Subscribing to topics in Kafka - Step-by-Step Execution

Process Flow - Subscribing to topics

Create Kafka Consumer

↓

Configure Consumer Settings

↓

Subscribe to Topic(s)

↓

Poll for Messages

↓

Process Messages

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Repeat Polling or Exit

This flow shows how a Kafka consumer is created, configured, subscribes to topics, polls messages, and processes them repeatedly.

Execution Sample

Kafka

from kafka import KafkaConsumer
consumer = KafkaConsumer('my_topic', bootstrap_servers=['localhost:9092'])
for message in consumer:
    print(message.value.decode('utf-8'))

This code creates a Kafka consumer that subscribes to 'my_topic' and prints each message received.

Process Table

Step	Action	Evaluation	Result
1	Create KafkaConsumer object	KafkaConsumer('my_topic', ...)	Consumer created, ready to subscribe
2	Subscribe to 'my_topic'	Subscription set	Consumer listens to 'my_topic'
3	Poll for messages	Check if messages available	No messages yet, wait
4	Receive message	Message received from 'my_topic'	Message object with value bytes
5	Decode message.value	message.value.decode('utf-8')	String message content
6	Print message content	print output	Message content shown on screen
7	Repeat polling	Loop continues	Wait for next message or exit

💡 Consumer stops when program ends or interrupted

Status Tracker

Variable	Start	After Step 1	After Step 4	After Step 5	Final
consumer	None	KafkaConsumer object	KafkaConsumer with message ready	KafkaConsumer object	KafkaConsumer looping for messages
message	None	None	Message object with bytes	Message object with bytes	Next message or None

Key Moments - 3 Insights

Why do we decode message.value before printing?

Kafka messages are bytes, not strings. Decoding converts bytes to readable text, as shown in step 5 of the execution_table.

What happens if no messages are available when polling?

The consumer waits (blocks) until messages arrive, as seen in step 3 where polling checks but finds no messages yet.

How does the consumer know which topic to listen to?

The topic is specified when creating the consumer or by calling subscribe(), as in step 2 where 'my_topic' is set.

Visual Quiz - 3 Questions

Look at the execution_table, what is the state of 'message' after step 4?

A Decoded string message

B Message object with bytes

C None

D KafkaConsumer object

💡 Hint

Check the 'message' variable in variable_tracker after step 4 and execution_table row 4

At which step does the consumer start listening to 'my_topic'?

A Step 2

B Step 3

C Step 1

D Step 5

💡 Hint

Look at execution_table row 2 where subscription is set

If the topic name changes, which step in the execution_table would be affected?

A Step 3

B Step 5

C Step 1

D Step 6

💡 Hint

Topic is specified when creating the consumer object in step 1

Concept Snapshot

Kafka Consumer Subscription:
- Create KafkaConsumer with topic(s)
- Consumer subscribes to topics automatically
- Poll messages in a loop
- Messages are bytes, decode before use
- Process messages as they arrive
- Loop continues until stopped

Full Transcript

This visual execution shows how a Kafka consumer subscribes to topics and processes messages. First, a KafkaConsumer object is created with the topic name. Then the consumer subscribes to that topic and waits for messages. When messages arrive, they are received as bytes and decoded to strings before printing. The consumer keeps polling and processing messages until the program stops. Key points include decoding bytes to strings and understanding the subscription step.