Consumer Group

A consumer group is a set of consumers working together.

Key Rule

One partition → one consumer (within same group)

Example

Topic: order.created.v1 (3 partitions)

Consumer group: order-service-group

Consumers: Consumer 1 → Partition 0
Consumer 2 → Partition 1
Consumer 3 → Partition 2

Why Consumer Groups Exist

• load balancing
• parallel processing
• scalability

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Multiple Consumer Groups

Topic: order.created.v1

Group 1: notification-service
Group 2: audit-service

Both groups receive all messages independently.

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4. Putting It All Together

Producer
↓
Topic (order.created.v1)
↓
Partitions (0,1,2)
↓
Consumer Group
↓
Consumers process messages

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Real Banking Example

Transaction Service
↓
account.transaction.v1 topic
↓
Partition 0 / 1 / 2
↓
Consumer Group: notification-service
↓
Consumers process events

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Command

Produce Message

kafka-console-producer.sh   --topic order.created.v1   --bootstrap-server localhost:9092

Consume Message

kafka-console-consumer.sh   --topic order.created.v1   --from-beginning   --bootstrap-server localhost:9092

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Failure Scenarios

Consumer Crash

• Kafka reassigns partition to another consumer
• Messages reprocessed if not committed

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More Consumers than Partitions

Example: 3 partitions, 5 consumers

Result: 2 consumers remain idle

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Fewer Consumers than Partitions

Example: 3 partitions, 2 consumers

Result: - one consumer handles multiple partitions

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Key

• Topic = logical grouping
• Partition = scalability + ordering unit
• Consumer Group = parallel processing

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