CQRS pattern in Kafka - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of Kafka message flow in CQRS

Given the following Kafka producer and consumer setup in a CQRS pattern, what will be the output printed by the consumer?

Kafka

from kafka import KafkaProducer, KafkaConsumer
import json

producer = KafkaProducer(bootstrap_servers='localhost:9092', value_serializer=lambda v: json.dumps(v).encode('utf-8'))
consumer = KafkaConsumer('write_topic', bootstrap_servers='localhost:9092', value_deserializer=lambda m: json.loads(m.decode('utf-8')))

# Producer sends a command
producer.send('write_topic', {'command': 'create_order', 'order_id': 123})
producer.flush()

# Consumer reads the command
for message in consumer:
    print(f"Received command: {message.value['command']} with order_id {message.value['order_id']}")
    break

A Received command: create_order with order_id None

B Received command: create_order with order_id '123'

C No output, consumer times out

D Received command: create_order with order_id 123

💡 Hint

Remember that JSON serialization converts integers as numbers, not strings.

The producer sends a JSON message with order_id as an integer 123. The consumer deserializes it back to a Python dict with order_id as int. So the printed output shows 123 without quotes.

🧠 Conceptual

intermediate

CQRS pattern separation of concerns

In a CQRS pattern using Kafka, which statement best describes the role of the write and read sides?

A The write side handles queries; the read side handles commands and updates state.

B The write side handles commands and updates state; the read side handles queries and returns data.

C Both write and read sides handle commands and queries interchangeably.

D The write side only stores data; the read side only deletes data.

💡 Hint

Think about separating responsibilities for updating and reading data.

CQRS stands for Command Query Responsibility Segregation. Commands change state (write side), queries read state (read side).

🔧 Debug

advanced

Identify the error in Kafka consumer code for CQRS read model

What error will this Kafka consumer code produce when trying to read from the 'read_topic' in a CQRS pattern? consumer = KafkaConsumer('read_topic', bootstrap_servers='localhost:9092') for message in consumer: data = message.value print(f"Read model data: {data['id']}")

Kafka

consumer = KafkaConsumer('read_topic', bootstrap_servers='localhost:9092')
for message in consumer:
    data = message.value
    print(f"Read model data: {data['id']}")

A KeyError: 'id'

B KafkaTimeoutError: Failed to fetch messages

C TypeError: 'bytes' object is not subscriptable

D No error, prints the id correctly

💡 Hint

Check how KafkaConsumer returns message values by default.

By default, KafkaConsumer returns message.value as bytes. Accessing data['id'] causes TypeError because bytes are not subscriptable like dicts.

📝 Syntax

advanced

Correct Kafka producer code for sending a command in CQRS

Which option is the correct Kafka producer code snippet to send a command message with key 'order-1' and value {'action': 'update', 'order_id': 1} in CQRS?

A producer.send('commands', key=b'order-1', value=json.dumps({'action': 'update', 'order_id': 1}).encode('utf-8'))

B producer.send('commands', key='order-1', value=json.dumps({'action': 'update', 'order_id': 1}))

C producer.send('commands', key=b'order-1', value={'action': 'update', 'order_id': 1})

D producer.send('commands', key='order-1', value={'action': 'update', 'order_id': 1})

💡 Hint

Kafka keys and values must be bytes; JSON strings must be encoded.

Kafka producer expects key and value as bytes. Option A correctly encodes JSON string to bytes and uses bytes key.

🚀 Application

expert

Designing event sourcing with CQRS and Kafka

You want to implement event sourcing with CQRS using Kafka. Which approach correctly ensures that all state changes are captured and read models are updated asynchronously?

A Write side produces events to a Kafka topic; read side consumes events and updates read models asynchronously.

B Both write and read sides produce and consume events from the same Kafka topic simultaneously.

C Write side updates database directly; read side queries database without Kafka involvement.

D Read side produces events to Kafka; write side consumes events and updates state synchronously.

💡 Hint

Think about how event sourcing captures changes and how read models stay updated.

In event sourcing with CQRS, the write side produces events representing state changes. The read side consumes these events asynchronously to update read models, ensuring eventual consistency.