Event-driven architecture patterns in Blockchain / Solidity - Mini Project: Build & Apply

Building a Simple Event-Driven Blockchain Logger

📖 Scenario: You are creating a simple blockchain event logger. In blockchain systems, events like new blocks or transactions are emitted and other parts of the system listen and react to these events. This helps keep the system organized and responsive.Imagine you want to track when new blocks are added and when transactions happen, and log these events.

🎯 Goal: Build a simple event-driven system that listens for blockchain events and logs messages when new blocks or transactions occur.

📋 What You'll Learn

Create a dictionary called blockchain_events to hold event names and their listeners

Create a variable called new_block with the value 'block_added'

Write a function called emit_event that takes an event name and calls all listeners for that event

Print the log messages when events are emitted

💡 Why This Matters

🌍 Real World

Event-driven architecture is common in blockchain systems to handle asynchronous events like new blocks, transactions, or network changes efficiently.

💼 Career

Understanding event-driven patterns is important for blockchain developers to build scalable and maintainable decentralized applications and services.

1

Create the event listeners dictionary

Create a dictionary called blockchain_events with two keys: 'block_added' and 'transaction_made'. Each key should have an empty list as its value.

Blockchain / Solidity

# Create the blockchain_events dictionary with empty lists for 'block_added' and 'transaction_made'
# Your code here

Hint

Use curly braces {} to create a dictionary. Each key should be a string and the value an empty list [].

2

Define the event name variable

Create a variable called new_block and set it to the string 'block_added'.

Blockchain / Solidity

blockchain_events = {
    'block_added': [],
    'transaction_made': []
}

# Create the variable new_block with value 'block_added'
# Your code here

Hint

Assign the string 'block_added' to the variable new_block.

3

Write the event emitter function

Write a function called emit_event that takes one parameter event_name. Inside the function, use a for loop with variables listener to iterate over blockchain_events[event_name] and call each listener().

Blockchain / Solidity

blockchain_events = {
    'block_added': [],
    'transaction_made': []
}

new_block = 'block_added'

# Define the emit_event function to call all listeners for the given event_name
# Your code here

Hint

Use a for loop to go through each listener in the list for the event and call it like a function.

4

Add listeners and print event logs

Define two functions: log_block_added that prints 'New block added to the blockchain.' and log_transaction_made that prints 'New transaction recorded.'. Add log_block_added to blockchain_events['block_added'] and log_transaction_made to blockchain_events['transaction_made']. Then call emit_event(new_block) and emit_event('transaction_made') to print the logs.

Blockchain / Solidity

blockchain_events = {
    'block_added': [],
    'transaction_made': []
}

new_block = 'block_added'

def emit_event(event_name):
    for listener in blockchain_events[event_name]:
        listener()

# Define listener functions, add them to blockchain_events, and emit events
# Your code here

Hint

Define functions that print the messages, add them to the correct event lists, then call emit_event with the event names.

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