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
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
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.
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.
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
Get a free API key from CoinMarketCap.
Add a line to the
.envfile with the
COINMARKETCAP_API_KEY=variable and set it to your API key. For example:
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.
Functions-request-config.js. Note the
["1", "bitcoin", "btc-bitcoin"]. These arguments are BTC IDs at CoinMarketCap, CoinGecko, and Coinpaprika. You can adapt
argsto 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.
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.
functions-simulate task to run the source code locally and make sure
Functions-request-source.js are correctly written:
npx hardhat functions-simulate
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.
Send a request to the Decentralized Oracle Network to fetch the asset price. Run the
functions-request task with the
subid (subscription ID) and
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
$ 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:
executeRequestfunction was successfully called in the
FunctionsConsumercontract. The transaction in this example is 0x9fa43ee9e8d4ba61ef87bc164b88eb6a9a055140453c27d2b15b42bd4b91a56a.
- The request ID is
- The DON successfully fulfilled your request. The total cost was:
- The consumer contract received a response in
byteswith a value of
0x00000000000000000000000000000000000000000000000000000000002310ff. Decoding the response off-chain to
uint256gives you a result of
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
$ 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
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.
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
latestErrorare 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
OCRResponseevent 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
The two remaining functions are:
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.
It sends the request to the oracle by calling the
sendRequestfunction. You can read the API reference for sending a request. Finally, it stores the request id in
bytes32 assignedReqID = sendRequest(req, subscriptionId, gasLimit); latestRequestId = assignedReqID;
fulfillRequestto be invoked during the callback. This function is defined in
fulfillRequestAPI 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
latestErrorbefore emitting the
latestResponse = response; latestError = err; emit OCRResponse(requestId, response, err);
Read the Request Configuration section for a detailed description of each setting. In this example, the settings are the following:
secretsLocation: Location.Inline: The secrets are provided within the request.
source: fs.readFileSync("./Functions-request-source.js").toString(): The source code must be a script object. This example uses
toString()to get the content as a
COINMARKETCAP_API_KEYis fetched from the environment variables. Make sure to set
secretsis 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/USDprice. These arguments are BTC IDs at CoinMarketCap, CoinGecko, and Coinpaprika. You can adapt
argsto 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.uint256to inform users how to decode the response received by the DON.
To check the expected API responses, run these commands in your terminal:
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'
curl -X 'GET' \ 'https://api.coingecko.com/api/v3/simple/price?vs_currencies=USD&ids=bitcoin' \ -H 'accept: application/json'
curl -X 'GET' \ 'https://api.coinpaprika.com/v1/tickers/btc-bitcoin' \ -H 'accept: application/json'
The price is located at:
The code is self-explanatory and has comments to help you understand all the steps. The main steps are:
- Construct the HTTP objects
Functions.makeHttpRequest. The values for
coinPaprikaCoinIdare 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.encodeUint256helper function. Because solidity doesn’t support decimals, multiply the result by