Spring Boot - Start/Stop a Kafka Listener Dynamically

Apache Kafka is a distributed streaming platform that's widely used for building real-time data pipelines. With Spring Boot, it's straightforward to set up Kafka consumers using the Spring Kafka project. Sometimes, there might be a requirement to start or stop Kafka listeners dynamically, based on specific conditions or external commands. This can be achieved using the KafkaListenerEndpointRegistry provided by Spring Kafka.

Here's a basic guide on how to start/stop a Kafka listener dynamically:

Add Dependencies: First, ensure that you have the necessary dependencies in your Maven pom.xml:

<dependency>
    <groupId>org.springframework.kafka</groupId>
    <artifactId>spring-kafka</artifactId>
</dependency>

Create a Kafka Listener: Define a Kafka listener using the @KafkaListener annotation.

@Service
public class KafkaConsumerService {

    @KafkaListener(id = "myKafkaListener", topics = "myTopic")
    public void listen(String message) {
        System.out.println("Received Message: " + message);
    }

}

Control the Listener Using KafkaListenerEndpointRegistry: You can autowire the KafkaListenerEndpointRegistry and use it to control the listener's lifecycle.

@Service
public class KafkaListenerControlService {

    @Autowired
    private KafkaListenerEndpointRegistry kafkaListenerEndpointRegistry;

    public void startListener() {
        MessageListenerContainer listenerContainer = kafkaListenerEndpointRegistry.getListenerContainer("myKafkaListener");
        listenerContainer.start();
    }

    public void stopListener() {
        MessageListenerContainer listenerContainer = kafkaListenerEndpointRegistry.getListenerContainer("myKafkaListener");
        listenerContainer.stop();
    }

    public boolean isListenerRunning() {
        MessageListenerContainer listenerContainer = kafkaListenerEndpointRegistry.getListenerContainer("myKafkaListener");
        return listenerContainer.isRunning();
    }
}

Use the Service: You can now use KafkaListenerControlService to start, stop, or check the status of the Kafka listener as needed.

For instance, you can bind these control methods to API endpoints, scheduled tasks, or other triggers based on your application's requirements. This gives you the flexibility to control when your application starts or stops consuming messages from Kafka.

Controlling Kafka listener lifecycle in Spring Boot:
- Description: Spring Kafka allows you to control the lifecycle of Kafka listeners, enabling customization during initialization, start, and shutdown.
- Code:
```
@KafkaListener(id = "myListener", topics = "myTopic")
public void listen(String message) {
    // Kafka listener logic
}
```

Dynamically enabling and disabling Kafka listeners in Spring Boot:

Description: Use conditional flags or properties to dynamically enable or disable Kafka listeners based on runtime conditions.

Code:

# application.yml
kafka:
  listener:
    enabled: true

@KafkaListener(id = "myListener", topics = "myTopic", autoStartup = "${kafka.listener.enabled}")
public void listen(String message) {
    // Kafka listener logic
}

Conditional Kafka listener activation in Spring Boot:

Description: Apply conditions based on certain criteria for Kafka listener activation, such as the presence of a specific property.

Code:

@ConditionalOnProperty(name = "kafka.listener.enabled", havingValue = "true")
@KafkaListener(id = "myListener", topics = "myTopic")
public void listen(String message) {
    // Kafka listener logic
}

Programmatic control of Kafka listener in Spring Boot:

Description: Use the KafkaListenerEndpointRegistry to programmatically control the state of Kafka listeners.

Code:

@Autowired
private KafkaListenerEndpointRegistry registry;

// Programmatically stop a listener
registry.getListenerContainer("myListener").stop();

// Programmatically start a listener
registry.getListenerContainer("myListener").start();

Pause and resume Kafka listeners dynamically in Spring Boot:

Description: Pause and resume Kafka listeners dynamically using the KafkaListenerEndpointRegistry.

Code:

@Autowired
private KafkaListenerEndpointRegistry registry;

// Pause a listener
registry.getListenerContainer("myListener").pause();

// Resume a listener
registry.getListenerContainer("myListener").resume();

Custom annotations for dynamic Kafka listener management in Spring Boot:

Description: Create custom annotations to encapsulate dynamic listener management behavior and apply them to Kafka listener methods.

Code:

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.METHOD)
public @interface DynamicKafkaListener {
    boolean enabled() default true;
}

@DynamicKafkaListener
@KafkaListener(id = "myListener", topics = "myTopic", autoStartup = "#{@dynamicKafkaListener.enabled}")
public void listen(String message) {
    // Kafka listener logic
}

Using Actuator endpoints to start/stop Kafka listeners in Spring Boot:
- Description: Utilize Spring Boot Actuator endpoints to manage Kafka listeners dynamically.
- Code:
```
# application.yml
management:
  endpoints:
    web:
      exposure:
        include: kafka-listeners
```
```
@KafkaListener(id = "myListener", topics = "myTopic")
public void listen(String message) {
    // Kafka listener logic
}
```
  Access the /actuator/kafka-listeners endpoint to start, stop, or obtain information about Kafka listeners.
Graceful shutdown of Kafka listeners in Spring Boot:
- Description: Ensure graceful shutdown of Kafka listeners during application shutdown to avoid message loss.
- Code:
```
@PreDestroy
public void onDestroy() {
    registry.getListenerContainer("myListener").stop();
}
```