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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 tutorials branch of the Chainlink Functions Starter Kit. You can locate this tutorial in the /tutorials/6-multiple-apis directory.

    git checkout tutorials

  4. Get a free API key from CoinMarketCap and note your API key.

  5. The starter kit stores encrypted secrets as gists to share them off-chain with the Decentralized Oracle Network. To allow the starter kit to write gists on your behalf, create a github fine-grained personal access token.

    1. Visit Github tokens settings page.
    2. Click on Generate new token.
    3. Provide a name to your token and define the expiration date.
    4. Under Account permissions, enable Read and write for Gists. Note: Do not enable additional settings.
    5. Click on Generate token and copy your fine-grained personal access token.

  6. Run npx env-enc set to add an encrypted GITHUB_API_TOKEN and COINMARKETCAP_API_KEY to your .env.enc file.

    npx env-enc set

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 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 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 config.js and source.js are correctly written:

npx hardhat functions-simulate --configpath REPLACE_CONFIG_PATH

Example:

$ npx hardhat functions-simulate --configpath tutorials/6-multiple-apis/config.js
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: $28354.00

__Output from sandboxed source code__
Output represented as a hex string: 0x00000000000000000000000000000000000000000000000000000000002b43c8
Decoded as a uint256: 2835400

__Simulated On-Chain Response__
Response returned to client contract represented as a hex string: 0x00000000000000000000000000000000000000000000000000000000002b43c8
Decoded as a uint256: 2835400

Gas used by sendRequest: 398311
Gas used by client callback function: 75029

Reading the output of the example above, you can note that the BTC/USD median price is: 28354.00 USD. Because Solidity does not support decimals, we move the decimal point so that the value looks like the integer 2835400 before returning the bytes encoded value 0x00000000000000000000000000000000000000000000000000000000002b43c8 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 --configpath REPLACE_CONFIG_PATH

Example:

$ npx hardhat functions-request --subid 443 --contract 0x4B4BA2Fd6b93aDF8d6b6002E10540E58394388Ea --network polygonMumbai --configpath tutorials/6-multiple-apis/config.js
secp256k1 unavailable, reverting to browser version
Estimating cost if the current gas price remains the same...

The transaction to initiate this request will charge the wallet (0x9d087fC03ae39b088326b67fA3C788236645b717):
0.000561571505990096 MATIC, which (using mainnet value) is $0.0006232320573478085

If the request's callback uses all 100,000 gas, this request will charge the subscription:
0.20014806820954737 LINK

Continue? Enter (y) Yes / (n) No
y
Simulating Functions request locally...

__Console log messages from sandboxed code__
Median Bitcoin price: $28333.58

__Output from sandboxed source code__
Output represented as a hex string: 0x00000000000000000000000000000000000000000000000000000000002b3bce
Decoded as a uint256: 2833358

Successfully created encrypted secrets Gist: https://gist.github.com/aelmanaa/482d4308b57cf9f39ed1e24efaae0a4f

⣾ Request 0x80d692db8ae330e9e014b44bb5e41f239af543084f099225846d9cb732c3daae has been initiated. Waiting for fulfillment from the Decentralized Oracle Network...

ℹ Transaction confirmed, see https://mumbai.polygonscan.com/tx/0x4efe45aa9a21dd33e34a62af879275579a06ab34910abb7676de07991bb8c223 for more details.

✔ Request 0x80d692db8ae330e9e014b44bb5e41f239af543084f099225846d9cb732c3daae fulfilled! Data has been written on-chain.

Response returned to client contract represented as a hex string: 0x00000000000000000000000000000000000000000000000000000000002b4170
Decoded as a uint256: 2834800

Actual amount billed to subscription #443:
┌──────────────────────┬─────────────────────────────┐
│         Type         │           Amount            │
├──────────────────────┼─────────────────────────────┤
│  Transmission cost:  │  0.000092593367121997 LINK  │
│      Base fee:       │          0.2 LINK           │
│                      │                             │
│     Total cost:      │  0.200092593367121997 LINK  │
└──────────────────────┴─────────────────────────────┘


Off-chain secrets Gist https://gist.github.com/aelmanaa/482d4308b57cf9f39ed1e24efaae0a4f deleted successfully

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 0x4efe45aa9a21dd33e34a62af879275579a06ab34910abb7676de07991bb8c223.
  • The request ID is 0x80d692db8ae330e9e014b44bb5e41f239af543084f099225846d9cb732c3daae.
  • The DON successfully fulfilled your request. The total cost was: 0.200092593367121997 LINK.
  • The consumer contract received a response in bytes with a value of 0x00000000000000000000000000000000000000000000000000000000002b4170. Decoding the response off-chain to uint256 gives you a result of 2834800.
  • The starter kit created a gist https://gist.github.com/aelmanaa/482d4308b57cf9f39ed1e24efaae0a4f containing the encrypted secrets. This gist is shared with the DON when making the request.
  • After request fulfillment, the starter kit deleted the gist https://gist.github.com/aelmanaa/482d4308b57cf9f39ed1e24efaae0a4f.

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 --configpath REPLACE_CONFIG_PATH

Example:

$ npx hardhat functions-read  --contract 0x4B4BA2Fd6b93aDF8d6b6002E10540E58394388Ea  --network polygonMumbai --configpath tutorials/6-multiple-apis/config.js
secp256k1 unavailable, reverting to browser version
Reading data from Functions client contract 0x4B4BA2Fd6b93aDF8d6b6002E10540E58394388Ea on network mumbai

On-chain response represented as a hex string: 0x00000000000000000000000000000000000000000000000000000000002b4170
Decoded as a uint256: 2834800

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 {Functions, FunctionsClient} from "./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) {
  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);

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.
  • codeLanguage: CodeLanguage.JavaScript: The source code is developed in the JavaScript language.
  • source: fs.readFileSync(path.resolve(__dirname, "source.js")).toString(): The source code must be a script object. This example uses fs.readFileSync to read 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. 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.

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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