# Overview of Blockchain Oracle Research
## 5 Converging Studies, Research Themes, and Research Directions
### Oracle Applied

Table 12: _Cont._Another group of studies discussed how oracles intervene according to a specific financial application (e.g., loans, trading platforms, trust services) [46; 86]; however, apart from two papers from Khalifa University and the University of Clermont Auvergne, which both investigated e-auctions, the rest had heterogeneous aims. Both studies on e-auction had an experimental approach and proposed a new auction service based on the Ethereum blockchain, specifying the role of oracles and how to overcome possible security issues [87; 88]. Three studies also investigated the role of oracles in cross-chain asset transfers. Whereas the study from the University of Lisbon provides an overview of different cross-chain techniques, the work from the University of Verona discusses the utilities of the transferred tokens based on their provenance [51; 89]. Another study from Beihang University proposes PracticalAgentChain, an intermediary between the data oracle and provenance blockchains (e.g., Bitcoin and Ethereum) [90]. The system works as a reputation-based trading pool and utilizes Town Crier for reliable oracle service. Business process management research is mainly built toward the ability of blockchains to monitor business processes efficiently. [PERSON] et al. [91] provided an overview of how the monitoring business process with blockchain can be achieved and discussed the challenges eventually faced. An extensive description of oracle implications is provided, first by discussing how oracles should be synchronized (time management) to avoid delay of reports. Second, the reliability of oracles is discussed to ensure that the data are not manipulated. Third, the flexibility of oracles should be guaranteed so that the smart contract can select the best data source according to the monitored event. Fourth, blockchain data are aligned with real-world data so that the event sequence is not misrepresented. Concerning the timeliness and alignment of oracles, a group of works by the University of Potsdam [92; 93; 94] proposed a \"deferred choice pattern\", given that the time of transaction is not known in advance. Their model involves an extended oracle architecture to make all historical process data available (history oracle), sensitive to any unexpected change (publish-subscribe oracle), and preserve privacy and data efficiency. The last is achieved by performing part of the computation and variable evaluation off-chain (conditional oracle variants). More focused on the privacy of business process execution, works from the University of Insubria have proposed an encryption mechanism to ensure data confidentiality, even in the presence of an untrusted oracle. The works also verify the encryption data consequences on smart contracts and transaction overhead [95; 96]. As for the supply chain and traceability fields, the works seem heterogeneous, although eight entries were retrieved. Construction, fashion, and food supply chains were investigated, as well as the traceability of vehicles and COVID-19 infections [97; 98; 99]. [PERSON] et al. proposed that, for a blockchain traceability solution to work, it must operate on a dedicated layer/network, with traceability data separated from the blockchain data in which a reliable data verification mechanism should be implemented. Oracles and external APIs, in their design, play a critical role [100]. Following this approach, [PERSON] et al. [99] proposed collecting traceability data on a cloud, where a network of trusted oracles (recognized by the signature) approves the most reliable information. Concerning the reliability of oracles, a study by [PERSON] et al. [101] proposed rules to evaluate an oracle's reputation in tracing COVID-19 cases. Thresholds also determine the oracles' inclusion or exclusion from the trusted network. Focusing on the construction supply chain, [PERSON] et al. [97] proposed the use of smart construction objects (SCOs) as blockchain oracles given their intrinsic characteristics.