Oracle integration (Chainlink) in Blockchain / Solidity - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Chainlink oracle request event?

Consider the following Solidity event emitted after a Chainlink oracle request is sent. What will be the output in the transaction logs when the request is made?

Blockchain / Solidity

event ChainlinkRequested(bytes32 indexed requestId);

function requestData() public returns (bytes32 requestId) {
    requestId = sendChainlinkRequestTo(oracle, jobId, payment);
    emit ChainlinkRequested(requestId);
}

A No event is emitted because events need to be declared outside functions

B An event with the requestId value logged in the transaction logs

C A compilation error due to missing event parameters

D The function returns void and no logs are created

💡 Hint

Remember that events declared outside functions can be emitted inside functions.

The event ChainlinkRequested is declared correctly and emitted with the requestId. This will appear in the transaction logs as an event with the requestId indexed.

🧠 Conceptual

intermediate

Which Chainlink component is responsible for fetching off-chain data?

In the Chainlink architecture, which component actually retrieves data from outside the blockchain?

A Chainlink Aggregator Contract

B Smart Contract on Ethereum

C Chainlink Token (LINK)

D Chainlink Oracle Node

💡 Hint

Think about which part connects to the real world outside the blockchain.

Chainlink Oracle Nodes are off-chain services that fetch data from external APIs and deliver it to smart contracts.

🔧 Debug

advanced

Why does this Chainlink callback function fail to update the state?

Given the following Solidity code snippet, why does the state variable not update after the oracle response?

Blockchain / Solidity

bytes32 public data;

function fulfill(bytes32 _requestId, bytes32 _data) external recordChainlinkFulfillment(_requestId) {
    data = _data;
}

function requestData() public {
    Chainlink.Request memory req = buildChainlinkRequest(jobId, address(this), this.fulfill.selector);
    sendChainlinkRequestTo(oracle, req, payment);
}

A The data variable is not declared as public, so it cannot be updated

B The recordChainlinkFulfillment modifier is missing, causing the function to revert

C The fulfill function is not marked as external, so it cannot be called by the oracle

D The fulfill function does not emit an event, so the update is invisible

💡 Hint

Oracle calls to fulfill functions require a specific visibility.

Chainlink oracle nodes call the fulfill function externally, so it must be marked external. If it is public, the call will fail.

📝 Syntax

advanced

Which option correctly initializes a Chainlink.Request struct?

Select the correct way to initialize a Chainlink.Request struct in Solidity for a job with jobId and callback function fulfill.

A Chainlink.Request memory req = buildChainlinkRequest(jobId, address(this), this.fulfill.selector);

B Chainlink.Request req = new Chainlink.Request(jobId, address(this), this.fulfill);

C Chainlink.Request req = Chainlink.Request(jobId, address(this), fulfill);

D Chainlink.Request memory req = Chainlink.Request(jobId, this, fulfill.selector);

💡 Hint

Look for the correct use of memory keyword and selector syntax.

The buildChainlinkRequest function returns a properly initialized Chainlink.Request struct with memory keyword and the fulfill function selector.

🚀 Application

expert

What is the final value of `result` after this Chainlink oracle response?

Given the following Solidity contract snippet, what will be the value of the public variable `result` after the oracle calls fulfill with _data = 42?

Blockchain / Solidity

uint256 public result;

function requestData() public returns (bytes32 requestId) {
    Chainlink.Request memory req = buildChainlinkRequest(jobId, address(this), this.fulfill.selector);
    requestId = sendChainlinkRequestTo(oracle, req, payment);
}

function fulfill(bytes32 _requestId, uint256 _data) public recordChainlinkFulfillment(_requestId) {
    result = _data * 2;
}

A 84

B 42

C 0

D The contract will revert and result remains unchanged

💡 Hint

Check how the fulfill function processes the _data parameter.

The fulfill function doubles the _data value and assigns it to result, so 42 * 2 = 84.