Event-driven architecture patterns in Blockchain / Solidity - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this event handler code snippet?

Consider a blockchain smart contract that emits an event when a token is transferred. What will be the output of the following event listener code when a transfer event is emitted?

Blockchain / Solidity

contract Token {
  event Transfer(address indexed from, address indexed to, uint amount);

  function transfer(address to, uint amount) public {
    emit Transfer(msg.sender, to, amount);
  }
}

// Event listener pseudocode
Token.on('Transfer', (from, to, amount) => {
  console.log(`Transfer from ${from} to ${to} of ${amount} tokens`);
});

// Simulate transfer
Token.transfer('0xABC', 100);

A Transfer from 0xSender to 0xABC of 100 tokens

B Transfer from 0xABC to 0xSender of 100 tokens

C Transfer from 0xSender to 0xABC of 0 tokens

D No output, event listener not triggered

💡 Hint

Remember that the 'from' address is the sender of the tokens, usually msg.sender.

The event emits the sender's address as 'from', the recipient as 'to', and the amount transferred. The listener logs these values accordingly.

🧠 Conceptual

intermediate

Which pattern best describes decoupling components in blockchain event-driven systems?

In blockchain event-driven architecture, which pattern helps to decouple the smart contract logic from off-chain services by using events?

A Factory pattern

B Singleton pattern

C Observer pattern

D Decorator pattern

💡 Hint

Think about how listeners subscribe to events emitted by contracts.

The Observer pattern allows off-chain services to listen to events emitted by smart contracts, enabling decoupling.

🔧 Debug

advanced

Identify the error in this event subscription code

This code subscribes to a blockchain event but does not receive any events. What is the error?

Blockchain / Solidity

contract Token {
  event Approval(address indexed owner, address indexed spender, uint value);
}

// Subscription code
Token.on('approval', (owner, spender, value) => {
  console.log(`Approved ${value} tokens from ${owner} to ${spender}`);
});

A Event name is case-sensitive; should be 'Approval' not 'approval'

B Callback function parameters are in wrong order

C Missing await keyword before Token.on

D Event cannot have indexed parameters

💡 Hint

Check the exact event name spelling and casing.

Event names in blockchain are case-sensitive. Using 'approval' instead of 'Approval' means no events are caught.

📝 Syntax

advanced

Which option correctly defines an event with multiple indexed parameters in Solidity?

Select the correct Solidity event declaration with two indexed parameters.

A event Transfer(indexed address from, indexed address to, uint amount);

B event Transfer(address indexed from, address indexed to, uint amount);

C event Transfer(address from, address to, uint indexed amount);

D event Transfer(address from indexed, address to indexed, uint amount);

💡 Hint

Indexed keyword comes immediately after the type.

In Solidity, 'indexed' must come after the type and before the parameter name.

🚀 Application

expert

How many events will be processed by the listener in this scenario?

A smart contract emits an event 'DataUpdated' every time data changes. The listener subscribes once and processes events from block 100 to 105. The contract emitted 'DataUpdated' events at blocks 101, 103, 103, 104, and 106. How many events will the listener process?

A 6

B 5

C 3

D 4

💡 Hint

Count only events emitted between blocks 100 and 105 inclusive.

Events at blocks 101, 103 (two events), and 104 are within the range, totaling 4 events. The event at block 106 is outside the range.

Practice

1. What is the main purpose of event-driven architecture in blockchain applications?

easy

A. To allow smart contracts to emit events that trigger automatic responses

B. To store large amounts of data on the blockchain

C. To speed up the mining process

D. To encrypt transactions for security

Solution

1. Step 1: Understand event-driven architecture basics

   Event-driven architecture means systems react automatically to events or actions.

2. Step 2: Apply this to blockchain context

   In blockchain, smart contracts emit events that apps listen to and respond automatically.

3. Final Answer:

   To allow smart contracts to emit events that trigger automatic responses -> Option A

4. Quick Check:

   Event-driven architecture = automatic reactions to events [OK]

Hint: Event-driven means automatic reaction to smart contract events [OK]

Common Mistakes:

• Confusing event-driven with data storage
• Thinking it speeds up mining
• Mixing encryption with event handling

2. Which of the following is the correct syntax to listen for an event named Transfer in a blockchain app using JavaScript?

easy

A. contract.listen('Transfer', function(from, to, amount) {});

B. contract.emit('Transfer', (from, to, amount) => {});

C. contract.on('Transfer', (from, to, amount) => { /* handle event */ });

D. contract.catch('Transfer', (from, to, amount) => {});

Solution

1. Step 1: Identify correct event listening method

   In JavaScript blockchain libraries, on is used to listen for events.

2. Step 2: Check syntax for event callback

   The callback receives event parameters like from, to, and amount.

3. Final Answer:

   contract.on('Transfer', (from, to, amount) => { /* handle event */ }); -> Option C

4. Quick Check:

   Use on to listen for events [OK]

Hint: Use .on() method to listen for blockchain events in JS [OK]

Common Mistakes:

• Using .emit() instead of .on() to listen
• Using .listen() which is invalid
• Using .catch() which is for errors

3. Given this Solidity event and JavaScript listener:

event Deposit(address indexed user, uint amount);
contract.on('Deposit', (user, amount) => { console.log(amount.toString()); });

What will be printed if the event is emitted with amount = 500?

medium

A. 0x1f4

B. 500

C. undefined

D. Error: amount.toString is not a function

Solution

1. Step 1: Understand event parameters

   The event emits amount as a number (uint) which is received in JS as a BigNumber or number.

2. Step 2: Check the listener code

   The listener calls amount.toString() which converts the number 500 to string "500".

3. Final Answer:

   500 -> Option B

4. Quick Check:

   amount.toString() converts number 500 to string '500' [OK]

Hint: toString() converts number to string for console output [OK]

Common Mistakes:

• Thinking amount prints as hex
• Assuming amount is undefined
• Confusing toString() usage

4. Identify the error in this JavaScript event listener code for a blockchain contract:

contract.on('Approval', (owner, spender, value) => {
  console.log('Approved:', owner, spender, value.toString);
});

medium

A. contract.on is not a function

B. Wrong event name 'Approval'

C. Callback parameters are incorrect

D. Missing parentheses after value.toString

Solution

1. Step 1: Check usage of toString method

   The code uses value.toString without parentheses, so it logs the function reference, not the string.

2. Step 2: Correct usage requires parentheses

   It should be value.toString() to convert the value to a string for logging.

3. Final Answer:

   Missing parentheses after value.toString -> Option D

4. Quick Check:

   toString() needs () to call method [OK]

Hint: Call toString() with parentheses to convert value [OK]

Common Mistakes:

• Forgetting () after toString
• Assuming event name is wrong
• Misunderstanding callback parameters

5. You want to build a blockchain app that updates user balances automatically when a BalanceUpdated event is emitted by a smart contract. Which event-driven pattern best fits this use case?

hard

A. Event Listener pattern to react and update balances on event emission

B. Command Query Responsibility Segregation (CQRS) to separate reads and writes

C. Event Sourcing to store all balance changes as events

D. Batch Processing to update balances periodically

Solution

1. Step 1: Understand the requirement

   The app must update balances automatically when an event occurs.

2. Step 2: Match the pattern to the need

   The Event Listener pattern listens for events and triggers immediate updates, fitting this use case perfectly.

3. Final Answer:

   Event Listener pattern to react and update balances on event emission -> Option A

4. Quick Check:

   Use Event Listener for automatic reaction to events [OK]

Hint: Use Event Listener to react instantly to blockchain events [OK]

Common Mistakes:

• Confusing Event Sourcing with immediate updates
• Choosing CQRS which separates reads/writes but not event reaction
• Using Batch Processing which delays updates