Consumer poll loop in Kafka - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to poll messages from the Kafka consumer.

Kafka

ConsumerRecords<String, String> records = consumer.[1](Duration.ofMillis(100));

Drag options to blanks, or click blank then click option'

AcommitSync

Bsubscribe

Cpoll

Dclose

Attempts:

3 left

💡 Hint

Common Mistakes

Using subscribe instead of poll to fetch messages.

Calling commitSync before polling messages.

The poll method is used to fetch data from Kafka topics.

2fill in blank

medium

Complete the code to iterate over the polled records.

Kafka

for (ConsumerRecord<String, String> record : [1]) {
    System.out.println(record.value());
}

Drag options to blanks, or click blank then click option'

Arecords

Bconsumer

Ctopic

Dpartition

Attempts:

3 left

💡 Hint

Common Mistakes

Trying to iterate over the consumer object instead of the records.

Using topic or partition which are not iterable collections.

The records variable holds the polled messages to iterate over.

3fill in blank

hard

Fix the error in committing offsets synchronously after processing records.

Kafka

consumer.[1]();

Drag options to blanks, or click blank then click option'

AcommitAsync

BcommitSync

Cpoll

Dsubscribe

Attempts:

3 left

💡 Hint

Common Mistakes

Using commitAsync when synchronous commit is required.

Calling poll or subscribe instead of commit methods.

commitSync() commits the offsets synchronously to Kafka after processing.

4fill in blank

hard

Fill both blanks to create a consumer poll loop that processes messages and commits offsets.

Kafka

while (true) {
    ConsumerRecords<String, String> records = consumer.[1](Duration.ofMillis(100));
    for (ConsumerRecord<String, String> record : records) {
        System.out.println(record.value());
    }
    consumer.[2]();
}

Drag options to blanks, or click blank then click option'

Apoll

Bsubscribe

CcommitSync

Dclose

Attempts:

3 left

💡 Hint

Common Mistakes

Using subscribe instead of poll to fetch messages.

Using commitAsync instead of commitSync when synchronous commit is needed.

The loop polls messages with poll() and commits offsets synchronously with commitSync().

5fill in blank

hard

Fill all three blanks to create a consumer poll loop that subscribes to a topic, polls messages, and commits offsets.

Kafka

consumer.[1](Collections.singletonList("my-topic"));

while (true) {
    ConsumerRecords<String, String> records = consumer.[2](Duration.ofMillis(100));
    for (ConsumerRecord<String, String> record : records) {
        System.out.println(record.value());
    }
    consumer.[3]();
}

Drag options to blanks, or click blank then click option'

Asubscribe

Bpoll

CcommitSync

Dclose

Attempts:

3 left

💡 Hint

Common Mistakes

Calling poll before subscribe.

Using commitAsync instead of commitSync.

Calling close inside the loop.

The consumer first subscribes to the topic with subscribe(), then polls messages with poll(), and finally commits offsets synchronously with commitSync().

Practice

1. What is the main purpose of using a poll() method inside a Kafka consumer loop?

easy

A. To send messages to Kafka topics

B. To continuously fetch new messages from Kafka topics

C. To close the consumer connection

D. To create a new Kafka topic

Solution

1. Step 1: Understand the role of poll() in Kafka consumer

   The poll() method is used to fetch messages from Kafka topics.

2. Step 2: Recognize the loop usage

   Using poll() inside a loop allows the consumer to keep listening and fetching new messages continuously.

3. Final Answer:

   To continuously fetch new messages from Kafka topics -> Option B

4. Quick Check:

   poll() fetches messages repeatedly [OK]

Hint: poll() inside loop fetches messages repeatedly [OK]

Common Mistakes:

• Thinking poll() sends messages
• Confusing poll() with closing connection
• Assuming poll() creates topics

2. Which of the following is the correct syntax to start a Kafka consumer poll loop in Java?

easy

A. consumer.poll(Duration(100));

B. for(int i=0; i<10; i++) consumer.poll(100);

C. consumer.poll(100); while(true) {}

D. while(true) { consumer.poll(Duration.ofMillis(100)); }

Solution

1. Step 1: Check correct poll method usage

   The poll method requires a Duration argument like Duration.ofMillis(100) in Java.

2. Step 2: Verify loop structure

   A continuous loop like while(true) is used to keep polling messages repeatedly.

3. Final Answer:

   while(true) { consumer.poll(Duration.ofMillis(100)); } -> Option D

4. Quick Check:

   Correct poll syntax with continuous loop [OK]

Hint: Use while(true) with poll(Duration.ofMillis(x)) [OK]

Common Mistakes:

• Using poll with int instead of Duration
• Not using a loop for continuous polling
• Incorrect Duration constructor syntax

3. What will be the output of the following Kafka consumer poll loop snippet?

while (true) {
  var records = consumer.poll(Duration.ofMillis(100));
  System.out.println(records.count());
  break;
}

medium

A. Prints the number of records fetched in one poll and exits

B. Prints zero and continues infinitely

C. Throws an exception because of missing commit

D. Does not compile due to syntax error

Solution

1. Step 1: Analyze the poll call and record count

   The poll() fetches records and records.count() returns how many were fetched.

2. Step 2: Understand the loop and break

   The loop runs once because of break; after printing count, so it prints the count once and stops.

3. Final Answer:

   Prints the number of records fetched in one poll and exits -> Option A

4. Quick Check:

   poll() fetches records, break stops loop [OK]

Hint: Break after poll prints count once [OK]

Common Mistakes:

• Assuming infinite loop without break
• Thinking commit is mandatory to print count
• Confusing syntax causing compile error

4. Identify the error in this Kafka consumer poll loop code snippet:

while (true) {
  var records = consumer.poll(100);
  for (var record : records) {
    System.out.println(record.value());
  }
}

medium

A. poll() argument should be a Duration, not an int

B. for-each loop syntax is incorrect

C. Missing consumer.subscribe() call before loop

D. System.out.println cannot print record values

Solution

1. Step 1: Check poll method argument type

   The poll() method requires a Duration argument, not a plain integer.

2. Step 2: Confirm other code parts

   The for-each loop and print statement are correct; missing subscribe is not shown as error here.

3. Final Answer:

   poll() argument should be a Duration, not an int -> Option A

4. Quick Check:

   poll() needs Duration argument [OK]

Hint: poll() needs Duration, not int [OK]

Common Mistakes:

• Passing int instead of Duration to poll()
• Confusing for-each syntax
• Assuming subscribe() missing causes compile error

5. You want to process messages from multiple Kafka topics continuously and commit offsets manually after processing. Which code snippet correctly implements this using a consumer poll loop?

hard

A. consumer.poll(Duration.ofMillis(200));
consumer.commitSync();

B. while(true) {
var records = consumer.poll(200);
for(var record : records) {
process(record);
}
consumer.commitAsync();
}

C. while(true) {
var records = consumer.poll(Duration.ofMillis(200));
for(var record : records) {
process(record);
}
consumer.commitSync();
}

D. for(int i=0; i<10; i++) {
var records = consumer.poll(Duration.ofMillis(200));
consumer.commitSync();
}

Solution

1. Step 1: Check poll argument and loop structure

   while(true) { var records = consumer.poll(Duration.ofMillis(200)); for(var record : records) { process(record); } consumer.commitSync(); } uses poll(Duration.ofMillis(200)) inside a continuous while(true) loop, which is correct.

2. Step 2: Verify message processing and manual commit

   It processes each record and calls commitSync() after processing, ensuring offsets are committed manually.

3. Final Answer:

   while(true) { var records = consumer.poll(Duration.ofMillis(200)); for(var record : records) { process(record); } consumer.commitSync(); } -> Option C

4. Quick Check:

   Continuous poll, process, then manual commit [OK]

Hint: Use while(true), poll with Duration, then commitSync() [OK]

Common Mistakes:

• Passing int instead of Duration to poll()
• Using for loop instead of continuous loop
• Not processing records before commit
• Missing loop causing single poll only