  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.
