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Call Multiple Data Sources

This tutorial shows you how make multiple API calls from your smart contract to a Decentralized Oracle Network. After OCR completes off-chain computation and aggregation, the DON returns the asset price to your smart contract. This example returns the BTC/USD price.

This guide assumes that you know how to build HTTP requests and how to use secrets. Read the API query parameters and API use secrets guides before you follow the example in this document. To build a decentralized asset price, send a request to the DON to fetch the price from many different API providers. Then, calculate the median price. The API providers in this example are:

Before you begin

  1. Complete the setup steps in the Getting Started guide: The Getting Started Guide shows you how to set up your environment with the necessary tools for these tutorials. You can re-use the same consumer contract for each of these tutorials.

  2. Make sure your subscription has enough LINK to pay for your requests. Read Get Subscription details to learn how to check your subscription balance. If your subscription runs out of LINK, follow the Fund a Subscription guide.

  3. Check out the correct branch before you try this tutorial: Each tutorial is stored in a separate branch of the Chainlink Functions Starter Kit repository.

    git checkout tutorial-6

  4. Get a free API key from CoinMarketCap.

  5. Open your .env file.

  6. Add a line to the .env file with the COINMARKETCAP_API_KEY= variable and set it to your API key. For example: COINMARKETCAP_API_KEY="78143127-fe7e-d5fe-878f-143notarealkey"

  7. Save your .env file.

Tutorial

This tutorial is configured to get the median BTC/USD price from multiple data sources. For a detailed explanation of the code example, read the Explanation section.

  • Open Functions-request-config.js. Note the args value is ["1", "bitcoin", "btc-bitcoin"]. These arguments are BTC IDs at CoinMarketCap, CoinGecko, and Coinpaprika. You can adapt args to fetch other asset prices. See the API docs for CoinMarketCap, CoinGecko, and CoinPaprika for details. For more information about the request, read the request config section.
  • Open Functions-request-source.js to analyze the JavaScript source code. Read the source code explanation for a more detailed explanation of the request source file.

Simulation

The Chainlink Functions Hardhat Starter Kit includes a simulator to test your Functions code on your local machine. The functions-simulate command executes your code in a local runtime environment and simulates an end-to-end fulfillment. This helps you to fix issues before you submit functions to the Decentralized Oracle Network.

Run the functions-simulate task to run the source code locally and make sure Functions-request-config.js and Functions-request-source.js are correctly written:

npx hardhat functions-simulate

Example:

$ npx hardhat functions-simulate
secp256k1 unavailable, reverting to browser version

__Compiling Contracts__
Nothing to compile
Duplicate definition of Transfer (Transfer(address,address,uint256,bytes), Transfer(address,address,uint256))

Executing JavaScript request source code locally...

__Console log messages from sandboxed code__
Median Bitcoin price: $22975.59

__Output from sandboxed source code__
Output represented as a hex string: 0x0000000000000000000000000000000000000000000000000000000000230ed7
Decoded as a uint256: 2297559

__Simulated On-Chain Response__
Response returned to client contract represented as a hex string: 0x0000000000000000000000000000000000000000000000000000000000230ed7
Decoded as a uint256: 2297559

Estimated transmission cost: 0.000045536612837717 LINK (This will vary based on gas price)
Base fee: 0.0 LINK
Total estimated cost: 0.000045536612837717 LINK

Reading the output of the example above, you can note that the BTC/USD median price is: 22975.59 USD. Because Solidity does not support decimals, we move the decimal point so that the value looks like the integer 2297559 before returning the bytes encoded value 0x0000000000000000000000000000000000000000000000000000000000230ed7 in the callback. Read the source code explanation for a more detailed explanation.

Request

Send a request to the Decentralized Oracle Network to fetch the asset price. Run the functions-request task with the subid (subscription ID) and contract parameters. This task passes the functions JavaScript source code and any arguments and secrets when calling the executeRequest function in your deployed FunctionsConsumer contract. Read the functionsConsumer section for a more detailed explanation about the consumer contract.

npx hardhat functions-request --subid REPLACE_SUBSCRIPTION_ID --contract REPLACE_CONSUMER_CONTRACT_ADDRESS --network REPLACE_NETWORK

Example:

$ npx hardhat functions-request --subid 6 --contract 0xa9b286E892d579dc727c79D3be9b01949796240A  --network mumbai
secp256k1 unavailable, reverting to browser version
Simulating Functions request locally...

__Console log messages from sandboxed code__
Median Bitcoin price: $22981.11

__Output from sandboxed source code__
Output represented as a hex string: 0x00000000000000000000000000000000000000000000000000000000002310ff
Decoded as a uint256: 2298111


If all 100000 callback gas is used, this request is estimated to cost 0.000054961325570353 LINK
Continue? (y) Yes / (n) No
y

Requesting new data for FunctionsConsumer contract 0xa9b286E892d579dc727c79D3be9b01949796240A on network mumbai
Waiting 2 blocks for transaction 0x9fa43ee9e8d4ba61ef87bc164b88eb6a9a055140453c27d2b15b42bd4b91a56a to be confirmed...

Request 0x68014e0a20daafe82cc65797222943e0bb5ff3123ff80d7612523945f722c9fb initiated
Waiting for fulfillment...

Request 0x68014e0a20daafe82cc65797222943e0bb5ff3123ff80d7612523945f722c9fb fulfilled!
Response returned to client contract represented as a hex string: 0x00000000000000000000000000000000000000000000000000000000002310ff
Decoded as a uint256: 2298111

Transmission cost: 0.000119462925581673 LINK
Base fee: 0.0 LINK
Total cost: 0.000119462925581673 LINK

The output of the example above gives you the following information:

  • The executeRequest function was successfully called in the FunctionsConsumer contract. The transaction in this example is 0x9fa43ee9e8d4ba61ef87bc164b88eb6a9a055140453c27d2b15b42bd4b91a56a.
  • The request ID is 0x68014e0a20daafe82cc65797222943e0bb5ff3123ff80d7612523945f722c9fb.
  • The DON successfully fulfilled your request. The total cost was: 0.000119462925581673 LINK.
  • The consumer contract received a response in bytes with a value of 0x00000000000000000000000000000000000000000000000000000000002310ff. Decoding the response off-chain to uint256 gives you a result of 2298111.

At any time, you can run the functions-read task with the contract parameter to read the latest received response.

npx hardhat functions-read  --contract REPLACE_CONSUMER_CONTRACT_ADDRESS --network REPLACE_NETWORK

Example:

$ npx hardhat functions-read  --contract 0xa9b286E892d579dc727c79D3be9b01949796240A --network mumbai
secp256k1 unavailable, reverting to browser version
Reading data from Functions client contract 0xa9b286E892d579dc727c79D3be9b01949796240A on network mumbai

On-chain response represented as a hex string: 0x00000000000000000000000000000000000000000000000000000000002310ff
Decoded as a uint256: 2298111

Explanation

FunctionsConsumer.sol

  • To write a Chainlink Functions consumer contract, your contract must import FunctionsClient.sol. You can read the API reference: FunctionsClient.

    This contract is not available in an NPM package, so you must download and import it from within your project.

    import "./dev/functions/FunctionsClient.sol";

  • Use the Functions.sol library to get all the functions needed for building a Chainlink Functions request. You can read the API reference: Functions.

    using Functions for Functions.Request;

  • The latest request id, latest received response, and latest received error (if any) are defined as state variables. Note latestResponse and latestError are encoded as dynamically sized byte array bytes, so you will still need to decode them to read the response or error:

    bytes32 public latestRequestId;
bytes public latestResponse;
bytes public latestError;

  • We define the OCRResponse event that your smart contract will emit during the callback

    event OCRResponse(bytes32 indexed requestId, bytes result, bytes err);

  • Pass the oracle address for your network when you deploy the contract:

    constructor(address oracle) FunctionsClient(oracle)

  • At any time, you can change the oracle address by calling the updateOracleAddress function.

  • The two remaining functions are:

    • executeRequest for sending a request. It receives the JavaScript source code, encrypted secrets, list of arguments to pass to the source code, subscription id, and callback gas limit as parameters. Then:

      • It uses the Functionslibrary to initialize the request and add any passed encrypted secrets or arguments. You can read the API Reference for Initializing a request, adding secrets, and adding arguments.

        Functions.Request memory req;
req.initializeRequest(Functions.Location.Inline, Functions.CodeLanguage.JavaScript, source);
if (secrets.length > 0) {
  if (secretsLocation == Functions.Location.Inline) {
     req.addInlineSecrets(secrets);
  } else {
    req.addRemoteSecrets(secrets);
  }
}
if (args.length > 0) req.addArgs(args);

      • It sends the request to the oracle by calling the FunctionsClient sendRequest function. You can read the API reference for sending a request. Finally, it stores the request id in latestRequestId.

        bytes32 assignedReqID = sendRequest(req, subscriptionId, gasLimit);
latestRequestId = assignedReqID;

    • fulfillRequest to be invoked during the callback. This function is defined in FunctionsClient as virtual (read fulfillRequest API reference). So, your smart contract must override the function to implement the callback. The implementation of the callback is straightforward: the contract stores the latest response and error in latestResponse and latestError before emitting the OCRResponse event.

      latestResponse = response;
latestError = err;
emit OCRResponse(requestId, response, err);

Functions-request-config.js

Read the Request Configuration section for a detailed description of each setting. In this example, the settings are the following:

  • codeLocation: Location.Inline: The JavaScript code is provided within the request.
  • secretsLocation: Location.Inline: The secrets are provided within the request.
  • codeLanguage: CodeLanguage.JavaScript: The source code is developed in the JavaScript language.
  • source: fs.readFileSync("./Functions-request-source.js").toString(): The source code must be a script object. This example uses fs.readFileSync to read Functions-request-source.js and calls toString() to get the content as a string object.
  • secrets: { apiKey: process.env.COINMARKETCAP_API_KEY }: JavaScript object which contains secret values. Before making the request, these secrets are encrypted using the DON public key. The process.env.COINMARKETCAP_API_KEY setting means COINMARKETCAP_API_KEY is fetched from the environment variables. Make sure to set COINMARKETCAP_API_KEY in your .env file. Note: secrets is limited to a key-value map that can only contain strings. It cannot include any other types or nested parameters.
  • walletPrivateKey: process.env["PRIVATE_KEY"]: This is your EVM account private key. It is used to generate a signature for the encrypted secrets such that an unauthorized third party cannot reuse them.
  • args: ["1", "bitcoin", "btc-bitcoin"]: These arguments are passed to the source code. This example requests the BTC/USD price. These arguments are BTC IDs at CoinMarketCap, CoinGecko, and Coinpaprika. You can adapt args to fetch other asset prices. See the API docs for CoinMarketCap, CoinGecko, and CoinPaprika for details.
  • expectedReturnType: ReturnType.uint256: The response received by the DON is encoded in bytes. Because the asset price is uint256, you must define ReturnType.uint256 to inform users how to decode the response received by the DON.

Functions-request-source.js

To check the expected API responses, run these commands in your terminal:

  • CoinMarketCap:

    curl -X 'GET' \
'https://pro-api.coinmarketcap.com/v1/cryptocurrency/quotes/latest?id=1&convert=USD' \
-H 'accept: application/json' \
-H 'X-CMC_PRO_API_KEY: REPLACE_WITH_YOUR_API_KEY'

  • CoinGecko:

    curl -X 'GET' \
'https://api.coingecko.com/api/v3/simple/price?vs_currencies=USD&ids=bitcoin' \
-H 'accept: application/json'

  • Coinpaprika:

    curl -X 'GET' \
'https://api.coinpaprika.com/v1/tickers/btc-bitcoin' \
-H 'accept: application/json'

The price is located at:

  • CoinMarketCap: data,1,quote,USD,price
  • CoinGecko: bitcoin,usd
  • Coinpaprika: quotes,USD,price

Read the JavaScript code section for a detailed explanation of how to write a compatible JavaScript source code. This JavaScript source code uses Functions.makeHttpRequest to make HTTP requests.

The code is self-explanatory and has comments to help you understand all the steps. The main steps are:

  • Construct the HTTP objects coinMarketCapRequest, coinGeckoRequest, and coinPaprikaRequest using Functions.makeHttpRequest. The values for coinMarketCapCoinId, coinGeckoCoinId, and coinPaprikaCoinId are fetched from the args. See the request config section for details.
  • Make the HTTP calls.
  • Read the asset price from each response.
  • Calculate the median of all the prices.
  • Return the result as a buffer using the Functions.encodeUint256 helper function. Because solidity doesn’t support decimals, multiply the result by 100 and round the result to the nearest integer. Note: Read this article if you are new to Javascript Buffers and want to understand why they are important.

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