How to use oracles in OpTrust

Oracles are web services that provide a connection between smart contracts and the outside world. They let decentralized apps (dApps) interact with off-chain data and services.

Chainlink​

Chainlink is a widely-recognized Web3 services platform that specializes in decentralized oracle networks. It lets you build Ethereum and Arbitrum dApps that connect to a variety of off-chain data feeds and APIs, including those that provide asset prices, weather data, random number generation, and more.

Querying the price of $ARB through Chainlink​

Here’s an example on how to use a price feed from Chainlink to query the current price of $ARB on-chain. We’ll use an interface provided by Chainlink that can be configured with the address of the proxy that holds the information we want to request, and wrap the operation in a contract.

Chainlink provides an npm package with the contracts needed to access their feeds. We first install that package in our project:

yarn add @chainlink/contracts

To use a data feed, we retrieve the information through the AggregatorV3Interface and the proxy address of the feed we want to query.

import "@chainlink/contracts/src/v0.8/interfaces/AggregatorV3Interface.sol";

In this case, we want to obtain the current price of $ARB in $USD in Arbitrum One, so we need to know the address of the proxy that will provide that information. Chainlink maintains a list of price feed address here. For $ARB/$USD, we’ll use the address 0xb2A824043730FE05F3DA2efaFa1CBbe83fa548D6.

We can now build the function to get the latest price of $ARB. We’ll use this example contract:

contract ARBPriceConsumer {
    AggregatorV3Interface internal priceFeed;

    /**
     * Network: Arbitrum One
     * Aggregator: ARB/USD
     * Address: 0xb2A824043730FE05F3DA2efaFa1CBbe83fa548D6
     */
        address constant PROXY = 0xb2A824043730FE05F3DA2efaFa1CBbe83fa548D6;

    constructor() {
        priceFeed = AggregatorV3Interface(PROXY);
    }

    /**
     * Returns the latest price.
     */
    function getLatestPrice() public view returns (int) {
        (
            /* uint80 roundID */,
            int price,
            /*uint startedAt*/,
            /*uint timeStamp*/,
            /*uint80 answeredInRound*/
        ) = priceFeed.latestRoundData();
        return price;
    }
}

You can adapt this contract to your needs. Just remember to use the address of the asset you want to request the price for in the appropriate network, and to deploy your contract to the same network. Remember we have a Quickstart available that goes through the process of compiling and deploying a contract.

More examples​

Refer to Chainlink’s documentation for more examples of querying price feeds plus other data feeds available.

API3​

API3 is a collaborative project to deliver traditional API services to smart contract platforms in a decentralized and trust-minimized way. API3 provides the technology for Airnodes to push off-chain data to on-chain contracts. This data can then be queried directly through the Airnode (initiating a “pull-type” request) or through dAPIs (data feeds of up-to-date off-chain data).

Querying the price of $ARB through API3​

Here’s an example on how to use an API3 data feed to query the current price of $ARB on-chain. The API3 market provides a list of all the dAPIs available across multiple chains including testnets. These dAPIs are self-funded so, before querying it, we must make sure they have enough funds to cover our test.

API3 provides an npm package with the contracts needed to access their feeds. We first install that package in our project:

yarn add @api3/contracts

To use a data feed, we retrieve the information through the specific proxy address for that feed. We’ll use the IProxy interface to do so.

import "@api3/contracts/v0.8/interfaces/IProxy.sol";

In this case, we want to obtain the current price of $ARB in $USD in Arbitrum One, so we need to know the address of the proxy that will provide that information. We will search the feed on the API3 Market, connect our wallet and click on Get Proxy. The ARB/USD proxy address is 0x0cB281EC7DFB8497d07196Dc0f86D2eFD21066A5.

We can now build the function to get the latest price of $ARB. We’ll use this example contract:

contract ARBPriceConsumer {
    /**
     * Network: Arbitrum One
     * Aggregator: ARB/USD
     * Proxy: 0x0cB281EC7DFB8497d07196Dc0f86D2eFD21066A5
     */
    address constant PROXY = 0x0cB281EC7DFB8497d07196Dc0f86D2eFD21066A5;

    /**
     * Returns the latest price.
     */
    function getLatestPrice()
        external
        view
        returns (int224 value, uint256 timestamp)
    {
        (value, timestamp) = IProxy(PROXY).read();
        // If you have any assumptions about `value` and `timestamp`, make sure
        // to validate them right after reading from the proxy.
    }
}

You can adapt this contract to your needs. Just remember to use the address of the asset you want to request the price for in the appropriate network, and to deploy your contract to the same network. Remember we have a Quickstart available that goes through the process of compiling and deploying a contract.

Querying a random number through API3​

API3 QRNG is a public utility provided with the courtesy of Australian National University (ANU). It is served as a public good, it is free of charge (apart from the gas costs), and it provides quantum randomness when requiring RNG on-chain.

To request randomness on-chain, the requester submits a request for a random number to AirnodeRrpV0. The ANU Airnode gathers the request from the AirnodeRrpV0 protocol contract, retrieves the random number off-chain, and sends it back to AirnodeRrpV0. Once received, it performs a callback to the requester with the random number.

Here’s an example of a basic QrngRequester that requests a random number.

API3 provides an npm package with the contracts needed to access the ANU qrng airnode. We first install that package in our project:

yarn add @api3/airnode-protocol

We’ll need several information to request a random number:

• address airnodeRrp: Address of the protocol contract. See the Chains page for a list of addresses on different chains. For Arbitrum, we’ll use 0xb015ACeEdD478fc497A798Ab45fcED8BdEd08924.

• address airnode: The address that belongs to the Airnode that will be called to get the QRNG data via its endpoints. See the Providers page for a list of addresses on different chains. For Arbitrum we’ll use 0x9d3C147cA16DB954873A498e0af5852AB39139f2.

• bytes32 endpointId: Endpoint ID known by the Airnode that will map to an API provider call (allowed to be bytes32(0)). You can also find that information in the Providers page. For Arbitrum we’ll use 0xfb6d017bb87991b7495f563db3c8cf59ff87b09781947bb1e417006ad7f55a78.

We can now build the function to get a random number. We’ll use this example contract:

import "@api3/airnode-protocol/contracts/rrp/requesters/RrpRequesterV0.sol";

contract QrngRequester is RrpRequesterV0 {
    event RequestedUint256(bytes32 indexed requestId);
    event ReceivedUint256(bytes32 indexed requestId, uint256 response);

    /**
     * Network: Arbitrum One
     * AirnodeRrpV0 Address: 0xb015ACeEdD478fc497A798Ab45fcED8BdEd08924
     * Airnode: 0x9d3C147cA16DB954873A498e0af5852AB39139f2
     * Endpoint ID: 0xfb6d017bb87991b7495f563db3c8cf59ff87b09781947bb1e417006ad7f55a78
     */
    address constant _airnodeRrp = 0xb015ACeEdD478fc497A798Ab45fcED8BdEd08924;
    address constant airnode = 0x9d3C147cA16DB954873A498e0af5852AB39139f2;
    bytes32 constant endpointIdUint256 = 0xfb6d017bb87991b7495f563db3c8cf59ff87b09781947bb1e417006ad7f55a78;
    mapping(bytes32 => bool) public waitingFulfillment;

    constructor() RrpRequesterV0(_airnodeRrp) {}

    function makeRequestUint256() external {
        bytes32 requestId = airnodeRrp.makeFullRequest(
            airnode,
            endpointIdUint256,
            address(this),
            msg.sender,
            address(this),
            this.fulfillUint256.selector,
            ""
        );
        waitingFulfillment[requestId] = true;
        emit RequestedUint256(requestId);
    }

    function fulfillUint256(bytes32 requestId, bytes calldata data)
        external
        onlyAirnodeRrp
    {
        require(
            waitingFulfillment[requestId],
            "Request ID not known"
        );
        waitingFulfillment[requestId] = false;
        uint256 qrngUint256 = abi.decode(data, (uint256));

        // Use `qrngUint256` here...

        emit ReceivedUint256(requestId, qrngUint256);
    }
}

You can adapt this contract to your needs. Just remember to use the addresses of the appropriate network, and to deploy your contract to the same network. Remember we have a Quickstart available that goes through the process of compiling and deploying a contract.

More examples​

Refer to API3’s documentation for more examples of querying other data feeds and Airnodes.

Tellor​

Tellor is a decentralized oracle network that incentivizes an open, permissionless network of data reporting and validation, ensuring that any verifiable data can be brought on-chain. It supports basic spot prices, sophisticated pricing specs (TWAP/VWAP), Snapshot Vote Results, and custom data needs.

Querying the price of $ETH through Tellor​

Here’s an example on how to use a Tellor data feed to query the current price of $ETH on-chain. The way it works is that a query is crafted asking for the price of one currency against another and sent to the oracle contract. If the information for that query is available, it will be returned. Oracle contracts can be found in the Contracts Reference page.

Tellor provides an npm package with the contracts needed to query the contract. We first install that package in our project:

npm install usingtellor

Our function will just wrap the call to the oracle contract with the query we are interested in. In this case we want to obtain the “SpotPrice” of “eth” against “usd”. We will request this information to the Arbitrum oracle contract 0xD9157453E2668B2fc45b7A803D3FEF3642430cC0. We’ll use this example contract:

contract ARBPriceConsumer is UsingTellor {
    /**
     * Network: Arbitrum One
     * Aggregator: ARB/USD
     * Address: 0xD9157453E2668B2fc45b7A803D3FEF3642430cC0
     */
    constructor(address payable _tellorAddress) UsingTellor(_tellorAddress)
    {}

    /**
     * Returns the latest price.
     */
    function getLatestPrice() public view returns (uint256) {
        bytes memory _queryData = abi.encode("SpotPrice", abi.encode("eth", "usd"));
        bytes32 _queryId = keccak256(_queryData);

        (bytes memory _value, uint256 _timestampRetrieved) =
            getDataBefore(_queryId, block.timestamp - 20 minutes);
        if (_timestampRetrieved == 0) return 0;
        require(block.timestamp - _timestampRetrieved < 24 hours);
        return abi.decode(_value, (uint256));
    }
}

You can adapt this contract to your needs. Just remember to use the ticker of the assets you want to request the price for, and to deploy your contract to the appropriate network, with the address of the oracle contract in that network. Remember we have a Quickstart available that goes through the process of compiling and deploying a contract.