## 2 State-of-the-Art Literature Review
They did not deal with a situation where the tracing is a dynamic process of accumulation of parts and sub-assemblies. Such a process requires a very different approach as part of the information gathered throughout the assembly process. An exception that does deal with the complexity of the assembly process is [PERSON] et al. [8]. [PERSON] et al. [8] enabled extra flexibility in tracing parts by using fungible tokens in a system called \"TokenTrail\". This system enables the use of alternative parts that play an identical role in the sub-assembly. However, their focus was on the technological aspects of the blockchain implementation, rather than the operational aspects that are the focus of this paper. Complex assembly such as aircraft assembly or automotive assembly includes thousands of parts and sub-assemblies (see Figure 1). The complexity also stems from the complex hierarchy of the product structure. The construction of this hierarchical structure is a dynamic process in which a larger sub-assembly must verify adaptability and compliance of its sub-assembly constituents and their hierarchy of sub-assemblies. The complexity is extended by the variety of final assembly products (exacerbated by the customized mass production). Moreover, the variety of regulation requirements (including international and domestic regulations) and the need for their traceability make the process even more complex. The automotive industry assembles very complex products such as trucks, tractors and a variety of cars. Figure 1 illustrates some of the complexity of regular cars, made of thousands of parts and sub-assemblies. The assembly processes are hierarchical, generating a hierarchy of sub-assemblies [32]. In this hierarchy, each sub-assembly must have a full verification and traceability of its constituent parts and components. The traceability structure and execution are, therefore, totally different from those described in products such as bottles and cans, which have no internal complexity and have no dynamic structure of sub-assemblies. Based on the above discussion and references, it is clear that none of the previous research dealt with the traceability of dynamic structure of sub-assemblies that are becoming part of other sub-assemblies. The traceability of sub-assemblies of this form is much more complex and challenging than in the cases mentioned in the literature. Based on this finding our research question is: How could a complex assembly operation with a dynamic accumulative structure of its sub-assemblies be effectively traced? BOM documentation is the core data structure in the entire life cycle of assembled products. When the BOM contains enough information, it could be the primary data source for any investigation. The investigation can focus on the part, or on sub-assemblies, or processes, whether for knowledge extraction, or for simulation [4]. Therefore, in our proposed model we adopt the approach of centralizing the development of product data around the BOM. However, in complex assemblies, such as aircraft assembly, there are many thousands of parts and processes that complicate the construction of a unified BOM [33]. In some domains there are very different versions of BOMs: engineering BOM, static service BOM (SBOM) [34], process BOM (PBOM), manufacturing BOM (MBOM), and maintenance BOM (WBOM) [7]. In these cases, it is obvious that constructing a unified BOM is a considerable challenge [33]. We do, however, propose a unifying approach so that the product will have a single unique documentation database. In our proposed unified approach, we heavily deploy smart contracts as a mechanism for automating validation and enabling reliable traceability. Figure 1: Illustration of the complexity of automotive assembly.