monitoring of CPS component behavior and is used as the basis for integrating geo-spatial context. Now we specify the central elements of subject-oriented process models, as they are relevant for the digital twin architecture that utilize them. Subject-oriented modeling accounts for some of the unique characteristics of CPS, such as allowing to depict heterogeneous components in a unifying way by focusing on their behavior [8; 28]. Subject-oriented modeling originated from the structure of natural language, specifically the components of sentences, i.e., subject, predicate, and object [23]. This allows for modeling inspired by natural language sentence formulation. Furthermore, the minimal number of symbols used by the associated standard modeling notation makes it more easily accessible. This is indicated by a recent research effort [29] to empirically examine the properties of both a control flow modeling paradigm (e.g., BPMN) and a communication modeling paradigm (e.g., subject-oriented process models). The subject-oriented approach uses two types of models to specify processes [23]: Subject Interaction Diagrams (SIDs) and Subject Behavior Diagrams (SBDs). SIDs depict different subjects and their interactions with each other as part of a process. These interactions are modeled as messages that are exchanged between the different subjects. In this kind of depiction, a subject is understood as an encapsulation of a certain behavior that it executes as part of the process, so each subject in the SID has an associated SBD. Different subject behavior is executed in parallel, with messages serving to synchronize behavior. The instantiation of the subject is furthermore left open during the modeling process. Whether a subject behavior is executed by a human, an organization, a software component, a cyber-physical component, etc. is decided at a later point in time. SBDs now depict behavior as a sequence of states, including function states (performing some local action), send states (sending a message to another subject), and receive states (receiving a message from another subject). Furthermore, one state needs to be marked as the start state of the sequence and there needs to be at least one end state (multiple end states are possible). States are connected through transitions (usually notated as arrows labeled with the result of the previous state). Different tools have been proposed that support the creation, validation, and execution of subject-oriented models. Such run-time engines originate from both the commercial (see the suites Metasonic ([[https://www.metasonic.de/en/](https://www.metasonic.de/en/)]([https://www.metasonic.de/en/](https://www.metasonic.de/en/)), retrieved 9 June 2022) and Compunity ([[https://compunity.eu/](https://compunity.eu/)]([https://compunity.eu/](https://compunity.eu/)), retrieved 9 June 2022)) and academic sector (e.g.,: [30; 31]). Subject-oriented business processes are embedded in organizations within the framework of Subject-oriented Business Process Management (S-BPM) [23].