Blockchain is a form of distributed ledger technology (DLT) where data is shared among users connected over the internet. Transactions are data state changes on the blockchain that are permanently recorded in a secure and transparent way without the need of a third party. Besides, the introduction of smart contracts to the blockchain has added programmability to the blockchain and revolutionized the software ecosystem leading toward decentralized applications (DApps) attracting businesses and organizations to employ this technology. Although promising, blockchains and smart contracts have no access to the external systems (i.e., off-chain) where real-world data and events resides; consequently, the usability of smart contracts in terms of performance and programmability would be limited to the on-chain data. Hence, \emph{blockchain oracles} are introduced to mitigate the issue and are defined as trusted third-party services that send and verify the external information (i.e., feedback) and submit it to smart contracts for triggering state changes in the blockchain. In this paper, we will study and analyze blockchain oracles with regard to how they provide feedback to the blockchain and smart contracts. We classify the blockchain oracle techniques into two major groups such as voting-based strategies and reputation-based ones. The former mainly relies on participants' stakes for outcome finalization while the latter considers reputation in conjunction with authenticity proof mechanisms for data correctness and integrity. We then provide a structured description of patterns in detail for each classification and discuss research directions in the end.

中文翻译：

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区块链 Oracle 设计模式

区块链是分布式账本技术 (DLT) 的一种形式，其中数据在通过互联网连接的用户之间共享。交易是区块链上的数据状态变化，以安全透明的方式永久记录，无需第三方。此外，将智能合约引入区块链为区块链增加了可编程性，并彻底改变了软件生态系统，导致去中心化应用程序（DApps）吸引企业和组织采用该技术。尽管前景广阔，但区块链和智能合约无法访问现实世界数据和事件所在的外部系统（即链下）；因此，智能合约在性能和可编程性方面的可用性将仅限于链上数据。因此，\emph{blockchain oracles} 被引入以缓解该问题，并被定义为受信任的第三方服务，可发送和验证外部信息（即反馈）并将其提交给智能合约以触发区块链中的状态变化。在本文中，我们将研究和分析区块链预言机如何向区块链和智能合约提供反馈。我们将区块链预言机技术分为两大类，例如基于投票的策略和基于声誉的策略。前者主要依靠参与者的权益来确定结果，而后者则结合真实性证明机制来考虑声誉，以确保数据的正确性和完整性。然后，我们为每个分类详细提供了模式的结构化描述，并在最后讨论了研究方向。