Subscribing to topics in Kafka - Mini Project: Build & Apply

Subscribing to Kafka Topics

📖 Scenario: You are building a simple Kafka consumer application that listens to messages from a specific topic. This is common in real-world systems where services communicate by sending messages through Kafka topics.

🎯 Goal: Create a Kafka consumer that subscribes to a topic named test-topic and prints any messages it receives.

📋 What You'll Learn

Create a Kafka consumer configuration dictionary with the required settings

Create a variable called topic with the value 'test-topic'

Subscribe the consumer to the topic

Print the messages received from the topic

💡 Why This Matters

🌍 Real World

Kafka consumers are used in real systems to process streams of data like logs, user activity, or sensor data.

💼 Career

Understanding how to subscribe and consume messages from Kafka topics is essential for backend developers, data engineers, and anyone working with real-time data pipelines.

1

Create Kafka consumer configuration

Create a dictionary called consumer_config with these exact entries: 'bootstrap.servers': 'localhost:9092', 'group.id': 'my-group', and 'auto.offset.reset': 'earliest'.

Kafka

# Your code here

Hint

Use a Python dictionary with the exact keys and values as shown.

2

Set the topic name

Create a variable called topic and set it to the string 'test-topic'.

Kafka

consumer_config = {
    'bootstrap.servers': 'localhost:9092',
    'group.id': 'my-group',
    'auto.offset.reset': 'earliest'
}
# Your code here

Hint

Assign the string 'test-topic' to the variable named topic.

3

Subscribe the consumer to the topic

Import Consumer from confluent_kafka. Create a Consumer instance called consumer using consumer_config. Use consumer.subscribe([topic]) to subscribe to the topic.

Kafka

consumer_config = {
    'bootstrap.servers': 'localhost:9092',
    'group.id': 'my-group',
    'auto.offset.reset': 'earliest'
}
topic = 'test-topic'
# Your code here

Hint

Use the Consumer class and call subscribe with a list containing topic.

4

Print messages from the topic

Write a loop that calls consumer.poll(1.0) to get messages. If a message is received and no error, print message.value().decode('utf-8'). Break the loop after printing one message. Finally, call consumer.close().

Kafka

from confluent_kafka import Consumer

consumer_config = {
    'bootstrap.servers': 'localhost:9092',
    'group.id': 'my-group',
    'auto.offset.reset': 'earliest'
}
topic = 'test-topic'

consumer = Consumer(consumer_config)
consumer.subscribe([topic])
# Your code here

Hint

Use poll to get messages, check for errors, decode the message value, print it, and then close the consumer.

Practice

1. What is the main purpose of the subscribe() method in Kafka consumers?

easy

A. To close the Kafka consumer connection

B. To send messages to Kafka topics

C. To listen to one or more Kafka topics and receive messages automatically

D. To create new Kafka topics

Solution

1. Step 1: Understand the role of subscribe()

   The subscribe() method is used by Kafka consumers to specify which topics they want to listen to.

2. Step 2: Identify what subscribe() enables

   Once subscribed, the consumer automatically receives messages from those topics when calling poll().

3. Final Answer:

   To listen to one or more Kafka topics and receive messages automatically -> Option C

4. Quick Check:

   subscribe() = listen to topics [OK]

Hint: subscribe() means start listening to topics [OK]

Common Mistakes:

• Confusing subscribe() with sending messages
• Thinking subscribe() creates topics
• Assuming subscribe() closes connections

2. Which of the following is the correct way to subscribe a Kafka consumer to multiple topics named "topicA" and "topicB"?

easy

A. consumer.subscribe(["topicA", "topicB"])

B. consumer.subscribe("topicA", "topicB")

C. consumer.subscribe("topicA" + "," + "topicB")

D. consumer.subscribe({"topicA", "topicB"})

Solution

1. Step 1: Recall the subscribe() method signature

   In Kafka, subscribe() takes a list (or collection) of topic names as an argument.

2. Step 2: Identify correct syntax for multiple topics

   Passing a list like ["topicA", "topicB"] is the correct way to subscribe to multiple topics.

3. Final Answer:

   consumer.subscribe(["topicA", "topicB"]) -> Option A

4. Quick Check:

   subscribe() needs a list for multiple topics [OK]

Hint: Use a list to subscribe to multiple topics [OK]

Common Mistakes:

• Passing multiple string arguments instead of a list
• Concatenating topic names into one string
• Using curly braces instead of square brackets

3. Given the following Kafka consumer code snippet, what will be the output if the consumer is subscribed to topic "news" and there are two messages "Hello" and "World" in the topic?

consumer.subscribe(["news"])
records = consumer.poll(timeout_ms=1000)
for record in records:
    print(record.value)

medium

A. ['Hello', 'World']

B. Hello\nWorld

C. HelloWorld

D. No output because poll() is missing a loop

Solution

1. Step 1: Understand subscribe() and poll() behavior

   The consumer subscribes to "news" and calls poll() once, which fetches available messages.

2. Step 2: Analyze the for loop printing each record

   The loop prints each message value on its own line, so "Hello" and "World" appear separately.

3. Final Answer:

   Hello\nWorld -> Option B

4. Quick Check:

   poll() returns messages, loop prints each [OK]

Hint: poll() returns messages; loop prints each on new line [OK]

Common Mistakes:

• Thinking poll() returns a list printed directly
• Assuming messages print concatenated without newline
• Believing poll() needs a loop to work at all

4. What is wrong with the following Kafka consumer code snippet?

consumer.subscribe("topic1")
while True:
    records = consumer.poll(1000)
    for record in records:
        print(record.value)

medium

A. subscribe() expects a list, not a single string

B. poll() must be called outside the loop

C. print() cannot be used inside the loop

D. The loop should be a for loop, not while

Solution

1. Step 1: Check subscribe() argument type

   The subscribe() method requires a list of topics, even if subscribing to one topic.

2. Step 2: Identify the error in argument

   Passing a single string instead of a list causes a type error or unexpected behavior.

3. Final Answer:

   subscribe() expects a list, not a single string -> Option A

4. Quick Check:

   subscribe() needs list argument [OK]

Hint: Always pass a list to subscribe(), even for one topic [OK]

Common Mistakes:

• Passing a string instead of a list to subscribe()
• Thinking poll() can't be inside a loop
• Misunderstanding loop type requirements

5. You want to subscribe a Kafka consumer to all topics starting with "logs-" dynamically, without listing each topic name. Which approach correctly achieves this?

hard

A. Use consumer.assign() with topic partitions matching "logs-"

B. Manually list all topics starting with "logs-" in a list and pass to subscribe()

C. Call subscribe() multiple times, once per topic starting with "logs-"

D. Use consumer.subscribe(Pattern.compile("logs-.*")) to subscribe by regex pattern

Solution

1. Step 1: Understand dynamic topic subscription

   Kafka consumers can subscribe to topics matching a regex pattern using subscribe(Pattern).

2. Step 2: Identify correct method for pattern subscription

   Using consumer.subscribe(Pattern.compile("logs-.*")) subscribes to all topics starting with "logs-" automatically.

3. Final Answer:

   Use consumer.subscribe(Pattern.compile("logs-.*")) to subscribe by regex pattern -> Option D

4. Quick Check:

   subscribe() supports regex patterns [OK]

Hint: Use regex pattern with subscribe() for dynamic topic matching [OK]

Common Mistakes:

• Listing topics manually instead of using regex
• Calling subscribe() multiple times incorrectly
• Using assign() which requires partitions, not topics