# The Blockchain Oracle Problem in Decentralized Finance--A Multivocal Approach
## Appendix A
### Bluzzle DeFi Oracle  
Bluzzle is a project born in 2017 described in its whitepaper as the \"Decentralized Database for the future\". As known, the blockchain stores hashed data of transactions, and if documents are signed or registered with blockchain, \"only\" the hash is uploaded on the chain [133]. This is to limit the size of the blockchain, which is replicated in every node. An excessive increase of the blockchain may, in fact, determine a problem of centralization [41]. For that reason, blockchain applications involving data such as photos, documents, videos, and audio rely on external databases to balance immutability and blockchain size growth. This is where Bluzzle comes into play. Bluzzle offers a system of decentralized databases to supply data for blockchain applications. They claim to be the alternative to Oracle (the database management company) in the decentralized world [134]. As the Bluzzle database system provides any sort of data for blockchain applications, it is lately being used in Decentralized Finance to fetch the exchange rate of assets. However, as the company realized that they were not prepared to be price oracles, they decided to build an appropriate section to serve that purpose [135]. Considering the issues that may arise with unreliable price feeds, the Bluzzle oracles organize the work in five phases, which are described in Figure 11.  
First, the price feeders are chosen among the network contributors (validators). Those entities are the ones that contribute to the Bluzzle Network, providing storage and staking BLZ (the network native token). Thus, the position of price feeders is open only to those who actively contribute to the network. Unlike other platforms, Bluzzle does not impose a trusted data-source list and leaves the feeders the choice of the reliable source to draw upon. The voting power of the feeders is based on their storage supply and staking amount. The second phase consists of eliminating feeds of validators whose value exceeds the platform's maximum deviation index. The stake of outliers is then slashed. The third phase consists of a recalculation of the voting power of validators due to the exit of outliers in phase two. In the fourth phase, the feeds that stay inside the tolerance limits are blended to create a median value that is then passed to the fifth phase for the final evaluation. The last phase of the Bluzzle price oracle consists of putting the blended value under a statistical analysis that can discover price anomalies by confron  
Figure 10: Razor epoch description. Source: Razor whitepaper. feed processes. If the price is confirmed, the system transfers the value to the Bluzzle database and rewards the validators that provided the correct price [136]. Compared to other price oracles, Bluzzle has the characteristics of being updated every sixty seconds, which makes its resource adaptable to many DeFi projects. Furthermore, data stay on the DB, and they are transmitted on-chain only if requested by clients. At the time of writing the paper, Bluzzle offers its service for free and also free access to price history on their database [137]. * Thompson (2021) Thompson, P. The DeFi Hacks of 2020. Available online: [[https://coingeek.com/the-defi-hacks-of-2020/](https://coingeek.com/the-defi-hacks-of-2020/)]([https://coingeek.com/the-defi-hacks-of-2020/](https://coingeek.com/the-defi-hacks-of-2020/)) (accessed on 20 March 2021). * Thomson (2020) Thomson, C. The DAO of ETHEREUM: Analyzing the DAO hack, the Blockchain, Smart contracts, and the Law. Available online: [[https://medium.com/blockchain-review/the-dao-of-ethereum-e228b93](https://medium.com/blockchain-review/the-dao-of-ethereum-e228b93) afc79]([https://medium.com/blockchain-review/the-dao-of-ethereum-e228b93](https://medium.com/blockchain-review/the-dao-of-ethereum-e228b93) afc79) (accessed on 3 April 2020). * Ellis et al. (2017) Ellis, S.; Juels, A.; Nazarov, S. ChainLink: A Decentralized Oracle Network. _Retrieved March_**2017**, _11_, 2018. * Peaster (2020) Peaster, W.M.