# Structuring Reference Architectures for the Industrial Internet of Things
## 7 Findings and Best Practices
The work outlines the consequences of the emerging IoT and discusses the requirements and implications for the truely connected city. In addition to the already provided visualized relations between the reference frameworks, several outstanding commonalities can be identified. On the technical implementation level, most reference architectures promote the usage of container-based--mainly Docker--deployment methods. The advantages of having unified modules in the regarded distributed and heterogeneous environments has reached a common agreement. Moreover, the provisioning of APIs following the REST paradigm can be seen as a default baseline for endpoint interaction. While a significant ratio of use cases requires different interaction methods, for instance in event- and streaming-based interactions, RESTful APIs are a central building block of nearly every proposed reference architecture. In particular for automated deployment and configuration tasks of middleware services and the fast connectivity of external components. Another example for a commonly shared agreement in the used solutions is supported by the repetitive mentioning of certain technologies. In particular, OAuth for authorization, HTTP for basic data exchange and MQTT for publish/subscribe applications present widespread building blocks. For the industrial internet, OPC-UA becomes one of the de facto standards for machine-to-machine communication. Furthermore, certain areas of interest can be identified. While the IIoT-related architectures extensively describe technical and syntactical integration techniques, the higher-level processes and especially the consuming applications are only roughly described (see Figure 8). Since naturally the focus of IIoT is on the handling of the 'thing', the full application stack is able to exploit the potential. Commonly referred analytical services working with artificial intelligence are usually not further enriched with best practices and remain mostly a black box. One can also notice a difference in the prioritization between the different domains and communities. While the rather IIoT-focused approaches present in detail data security mechanisms, the data exchange approaches provide more details on data privacy and usage control. Aspects concerning connectivity are widely covered and are discussed by nearly every reference architecture. More visualizations supporting this finding are also available at the website. However, the blank spots, for instance the unambiguous description of things, devices and endpoints, are only slightly discussed. While not one single framework needs to cover all of this, references to certain well-defined specifications between the frameworks can express a better overview. Stronger and explicitly stated differentiation to other works will speed up the progress in the field and prevent inefficiencies. Furthermore, the focus on connectivity and security does not reflect the overall potential of the IIoT. While the industrial community currently examines the interlinking and communication between IoT devices, their implications are only roughly realized. Self-controlled devices, in combination with flexible edge and cloud computing architectures [17; 47], need to adjust and act autonomously in order to achieve the necessary scalability and robustness. However, the main target is still the achievement of interoperability between heterogeneous systems rather than a real new design of future IIoT networks. The already existing complexity in terms of device variety, velocity and volume will further increase with the ongoing deployment of more and more connected IIoT systems. Therefore, simply following existing coordination and orchestration patterns for the quickly growing number of digital components will not be sufficient.