Event sourcing pattern in Kafka - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Kafka event sourcing consumer code?

Consider a Kafka consumer that reads events from a topic and reconstructs the current state by applying all events in order. What will be the final state printed?

Kafka

from kafka import KafkaConsumer
import json

consumer = KafkaConsumer('user-events', bootstrap_servers='localhost:9092', auto_offset_reset='earliest')

state = {}
for message in consumer:
    event = json.loads(message.value)
    if event['type'] == 'UserCreated':
        state[event['user_id']] = {'name': event['name'], 'active': True}
    elif event['type'] == 'UserDeactivated':
        if event['user_id'] in state:
            state[event['user_id']]['active'] = False
    if len(state) == 2:
        break
print(state)

A {"u1": {"name": "Alice", "active": true}, "u2": {"name": "Bob", "active": true}}

B {"u1": {"name": "Alice", "active": false}, "u2": {"name": "Bob", "active": true}}

C {"u1": {"name": "Alice", "active": true}}

D SyntaxError

💡 Hint

Remember that events are applied in order and can change the state of existing users.

The consumer reads events in order. First, UserCreated for u1 (Alice) sets active true. Then UserDeactivated for u1 sets active false. Then UserCreated for u2 (Bob) sets active true. So final state has u1 inactive and u2 active.

🧠 Conceptual

intermediate

Which statement best describes event sourcing in Kafka?

Choose the statement that correctly explains the event sourcing pattern using Kafka.

A Event sourcing stores only the latest state snapshot in Kafka topics to reduce storage.

B Event sourcing uses Kafka to send commands directly to update the database state.

C Event sourcing requires Kafka to delete old events after processing to save space.

D Event sourcing stores all changes as events in Kafka topics, allowing state reconstruction by replaying events.

💡 Hint

Think about how event sourcing preserves history.

Event sourcing means storing every change as an event. Kafka topics hold these events, so the current state can be rebuilt by replaying them.

🔧 Debug

advanced

Why does this Kafka event sourcing consumer code raise an error?

Identify the cause of the error in this Kafka consumer code snippet that processes events for state reconstruction.

Kafka

from kafka import KafkaConsumer
import json

consumer = KafkaConsumer('orders', bootstrap_servers='localhost:9092', auto_offset_reset='earliest')

state = {}
for message in consumer:
    event = json.loads(message.value)
    if event['type'] == 'OrderPlaced':
        state[event['order_id']] = event['details']
    elif event['type'] == 'OrderCancelled':
        del state[event['order_id']]
print(state)

A KeyError when deleting a non-existent order_id in state

B JSONDecodeError due to invalid JSON in message.value

C KafkaConsumer raises TimeoutError due to no messages

D TypeError because event['details'] is not a dictionary

💡 Hint

Check what happens if an OrderCancelled event arrives for an order_id not in state.

If the code tries to delete an order_id that was never added, Python raises a KeyError because the key does not exist in the dictionary.

📝 Syntax

advanced

Which Kafka consumer code snippet correctly applies event sourcing to update user balances?

Choose the code snippet that correctly updates user balances by applying 'Deposit' and 'Withdrawal' events from Kafka.

A

for msg in consumer:
    event = json.loads(msg.value)
    if event['type'] == 'Deposit':
        state[event['user_id']] +=+ event['amount']
    elif event['type'] == 'Withdrawal':
        state[event['user_id']] -=- event['amount']

B

for msg in consumer:
    event = json.loads(msg.value)
    if event['type'] == 'Deposit':
        state[event['user_id']] =+ event['amount']
    elif event['type'] == 'Withdrawal':
        state[event['user_id']] =- event['amount']

C

for msg in consumer:
    event = json.loads(msg.value)
    if event['type'] == 'Deposit':
        state[event['user_id']] = state.get(event['user_id'], 0) + event['amount']
    elif event['type'] == 'Withdrawal':
        state[event['user_id']] = state.get(event['user_id'], 0) - event['amount']

D

for msg in consumer:
    event = json.loads(msg.value)
    if event['type'] == 'Deposit':
        state[event['user_id']] += event['amount']
    elif event['type'] == 'Withdrawal':
        state[event['user_id']] -= event['amount']

💡 Hint

Check for correct use of operators and handling missing keys.

Option C correctly initializes missing keys with 0 and uses += and -= logic properly. Options B and D have invalid operators (=+ and +=+), and C does not handle missing keys causing KeyError.

🚀 Application

expert

How many events are stored in Kafka to reconstruct the state after these operations?

A system uses Kafka for event sourcing. The following events are produced in order:
1. UserCreated (user_id: u1)
2. UserUpdated (user_id: u1, name changed)
3. UserDeactivated (user_id: u1)
4. UserCreated (user_id: u2)
5. UserDeleted (user_id: u1)

How many events must be replayed from Kafka to reconstruct the current state of all users?

A 5

B 4

C 3

D 2

💡 Hint

Event sourcing requires replaying all events to get the current state.

All 5 events are stored and must be replayed in order to reconstruct the current state, even if some events delete or deactivate users.