Consumer

A consumer reads messages from the messaging system and processes them.

Example:

Payment Service consumes OrderCreated event.

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How Consumers Work

Consumers pull messages from broker.

Flow:

Consumer → request messages → Broker

Broker sends available messages.

Kafka is pull-based (not push).

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Example Consumer (Spring Kafka)

Configuration

spring:
  kafka:
    consumer:
      group-id: order-group

Consumer Code

@KafkaListener(topics = "order-created")
public void consume(String message) {
    System.out.println(message);
}

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Consumer Example in Microservices

Event:

OrderCreated

Consumers:

Payment Service
Inventory Service
Email Service

Each service processes the event independently.

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What Happens If Consumer Crashes

Kafka guarantees at‑least‑once delivery.

If a consumer crashes before completing processing:

Message may be reprocessed.

Therefore consumers must be idempotent.

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Consumer Scaling

Consumers can scale horizontally.

Example topic partitions:

Partition 0
Partition 1
Partition 2

Consumers:

Consumer 1 → Partition 0
Consumer 2 → Partition 1
Consumer 3 → Partition 2

This is handled through consumer groups.

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