Blockchain Smart Contracts with Solidity · Leçon

Récupération de données hors chaîne

Mettez en œuvre des modèles de demande et de réception de données provenant d’API externes via Chainlink, tout en garantissant leur intégrité.

Récupération de données hors chaîne est une leçon Blockchain Smart Contracts with Solidity gratuite sur CoddyKit. Ceci est la leçon 3 sur 4. Tu peux lire la leçon complète ci-dessous gratuitement — puis la pratiquer en direct dans le navigateur avec un éditeur de code intégré et un tuteur IA 24/7. Elle fait partie du parcours d'apprentissage Blockchain Smart Contracts with Solidity, et ta progression se synchronise sur le web et l'application CoddyKit. Le cours Blockchain Smart Contracts with Solidity comprend 4 leçons au total.

Certaines parties de cette leçon n'ont pas encore été traduites et s'affichent en anglais.

Bridging On-chain & Off-chain

Smart contracts live on the blockchain, isolated from the outside world. But what if your contract needs real-time data, like a cryptocurrency price or a weather report?

This is where oracles come in. In this lesson, we'll learn how to implement patterns for requesting and receiving external data from APIs using Chainlink, ensuring the data is secure and reliable.

The Oracle Request-Response Model

Chainlink acts as a bridge, allowing your smart contract to securely interact with off-chain (external) data sources. It works using a request-response cycle:

Your smart contract sends a request to a Chainlink oracle network.
A Chainlink node fetches the data from the specified API.
The Chainlink node then sends a response back to your contract by calling a specific 'callback' function.

Your Contract: A Chainlink Client

To interact with Chainlink, your Solidity contract needs to inherit from the ChainlinkClient contract provided by Chainlink.

This base contract gives you access to essential functions for building and sending data requests, as well as security features for receiving responses.

Initiating a Data Request

To ask for data, you'll use functions like buildChainlinkRequest and sendChainlinkRequestTo.

buildChainlinkRequest: Prepares the request, specifying details like the Chainlink job ID, your contract's address, and the callback function the oracle should call.
sendChainlinkRequestTo: Sends the prepared request to the Chainlink oracle, along with the LINK token fee.

Code: Requesting Live Price Data

Here's a simplified contract showing how to initiate a Chainlink request for external data, like a cryptocurrency price. This contract would be deployed to a test network.

The requestEthPrice function builds the request, specifies the API to call, and sends it to the oracle.

/*
  This is a simplified example for explanation.
  For a real deployment, replace placeholders like
  oracle address, LINK address, jobId, and fee
  with values for your chosen Chainlink network.
*/
// SPDX-License-Identifier: MIT
pragma solidity ^0.8.7;

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

contract PriceConsumer is ChainlinkClient {
    bytes32 public lastRequestId; // Stores the ID of our last request
    uint256 public currentPrice; // Where we'll store the fetched price

    address private immutable i_oracle; // Chainlink oracle address
    bytes32 private immutable i_jobId;  // Specific job ID for the data source
    uint256 private immutable i_fee;    // Amount of LINK to pay for the request

    constructor(address _oracle, address _link, bytes32 _jobId, uint256 _fee) {
        setChainlinkToken(_link); // Set the LINK token address
        i_oracle = _oracle;
        i_jobId = _jobId;
        i_fee = _fee;
    }

    // Function to request a price from a Chainlink oracle
    function requestEthPrice() public returns (bytes32) {
        // 1. Build the Chainlink request
        Chainlink.Request memory request = buildChainlinkRequest(
            i_jobId,                      // The job ID for the oracle
            address(this),                // Our contract's address
            this.fulfillEthPrice.selector // The function the oracle will call back
        );

        // 2. Add parameters for the Chainlink node (e.g., API URL, path to data)
        request.add("get", "https://min-api.cryptocompare.com/data/price?fsym=ETH&tsyms=USD");
        request.add("path", "USD"); // Extract the USD value from the JSON response
        request.addInt("times", 100000000); // Multiply by 10^8 for fixed-point math

        // 3. Send the request to the oracle
        lastRequestId = sendChainlinkRequestTo(i_oracle, request, i_fee);
        return lastRequestId;
    }

    // The fulfillEthPrice function will be defined later!
}

Tracking Requests with IDs

When you call sendChainlinkRequestTo, it returns a unique identifier: a bytes32 value called the requestId.

This ID acts like a tracking number for your data request.
It's essential because multiple requests might be pending, and the requestId ensures that when the data comes back, your contract knows which request it belongs to.

Always store this requestId in a state variable!

Fulfilling the Data Request

Once the Chainlink node successfully fetches the data, it calls the callback function you specified (e.g., fulfillEthPrice) in your contract.

This function's purpose is to:

Receive the requestId (to match it with the original request).
Receive the actual data fetched by the oracle (e.g., uint256 _price).
Process and store the data within your contract's state.

Code: Implementing the Callback

Let's complete our PriceConsumer contract by adding the fulfillEthPrice function. This is the function that the Chainlink oracle will call back to with the fetched price.

The recordChainlinkFulfillment modifier and the requestId check are crucial for security and data integrity.

/*
  This is a simplified example for explanation.
  For a real deployment, replace placeholders like
  oracle address, LINK address, jobId, and fee
  with values for your chosen Chainlink network.
*/
// SPDX-License-Identifier: MIT
pragma solidity ^0.8.7;

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

contract PriceConsumer is ChainlinkClient {
    bytes32 public lastRequestId; // Stores the ID of our last request
    uint256 public currentPrice; // Where we'll store the fetched price

    address private immutable i_oracle; // Chainlink oracle address
    bytes32 private immutable i_jobId;  // Specific job ID for the data source
    uint256 private immutable i_fee;    // Amount of LINK to pay for the request

    constructor(address _oracle, address _link, bytes32 _jobId, uint256 _fee) {
        setChainlinkToken(_link); // Set the LINK token address
        i_oracle = _oracle;
        i_jobId = _jobId;
        i_fee = _fee;
    }

    // Function to request a price from a Chainlink oracle
    function requestEthPrice() public returns (bytes32) {
        Chainlink.Request memory request = buildChainlinkRequest(
            i_jobId,
            address(this),
            this.fulfillEthPrice.selector // Callback function
        );
        request.add("get", "https://min-api.cryptocompare.com/data/price?fsym=ETH&tsyms=USD");
        request.add("path", "USD");
        request.addInt("times", 100000000); // Multiply by 10^8
        lastRequestId = sendChainlinkRequestTo(i_oracle, request, i_fee);
        return lastRequestId;
    }

    // This is the callback function called by the Chainlink oracle
    function fulfillEthPrice(bytes32 _requestId, uint256 _price)
        public
        recordChainlinkFulfillment(_requestId) // Security modifier
    {
        // Crucial: Check if the requestId matches our last sent request
        require(lastRequestId == _requestId, "Request ID mismatch!");
        currentPrice = _price; // Store the fetched price
    }
}

Enhancing Data Integrity

Ensuring the integrity of off-chain data is paramount. Chainlink employs several mechanisms, and you should use them:

recordChainlinkFulfillment(_requestId): This modifier (from ChainlinkClient) verifies that the caller is the authorized Chainlink oracle for that specific requestId.
requestId Matching: Always include a require(lastRequestId == _requestId, ...) check in your callback. This prevents accidental or malicious fulfillment of an old or unrelated request.
Error Handling: Consider what happens if the oracle fails or returns invalid data. Implement checks and revert if necessary.

Quick Check: Oracle Flow

You've learned about requesting and receiving data from Chainlink oracles.

Consider the following steps involved in a Chainlink request-response cycle:

Recap: Off-chain Data Retrieval

In this lesson, we explored how to enable your smart contracts to interact with external data using Chainlink oracles.

We learned about the request-response model, where your contract requests data and an oracle node delivers it.
You saw how to use the ChainlinkClient base contract to build and send requests.
We implemented a callback function (e.g., fulfillEthPrice) to receive the data.
Finally, we discussed the importance of requestId matching and the recordChainlinkFulfillment modifier for data integrity and security.

You can now build contracts that react to real-world events!

Questions Fréquemment Posées

La leçon « Récupération de données hors chaîne » est-elle gratuite ?

Oui — le texte complet de « Récupération de données hors chaîne » est gratuit à lire ici sur le web. Pour la pratiquer de manière interactive (un éditeur de code intégré et un tuteur IA 24/7) et déverrouiller le reste du cours Blockchain Smart Contracts with Solidity, passe à CoddyKit PRO. Le cours Blockchain Smart Contracts with Solidity comprend 4 leçons au total.

Qu'est-ce que j'apprendrai dans « Récupération de données hors chaîne » ?

Mettez en œuvre des modèles de demande et de réception de données provenant d’API externes via Chainlink, tout en garantissant leur intégrité. Tu pratiques Blockchain Smart Contracts with Solidity avec du code pratique que tu exécutes directement dans le navigateur, et un tuteur IA 24/7 répond à tes questions au fur et à mesure que tu avances dans la leçon.

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