# Towards Integrated Digital Twins for Industrial Products: Case Study on an Overhead Crane
## 4 Discussion
### Lessons Learned  
During the study, we recognized the following six themes that should be given special attention when developing digital twins that have more than one component. APIs. Various types of exchanging data between systems proved to be an area with a lot of space for rapid development. APIs have been a basic tool in IT systems development for decades, but the DT world is only on the verge of recognizing them. There is a multitude of different API types in various dimensions: they can be local or remote, private or public, just a library, standardized or unstandardized, or between these categories. Documentation can often be understood only by IT experts. Hence, knowing everything about the relevant APIs seems impossible. Nevertheless, we made a basic flowchart for selecting APIs during DT application development based on the experiences of the project, shown in Figure 14.  
Figure 13: A case visualization of the API-based business network framework: The components of the Ilmatar DT are colored by the organization in charge of their operation. U: university in general, C1-C4: companies, UG1-UG6: university groups. Key takeaways from the project are that APIs should be used more in DT development, any properly implemented API services become a part of the company infrastructure that can be used in later projects, and APIs need to be easy to use for as many people as possible to be used efficiently. Further notes on the usability of APIs are given in Section 4.5. Standards. During the project it became apparent that a standard for describing digital twins on metadata level would be needed to make extendable digital twins. The digital twin of the crane consists of several parts built for different purposes with different tools, but there was no method on how to state in a machine-readable format that all these parts belong to the digital twin of our IImatar crane. Standards for metadata exchange are now being developed [63]. Tools. While there are tools for implementing specific components of digital twins, there were no tools for combining these. In addition, only some of the component tools can be launched as services with 24/7 availability, which is a practical requirement if the component is leveraged by other teams. Furthermore, launching even simple web services proved to be unnecessarily difficult during the study. Therefore, we have two recommendations for digital twin tool development: digital twin builders that combine multiple components and making it easier for non-coders to deploy the components as 24/7 services. A recently published open-source tool \"Digital Twins Explorer\" by Microsoft [64] is a good opening in the right direction. Open source. The majority of the cases of the project were built on top of open-source solutions: all the cases built by \"University groups\" in Figure 13 leveraged open source and for example, the MindSphere IoT platform is based on open-source software. Open solutions can be tested instantly and therefore they seem to be especially suitable for innovation. Internet and software companies have found ways to leverage this innovation potential by both using and offering open solutions, whereas industrial companies are still reserved towards openness. Digital twins are mostly software and leverage the Internet, so any company building them should get familiar with open solutions to stay competitive. It is also good to acknowledge that open source is a complex field: for example, there are a plethora of open-source licenses, multiple business model styles, and specific dynamics for community engagement. Skills. The digital twin applications of the project were developed mainly by mechanical engineers, although some of them were at least moderately experienced in programming, which proved to be an essential skill in several DT components. Even though creating many advanced features is certainly possible, they may not be implementable in a limited time frame.