Oracle

• A blockchain oracle is any device or entity that connects a deterministic blockchain with off-chain data
• Smart contracts cannot make API calls themselves because they are deterministic, therefore to get data onto the chain, we have to have an off-chain entity that makes the API call and creates a transaction on-chain with the data (from the API) posted.
• This process of posting data from the real world onto a blockchain is what oracles do
• Some famous oracle services,
  • Chainlink
  • Provable 
  • BandChain
  • Tellor

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Chainlink Example

  function getWinner() 
    public
    onlyOwner
  {
    Chainlink.Request memory req = buildChainlinkRequest(JOB, address(this), this.fulfill.selector);
    req.add("get", "example-winner.com/winner");
    req.add("path", "winner");
    sendChainlinkRequestTo(ORACLE, req, ORACLE_PAYMENT);
  }

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Related

• API calls from Contracts

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Refs

1. https://ethereum.stackexchange.com/questions/85178/what-is-a-blockchain-oracle