# Context-Aware Systems Architecture in Industry 4.0: A Systematic Literature Review
## 5 Analysis and Discussion
### Architecture Technologies, Norms, Models, and Standards
#### 5.3.3 Architecture Standards  
This analysis shows that the architectures predominantly adopt a structural model based on layers, with each layer performing operations that progressively come closer to the set of instructions at the machine level, as illustrated in Figure 6. In the topmost layer, components are dedicated to operations related to the user interface, i.e., applications. On the other hand, in the deepest layer, components interact with the physical domain of the plant. The intermediate layers, often referred to as middleware, play a crucial role by providing the core of the system and the support services that the other layers need. This layered methodology proves to be effective in managing the development of highly complex systems, allowing the segregation of independent features and functionalities [39]. According to [PERSON] and [PERSON] [100], cited by [PERSON] et al. [93], these architectural configurations are among the most recurrent ones implemented in the IoT domain. As indicated by these authors, the reference architectures, namely the IIRA and RAMI 4.0, incorporate elements from various high-level perspectives and can be broken down into six main layers: business, functional, information, communication, integration, and physical. Figure 6: Comparative list of architecture standards. In analysing various studies on high-level software architecture patterns, there is a predominance in adopting the event-driven model (40%). This preference is attributed to the dynamic nature of the scenarios in which the systems are inserted and the constant occurrence of events. This is followed by the service-oriented architecture (SOA), which accounts for 32% of the implementations analysed. The SOA stands out for its ability to offer a modular structure, facilitating the integration and flexibility of services. At the subsequent level, agent-based standards and the publisher-subscriber model (24%) were identified because they are recognised for their effectiveness in managing asynchronous communications and distributing information between decoupled components. Finally, microservices appear as an emerging trend that has evolved from SOA principles; they add granularity and capacity to provide more flexible development, deployment, and scalability of services. In the contemporary industrial context, the ability to respond in real time is imperative for the players involved, which requires them to react quickly to events triggered by other players [49]. In this context, the architectures designed by [PERSON] et al. [75] and [PERSON] et al. [92] aim to increase the ability to react to unexpected occurrences. The pervasive insertion of IoT devices in industrial scenarios gives the event-based pattern a prevalence in architectures that stand out for their effectiveness, as [PERSON] et al. [39] report. The event orchestration and composition approach suggested by [PERSON] et al. [84] also promotes remarkable adaptability, reactivity, and autonomy. Regarding strategies for the implementation of the event-based standard, [PERSON] et al. [83] and [PERSON] et al. [74] propose a context-sensitive service layer that forwards events to subscribing clients to provide the required information. At the same time, [PERSON] et al. [73], [PERSON] et al. [39], and [PERSON] et al. [93] make it possible to integrate events from the physical layer. With technological progress in the SOA, there is the possibility of a progressive transition from a centralised to a distributed architecture in industrial automation [77]. The SOA is emerging as a promising solution for interoperability between devices, attributing this capacity to its intrinsic properties, which include message-based communication, flexible coupling, and the adoption of open standards [76]. [PERSON] et al. [84] also point out that the SOA provides a solid foundation for the promotion of flexibility. The abstraction of IoT devices facilitates access to and the execution of operations. It has been recognised and adopted by [PERSON] et al. [81], [PERSON] et al.