Kafka integration basics in Spring Boot - Step-by-Step Execution

Concept Flow - Kafka integration basics

Start Spring Boot App

↓

Configure Kafka Producer

↓

Send Message to Topic

↓

Kafka Broker Receives Message

↓

Configure Kafka Consumer

↓

Consumer Listens to Topic

↓

Process Received Message

↓

End

This flow shows how a Spring Boot app configures Kafka producer and consumer, sends a message, and processes it.

Execution Sample

Spring Boot

1. @SpringBootApplication
2. @KafkaListener(topics = "test-topic")
3. public void listen(String msg) { System.out.println(msg); }
4. kafkaTemplate.send("test-topic", "Hello Kafka");

This code sends a message to Kafka and listens to the same topic to print received messages.

Execution Table

Step	Action	Component	Message/Value	Result
1	Start Spring Boot Application	Application	-	App initialized with Kafka configs
2	Configure Kafka Producer	Producer	-	Producer ready to send messages
3	Configure Kafka Consumer with @KafkaListener	Consumer	topic=test-topic	Consumer subscribed to topic
4	Send message 'Hello Kafka' to 'test-topic'	Producer	Hello Kafka	Message sent to Kafka broker
5	Kafka broker receives message	Kafka Broker	Hello Kafka	Message stored in topic partition
6	Consumer listens and receives message	Consumer	Hello Kafka	Message received by consumer
7	Consumer processes message	Consumer	Hello Kafka	Message printed to console
8	End	-	-	Message flow complete

💡 Message sent and received successfully; flow ends after processing.

Variable Tracker

Variable	Start	After Step 4	After Step 6	Final
message	null	"Hello Kafka" (sent)	"Hello Kafka" (received)	null (processed)
producerReady	false	true	true	true
consumerReady	false	true	true	true

Key Moments - 3 Insights

Why does the consumer receive the message after it is sent?

Because the consumer is subscribed to the same topic and listens continuously, as shown in execution_table step 6.

What happens if the producer sends a message before the consumer is ready?

The message stays in the Kafka broker until the consumer subscribes and receives it, ensuring no message loss.

Why do we use @KafkaListener annotation?

It tells Spring Boot which method should listen to messages from a specific Kafka topic, as seen in step 3.

Visual Quiz - 3 Questions

Look at the execution table, at which step does the Kafka broker store the message?

A Step 6

B Step 4

C Step 5

D Step 3

💡 Hint

Check the 'Kafka Broker' component action in the execution_table rows.

According to variable_tracker, what is the value of 'message' after step 6?

A "Hello Kafka" (received)

B null

C "Hello Kafka" (sent)

D undefined

💡 Hint

Look at the 'message' row under 'After Step 6' column in variable_tracker.

If the consumer was not configured, what would happen to the message?

A Message would be lost immediately

B Message would remain in Kafka broker until consumed

C Message would be printed to console anyway

D Application would crash

💡 Hint

Refer to key_moments explanation about message storage in Kafka broker.

Concept Snapshot

Kafka integration in Spring Boot:
- Configure producer and consumer beans
- Use kafkaTemplate.send(topic, message) to send
- Use @KafkaListener to receive messages
- Messages go through Kafka broker
- Consumer processes messages asynchronously
- Ensures reliable message delivery

Full Transcript

This visual execution trace shows how a Spring Boot application integrates with Kafka. The app starts and configures a Kafka producer and consumer. The producer sends a message 'Hello Kafka' to a topic named 'test-topic'. The Kafka broker receives and stores this message. The consumer, subscribed to the same topic via @KafkaListener, receives the message and processes it by printing to the console. Variables like 'message', 'producerReady', and 'consumerReady' track the state changes during execution. Key moments clarify why the consumer receives messages and the role of the Kafka broker. The quiz tests understanding of message flow and state changes. This trace helps beginners see step-by-step how Kafka messaging works in Spring Boot.