Blockchain Oracles: State-of-the-Art and Research Directions

Blockchain interoperability is an innovative feature of blockchain technology that is rapidly gaining momentum in various fields. The mass adoption of enterprise blockchains has not yet been achieved because blockchain networks act as isolated islands that cannot connect or exchange assets and/or information. In addition, the invocation of smart contracts is restricted only to on-chain executions because of the lack of connectivity between the blockchains. This paper aims to conduct a comprehensive and thorough literature review regarding blockchain interoperability, with special highlight on blockchain Oracles being state-of-the-art. Oracles have shown potential as an emerging technology that has revolutionized the blockchain ecosystem by acting as agents that fetch external information into the blockchain ecosystem. A detailed comparative discussion of different blockchain interoperability techniques is presented, showing the strengths and weaknesses of each technique. Being overlooked in the literature, the shortcomings of these techniques in comparison to Oracles are identified, addressing how Oracles have succeeded in overcoming many of these limitations. In addition, the literature lacks a focus on the latest market solutions adopting blockchain Oracles, and only a few studies have considered them in detail. This gap has been addressed through an in-depth assessment of the latest market solutions adopting Oracles in the past few years. Finally, design issues trying to achieve the best practices of Oracles, future directions, and identified research gaps are highlighted.