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.

Practice

1. What is the main purpose of using Chainlink oracles in smart contracts?

easy

A. To encrypt data stored on the blockchain

B. To fetch real-world data securely into the blockchain

C. To create new tokens automatically

D. To speed up transaction processing on the blockchain

Solution

1. Step 1: Understand what oracles do

   Oracles connect smart contracts to external data sources outside the blockchain.

2. Step 2: Identify the main use of Chainlink oracles

   Chainlink oracles securely fetch real-world data like prices or weather into smart contracts.

3. Final Answer:

   To fetch real-world data securely into the blockchain -> Option B

4. Quick Check:

   Oracle purpose = fetch external data [OK]

Hint: Oracles bring outside data inside smart contracts [OK]

Common Mistakes:

• Thinking oracles speed up blockchain transactions
• Confusing oracles with token creation
• Assuming oracles encrypt blockchain data

2. Which of the following is the correct way to declare a Chainlink request in Solidity?

easy

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

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

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

D. ChainlinkRequest req = buildRequest(jobId, this, fulfill.selector);

Solution

1. Step 1: Recall Chainlink request syntax

   The correct syntax uses Chainlink.Request memory and buildChainlinkRequest function.

2. Step 2: Match the correct parameters

   Parameters are jobId, contract address (address(this)), and fulfill function selector (this.fulfill.selector).

3. Final Answer:

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

4. Quick Check:

   Correct request syntax = Chainlink.Request memory req = buildChainlinkRequest(jobId, address(this), this.fulfill.selector); [OK]

Hint: Use buildChainlinkRequest with memory and fulfill.selector [OK]

Common Mistakes:

• Omitting 'memory' keyword
• Using 'new' keyword incorrectly
• Wrong function selector syntax

3. Given this Solidity snippet, what will be the value of data after fulfillment?

uint256 public data;

function fulfill(bytes32 _requestId, uint256 _value) public recordChainlinkFulfillment(_requestId) {
    data = _value;
}

// Assume fulfill is called with _value = 42

medium

A. 0

B. RequestId bytes32 value

C. Compilation error

D. 42

Solution

1. Step 1: Understand fulfill function behavior

   The fulfill function sets the contract's data variable to the passed _value.

2. Step 2: Apply given input value

   Since fulfill is called with _value = 42, data becomes 42.

3. Final Answer:

   42 -> Option D

4. Quick Check:

   fulfill sets data = _value = 42 [OK]

Hint: fulfill sets data to passed _value parameter [OK]

Common Mistakes:

• Confusing _requestId with _value
• Assuming data stays zero
• Thinking function causes error

4. Identify the error in this Chainlink oracle request code snippet:

Chainlink.Request memory req = buildChainlinkRequest(jobId, address(this), this.fulfill.selector);
req.add("get", "https://api.example.com/data");
req.add("path", "price");
sendChainlinkRequest(req, fee);

medium

A. Missing LINK token approval before sending request

B. Incorrect function name: should be sendChainlinkRequestTo

C. Missing import for Chainlink library

D. Using 'add' instead of 'addString' for parameters

Solution

1. Step 1: Review Chainlink request sending requirements

   Before sending a request, the contract must have LINK tokens approved to pay the oracle fee.

2. Step 2: Check code for LINK approval

   The snippet does not show LINK token approval, which is required to avoid failure.

3. Final Answer:

   Missing LINK token approval before sending request -> Option A

4. Quick Check:

   LINK approval needed before sendChainlinkRequest [OK]

Hint: Always approve LINK tokens before sending requests [OK]

Common Mistakes:

• Assuming sendChainlinkRequestTo is correct function
• Confusing add and addString methods
• Ignoring LINK token approval step

5. You want to fetch the current ETH/USD price using Chainlink in your smart contract. Which steps must you combine to do this correctly?

hard

A. Use Chainlink request but omit fulfill function to save gas

B. Directly call the price feed contract without Chainlink oracles

C. Build a Chainlink request with jobId and URL, send request with fee, implement fulfill to store price

D. Send request without specifying jobId and rely on default oracle

Solution

1. Step 1: Build and send Chainlink request

   Create a request specifying jobId and data source URL, then send it with the required fee.

2. Step 2: Implement fulfill function

   Define fulfill to receive the oracle response and store the ETH/USD price in your contract.

3. Final Answer:

   Build a Chainlink request with jobId and URL, send request with fee, implement fulfill to store price -> Option C

4. Quick Check:

   Request + fee + fulfill = correct Chainlink usage [OK]

Hint: Request data, pay fee, handle response in fulfill [OK]

Common Mistakes:

• Skipping fulfill function implementation
• Not specifying jobId in request
• Assuming default oracle without jobId