Partition key and routing in Kafka - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the partition chosen for this Kafka message?

Given a Kafka topic with 3 partitions and the following producer code, what partition will the message be sent to?

Kafka

from kafka import KafkaProducer

producer = KafkaProducer(bootstrap_servers='localhost:9092')

# Key is a string 'user42'
key = b'user42'
value = b'Hello Kafka'

# Send message
future = producer.send('my_topic', key=key, value=value)
record_metadata = future.get(timeout=10)

print(record_metadata.partition)

A Partition 0

B Partition 3

C Partition 2

D Partition 1

💡 Hint

Kafka uses the hash of the key modulo the number of partitions to decide the partition.

Kafka hashes the key 'user42' and then takes modulo 3 (number of partitions). The hash of 'user42' modulo 3 results in partition 1.

🧠 Conceptual

intermediate

Why is using a partition key important in Kafka?

Select the main reason why using a partition key is important when producing messages to Kafka.

A It ensures messages with the same key go to the same partition, preserving order.

B It encrypts the message content for security.

C It compresses the message to save bandwidth.

D It automatically retries sending messages on failure.

💡 Hint

Think about message ordering guarantees in Kafka.

Kafka guarantees order only within a partition. Using a key ensures all messages with that key go to the same partition, preserving order.

🔧 Debug

advanced

Why does this Kafka producer code cause uneven message distribution?

This Kafka producer sends messages with keys that are always None. What is the likely effect on partition distribution?

Kafka

from kafka import KafkaProducer

producer = KafkaProducer(bootstrap_servers='localhost:9092')

for i in range(10):
    producer.send('my_topic', key=None, value=f'Message {i}'.encode())

producer.flush()

A Messages are evenly distributed across all partitions.

B All messages go to the same partition, causing uneven load.

C Messages cause a runtime error because key cannot be None.

D Messages are dropped silently without sending.

💡 Hint

What happens when no key is provided in Kafka producer?

When key is None, Kafka uses a round-robin or default partitioner which may send all messages to the same partition if the producer is single-threaded or due to batching.

📝 Syntax

advanced

Which Kafka producer code snippet correctly sets a partition key?

Choose the code snippet that correctly sets a string key for Kafka message production.

A producer.send('topic', key='user1', value=b'data')

B producer.send('topic', key=123, value=b'data')

C producer.send('topic', key=b'user1', value=b'data')

D producer.send('topic', key=None, value=b'data')

💡 Hint

Kafka keys must be bytes, not strings or integers.

Kafka producer expects the key as bytes. Passing a string without encoding causes an error. Passing an integer is invalid. None means no key.

🚀 Application

expert

How many partitions will contain messages after this code runs?

A Kafka topic has 4 partitions. The producer sends messages with keys cycling through ['A', 'B', 'C']. How many partitions will have messages after sending 12 messages?

Kafka

keys = [b'A', b'B', b'C']
for i in range(12):
    key = keys[i % 3]
    producer.send('my_topic', key=key, value=b'data')
producer.flush()

A 3 partitions

B 4 partitions

C 1 partition

D 0 partitions

💡 Hint

Messages with the same key always go to the same partition.

There are 3 distinct keys cycling, so messages go to 3 partitions. The 4th partition gets no messages.