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POST Data to an API

This tutorial shows you how to send a request to a Decentralized Oracle Network to call the Countries information GraphQL API. After OCR completes off-chain computation and aggregation, it returns the name, capital, and currency for the specified country to your smart contract. Because the endpoint is a GraphQL API, write a function that sends a GraphQL query in a POST HTTP method.

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-4

Tutorial

This tutorial is configured to get the country name, capital, and currency from countries.trevorblades.com in one request. For a detailed explanation of the code example, read the Explanation section.

  • Open Functions-request-config.js. The args value is ["JP"] to fetch information about Japan. You can adapt args to retrieve information about other countries as well. Read the Trevorblades API docs to get the list of supported countries. For more information about the request config file, read the request config explanation.
  • Open Functions-request-source.js to analyze the JavaScript source code. Read the source code explanation for a more detailed explanation of the source code 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 your functions to a 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__
Get name, capital and currency for country code: JP
HTTP POST Request to https://countries.trevorblades.com/
country response { country: { name: 'Japan', capital: 'Tokyo', currency: 'JPY' } }

__Output from sandboxed source code__
Output represented as a hex string: 0x7b226e616d65223a224a6170616e222c226361706974616c223a22546f6b796f222c2263757272656e6379223a224a5059227d
Decoded as a string: {"name":"Japan","capital":"Tokyo","currency":"JPY"}

__Simulated On-Chain Response__
Response returned to client contract represented as a hex string: 0x7b226e616d65223a224a6170616e222c226361706974616c223a22546f6b796f222c2263757272656e6379223a224a5059227d
Decoded as a string: {"name":"Japan","capital":"Tokyo","currency":"JPY"}

Estimated transmission cost: 0.00004738239043141 LINK (This will vary based on gas price)
Base fee: 0.0 LINK
Total estimated cost: 0.00004738239043141 LINK

The output of this example tells you that the country name is Japan, capital is Tokyo, and the currency is JPY. Because the final result is a JSON object, convert it to a string and return the bytes encoded value 0x7b226e616d65223a224a6170616e222c226361706974616c223a22546f6b796f222c2263757272656e6379223a224a5059227d in the callback. Read the source code explanation for a more detailed explanation.

Request

Send a request to the Decentralized Oracle Network to run the GraphQL query. Run the functions-request task with the subid (subscription ID) and contract parameters. This task passes the JavaScript source code, arguments, and secrets when it calls the executeRequest function in your deployed FunctionsConsumer contract. Read the functionsConsumer section for a more detailed explanation of the 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__
Get name, capital and currency for country code: JP
HTTP POST Request to https://countries.trevorblades.com/
country response { country: { name: 'Japan', capital: 'Tokyo', currency: 'JPY' } }

__Output from sandboxed source code__
Output represented as a hex string: 0x7b226e616d65223a224a6170616e222c226361706974616c223a22546f6b796f222c2263757272656e6379223a224a5059227d
Decoded as a string: {"name":"Japan","capital":"Tokyo","currency":"JPY"}


If all 100000 callback gas is used, this request is estimated to cost 0.000053129152872322 LINK
Continue? (y) Yes / (n) No
y

Requesting new data for FunctionsConsumer contract 0xa9b286E892d579dc727c79D3be9b01949796240A on network mumbai
Waiting 2 blocks for transaction 0xc7f20ea644baeb8ab0c66ef1c2c2f86816f59ec03e4040bff212712a1ff41831 to be confirmed...

Request 0x5fc971e03dd0482264445acfeeeaca4c4a76b30f7d47fdb1fe70b8647ba25272 initiated
Waiting for fulfillment...

Request 0x5fc971e03dd0482264445acfeeeaca4c4a76b30f7d47fdb1fe70b8647ba25272 fulfilled!
Response returned to client contract represented as a hex string: 0x7b226e616d65223a224a6170616e222c226361706974616c223a22546f6b796f222c2263757272656e6379223a224a5059227d
Decoded as a string: {"name":"Japan","capital":"Tokyo","currency":"JPY"}

Transmission cost: 0.000072181713913155 LINK
Base fee: 0.0 LINK
Total cost: 0.000072181713913155 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 0xc7f20ea644baeb8ab0c66ef1c2c2f86816f59ec03e4040bff212712a1ff41831.
  • The request ID is 0x5fc971e03dd0482264445acfeeeaca4c4a76b30f7d47fdb1fe70b8647ba25272.
  • The DON successfully fulfilled your request. The total cost was: 0.000072181713913155 LINK.
  • The consumer contract received a response in bytes with a value of 0x7b226e616d65223a224a6170616e222c226361706974616c223a22546f6b796f222c2263757272656e6379223a224a5059227d. Decoding it off-chain to a string gives you a result: {"name":"Japan","capital":"Tokyo","currency":"JPY"}.

At any time, you can run the functions-read task again 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: 0x7b226e616d65223a224a6170616e222c226361706974616c223a22546f6b796f222c2263757272656e6379223a224a5059227d
Decoded as a string: {"name":"Japan","capital":"Tokyo","currency":"JPY"}

Decoding 0x7b226e616d65223a224a6170616e222c226361706974616c223a22546f6b796f222c2263757272656e6379223a224a5059227d from bytes to string gives you a string {"name":"Japan","capital":"Tokyo","currency":"JPY"}. Off-chain, you can use JSON.parse() to convert the JSON string to a JSON object.

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.
  • 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.
  • args: ["JP"]: These arguments are passed to the source code. You can change the country code to request information of another country.
  • expectedReturnType: ReturnType.string: The response the DON receives is encoded in bytes. Because the country name, capital, and currency are put in a JSON string, define ReturnType.string to inform users how to decode the response received by the DON. Read the source code explanation for more information.`

Functions-request-source.js

To check the expected API response:

  • In your browser, open the countries GraphQL playground: https://countries.trevorblades.com/

  • Write this query:

    {
country(code: "JP") {
    name
    capital
    currency
}
}

  • Click on play to get the answer :

    {
  "data": {
    "country": {
      "name": "Japan",
      "capital": "Tokyo",
      "currency": "JPY"
    }
  }
}

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. To request the JP country information, the source code calls the https://countries.trevorblades.com/ URL and provides the query data in the HTTP request body. If you read the Functions.makeHttpRequest documentation, you see that you must provide the following parameters:

  • url: https://countries.trevorblades.com/

  • data (HTTP body):

    {
    query: `{\
        country(code: "${countryCode}") { \
        name \
        capital \
        currency \
        } \
    }`,
}

The countryCode is fetched from the args. See the request config section for more information.

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

  • Construct the HTTP object countryRequest using Functions.makeHttpRequest.

  • Run the HTTP request.

  • Read the country name, capital, and currency from the response.

  • Construct a JSON object:

    const result = {
  name: countryData.country.name,
  capital: countryData.country.capital,
  currency: countryData.country.currency,
}

  • Convert the JSON object to a JSON string using JSON.stringify(result). This step is mandatory before encoding string to bytes.

  • Return the result as a buffer using the Functions.string helper function. Note: Read this article if you are new to Javascript Buffers and want to understand why they are important.

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