Controller broker in Kafka - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Role of the Controller Broker in Kafka

What is the primary responsibility of the controller broker in a Kafka cluster?

A Managing partition leader elections and cluster metadata updates

B Balancing network traffic between brokers

C Storing all topic data permanently on disk

D Serving client requests for producing and consuming messages

💡 Hint

Think about which broker coordinates cluster-wide decisions.

The controller broker is responsible for managing partition leader elections and updating cluster metadata to keep the cluster consistent.

❓ Predict Output

intermediate

Controller Broker Leader Election Output

Given the following Kafka controller election log snippet, what will be the output message indicating the new controller?

Kafka

INFO Controller 2 is now the leader for the cluster

A ERROR Controller 1 failed to become leader

B INFO Controller 2 is now the leader for the cluster

C WARN Controller 3 lost leadership

D DEBUG Controller election started

💡 Hint

Look for the message that confirms the new controller leader.

The log message confirms that broker 2 has become the new controller leader.

🔧 Debug

advanced

Identifying Controller Broker Failure Cause

Which of the following log entries indicates a controller broker failure due to session timeout?

A INFO Controller 1 successfully refreshed session

B WARN Controller 2 received heartbeat from broker 3

C ERROR Controller 1 session expired, triggering leader election

D DEBUG Controller 3 started election process

💡 Hint

Look for a message about session expiration or failure.

A session expiration error indicates the controller broker lost its leadership and triggers a new election.

📝 Syntax

advanced

Kafka Controller Broker Configuration Syntax

Which of the following is the correct syntax to configure a Kafka broker as a controller candidate in the server.properties file?

A controller.quorum.voters=1@broker1:9093,2@broker2:9093,3@broker3:9093

B controller.voters=broker1:9093,broker2:9093,broker3:9093

C controller.broker.ids=1,2,3

D controller.candidates=broker1,broker2,broker3

💡 Hint

Check the official Kafka configuration property for controller quorum voters.

The correct property is controller.quorum.voters with format id@host:port for each voter.

🚀 Application

expert

Controller Broker Failover Scenario

In a Kafka cluster with three brokers (1, 2, 3), broker 1 is the current controller. Broker 1 crashes unexpectedly. What is the expected sequence of events for controller failover?

A All brokers simultaneously try to become controller causing conflict

B Broker 3 immediately takes over without election

C Cluster stops processing until broker 1 recovers

D Broker 2 detects broker 1 failure, triggers leader election, becomes new controller

💡 Hint

Consider how Kafka handles controller failover with leader election.

When the controller broker fails, another broker detects the failure and triggers a leader election to select a new controller.

Practice

1.

What is the main role of the controller broker in a Kafka cluster?

easy

A. Manage leader election and cluster metadata updates

B. Store all the messages permanently

C. Serve as a client to produce messages

D. Handle consumer group offsets only

Solution

1. Step 1: Understand controller broker responsibilities

   The controller broker is responsible for managing cluster-wide tasks such as leader election and metadata updates.

2. Step 2: Compare other options

   Storing messages, producing messages, and handling consumer offsets are roles of other brokers or clients, not the controller.

3. Final Answer:

   Manage leader election and cluster metadata updates -> Option A

4. Quick Check:

   Controller broker = leader election manager [OK]

Hint: Controller broker manages cluster tasks like leader election [OK]

Common Mistakes:

• Confusing controller with brokers that store data
• Thinking controller handles only consumer offsets
• Assuming controller is a client

2.

Which of the following is the correct way to identify the controller broker using Kafka's command line tool?

kafka-metadata-quorum.sh --bootstrap-server localhost:9092 status

easy

A. Find the broker with the lowest port number

B. Check the broker with the highest broker ID

C. Look for the LeaderId in the output

D. Look for the broker with the 'leader' role in the output

Solution

1. Step 1: Understand the command output

   The kafka-metadata-quorum.sh status tool shows metadata quorum details including LeaderId, which is the ID of the current controller broker.

2. Step 2: Identify controller in output

   The controller broker is identified by the LeaderId field, not by broker ID, port number, or generic leader role.

3. Final Answer:

   Look for the LeaderId in the output -> Option C

4. Quick Check:

   LeaderId = controller broker ID [OK]

Hint: LeaderId in metadata quorum status identifies controller [OK]

Common Mistakes:

• Assuming highest broker ID means controller
• Using port number to find controller
• Confusing generic leader role with LeaderId

3.

Given the following code snippet using Kafka AdminClient in Java, what will controllerId hold after execution?

AdminClient admin = AdminClient.create(props);
Node controller = admin.describeCluster().controller().get();
int controllerId = controller.id();
admin.close();

medium

A. The total number of brokers in the cluster

B. The topic partition count

C. The port number of the controller broker

D. The broker ID of the current controller broker

Solution

1. Step 1: Analyze AdminClient describeCluster method

   The describeCluster().controller() returns the current controller broker as a Node object.

2. Step 2: Understand Node.id()

   The id() method returns the broker ID of that Node, which is the controller broker's ID.

3. Final Answer:

   The broker ID of the current controller broker -> Option D

4. Quick Check:

   controller().id() = controller broker ID [OK]

Hint: controller().id() returns controller broker's ID [OK]

Common Mistakes:

• Confusing controller ID with number of brokers
• Thinking it returns port or partition count
• Ignoring the blocking get() call

4.

Consider this snippet to get the controller broker ID using Kafka AdminClient in Java:

AdminClient admin = AdminClient.create(props);
Node controller = admin.describeCluster().controller().get();
int controllerId = controller.id();
admin.close();

What is the likely cause if this code throws a java.util.concurrent.ExecutionException?

medium

A. The close() method was called too early

B. The AdminClient failed to connect to any broker

C. The id() method returns a string, not int

D. The controller() method does not exist

Solution

1. Step 1: Understand ExecutionException cause

   An ExecutionException usually wraps an error during the async call, often connection failure.

2. Step 2: Check AdminClient connectivity

   If AdminClient cannot connect to any broker, the get() call on controller() future will fail with this exception.

3. Final Answer:

   The AdminClient failed to connect to any broker -> Option B

4. Quick Check:

   ExecutionException = connection failure [OK]

Hint: ExecutionException often means connection problem [OK]

Common Mistakes:

• Thinking method does not exist causes ExecutionException
• Confusing return types of id() method
• Assuming close() causes this error

5.

You want to write a Kafka monitoring tool that automatically detects when the controller broker changes and logs the new controller's broker ID. Which approach is best?

1. Use AdminClient to poll describeCluster().controller() periodically.
2. Subscribe to Kafka internal topics to listen for controller changes.
3. Manually check broker logs for controller election messages.
4. Restart all brokers to force controller re-election and detect changes.

hard

A. Use AdminClient to poll describeCluster().controller() periodically

B. Subscribe to Kafka internal topics to listen for controller changes

C. Manually check broker logs for controller election messages

D. Restart all brokers to force controller re-election and detect changes

Solution

1. Step 1: Evaluate monitoring approaches

   Polling describeCluster().controller() via AdminClient is a reliable, programmatic way to detect controller changes.

2. Step 2: Assess other options

   Subscribing to internal topics is complex and not guaranteed; checking logs is manual and slow; restarting brokers is disruptive and impractical.

3. Final Answer:

   Use AdminClient to poll describeCluster().controller() periodically -> Option A

4. Quick Check:

   Polling controller() is best for monitoring [OK]

Hint: Poll controller() with AdminClient for changes [OK]

Common Mistakes:

• Trying to read internal topics for controller changes
• Relying on manual log inspection
• Restarting brokers unnecessarily