BAEL-6406 - Understanding Kafka Topics and Partitions (#14324)

* consumer config. topic config. driver and calculator classes.

* basic app working. test structure

* final version

* wraping up

* optimizing imports

spring-kafka-2/src/main/java/com/baeldung/spring/kafka/topicsandpartitions/KafkaConsumerConfig.java

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+package com.baeldung.spring.kafka.topicsandpartitions;
+
+import org.apache.kafka.clients.consumer.ConsumerConfig;
+import org.apache.kafka.common.serialization.DoubleDeserializer;
+import org.apache.kafka.common.serialization.StringDeserializer;
+import org.springframework.beans.factory.annotation.Value;
+import org.springframework.context.annotation.Bean;
+import org.springframework.context.annotation.Configuration;
+import org.springframework.kafka.annotation.EnableKafka;
+import org.springframework.kafka.config.ConcurrentKafkaListenerContainerFactory;
+import org.springframework.kafka.core.ConsumerFactory;
+import org.springframework.kafka.core.DefaultKafkaConsumerFactory;
+
+import java.util.HashMap;
+import java.util.Map;
+
+@EnableKafka
+@Configuration
+public class KafkaConsumerConfig {
+
+    @Value(value = "${spring.kafka.bootstrap-servers}")
+    private String bootstrapAddress;
+
+    @Bean
+    public ConsumerFactory<String, Double> consumerFactory() {
+        Map<String, Object> props = new HashMap<>();
+        props.put(ConsumerConfig.BOOTSTRAP_SERVERS_CONFIG, bootstrapAddress);
+        props.put(ConsumerConfig.KEY_DESERIALIZER_CLASS_CONFIG, StringDeserializer.class);
+        props.put(ConsumerConfig.VALUE_DESERIALIZER_CLASS_CONFIG, DoubleDeserializer.class);
+        return new DefaultKafkaConsumerFactory<>(props);
+    }
+
+    @Bean
+    public ConcurrentKafkaListenerContainerFactory<String, Double> kafkaListenerContainerFactory() {
+        ConcurrentKafkaListenerContainerFactory<String, Double> factory = new ConcurrentKafkaListenerContainerFactory<>();
+        factory.setConsumerFactory(consumerFactory());
+        return factory;
+    }
+}

spring-kafka-2/src/main/java/com/baeldung/spring/kafka/topicsandpartitions/KafkaProducerConfig.java

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+package com.baeldung.spring.kafka.topicsandpartitions;
+
+import org.apache.kafka.clients.producer.ProducerConfig;
+import org.apache.kafka.common.serialization.DoubleSerializer;
+import org.apache.kafka.common.serialization.StringSerializer;
+import org.springframework.beans.factory.annotation.Value;
+import org.springframework.context.annotation.Bean;
+import org.springframework.context.annotation.Configuration;
+import org.springframework.kafka.core.DefaultKafkaProducerFactory;
+import org.springframework.kafka.core.KafkaTemplate;
+import org.springframework.kafka.core.ProducerFactory;
+
+import java.util.HashMap;
+import java.util.Map;
+
+@Configuration
+public class KafkaProducerConfig {
+
+    @Value(value = "${spring.kafka.bootstrap-servers}")
+    private String bootstrapAddress;
+
+    @Bean
+    public ProducerFactory<String, Double> producerFactory() {
+        Map<String, Object> configProps = new HashMap<>();
+        configProps.put(ProducerConfig.BOOTSTRAP_SERVERS_CONFIG, bootstrapAddress);
+        configProps.put(ProducerConfig.KEY_SERIALIZER_CLASS_CONFIG, StringSerializer.class);
+        configProps.put(ProducerConfig.VALUE_SERIALIZER_CLASS_CONFIG, DoubleSerializer.class);
+        return new DefaultKafkaProducerFactory<>(configProps);
+    }
+
+    @Bean
+    public KafkaTemplate<String, Double> kafkaTemplate() {
+        return new KafkaTemplate<>(producerFactory());
+    }
+}

spring-kafka-2/src/main/java/com/baeldung/spring/kafka/topicsandpartitions/KafkaTopicConfig.java

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+package com.baeldung.spring.kafka.topicsandpartitions;
+
+import org.apache.kafka.clients.admin.AdminClientConfig;
+import org.apache.kafka.clients.admin.NewTopic;
+import org.springframework.beans.factory.annotation.Value;
+import org.springframework.context.annotation.Configuration;
+import org.springframework.kafka.config.TopicBuilder;
+import org.springframework.kafka.core.KafkaAdmin;
+
+import java.util.HashMap;
+import java.util.Map;
+
+@Configuration
+public class KafkaTopicConfig {
+
+    @Value(value = "${spring.kafka.bootstrap-servers}")
+    private String bootstrapAddress;
+
+    public KafkaAdmin kafkaAdmin() {
+        Map<String, Object> configs = new HashMap<>();
+        configs.put(AdminClientConfig.BOOTSTRAP_SERVERS_CONFIG, bootstrapAddress);
+        return new KafkaAdmin(configs);
+    }
+
+    public NewTopic celciusTopic() {
+        return TopicBuilder.name("celcius-scale-topic")
+                .partitions(2)
+                .build();
+    }
+}

spring-kafka-2/src/main/java/com/baeldung/spring/kafka/topicsandpartitions/TemperatureConsumer.java

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+package com.baeldung.spring.kafka.topicsandpartitions;
+
+import org.apache.kafka.clients.consumer.ConsumerRecord;
+import org.springframework.kafka.annotation.KafkaListener;
+import org.springframework.stereotype.Service;
+
+import java.util.HashSet;
+import java.util.Map;
+import java.util.Set;
+import java.util.concurrent.ConcurrentHashMap;
+import java.util.concurrent.CountDownLatch;
+
+@Service
+public class TemperatureConsumer {
+
+    private CountDownLatch latch = new CountDownLatch(1);
+
+    Map<String, Set<String>> consumedRecords = new ConcurrentHashMap<>();
+
+    @KafkaListener(topics = "celcius-scale-topic", groupId = "group-1")
+    public void consumer1(ConsumerRecord<?, ?> consumerRecord) {
+        computeConsumedRecord("consumer-1", consumerRecord.partition());
+    }
+
+    private void computeConsumedRecord(String key, int consumerRecord) {
+        consumedRecords.computeIfAbsent(key, k -> new HashSet<>());
+        consumedRecords.computeIfPresent(key, (k, v) -> {
+            v.add(String.valueOf(consumerRecord));
+            return v;
+        });
+    }
+
+    public CountDownLatch getLatch() {
+        return latch;
+    }
+}

spring-kafka-2/src/main/java/com/baeldung/spring/kafka/topicsandpartitions/ThermostatApplicationKafkaApp.java

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+package com.baeldung.spring.kafka.topicsandpartitions;
+
+import org.springframework.boot.SpringApplication;
+import org.springframework.boot.autoconfigure.SpringBootApplication;
+import org.springframework.context.annotation.Import;
+
+@SpringBootApplication
+@Import(value = {KafkaTopicConfig.class, KafkaProducerConfig.class, KafkaConsumerConfig.class})
+public class ThermostatApplicationKafkaApp {
+    public static void main(String[] args) {
+        SpringApplication.run(ThermostatApplicationKafkaApp.class, args);
+    }
+}

spring-kafka-2/src/main/java/com/baeldung/spring/kafka/topicsandpartitions/ThermostatService.java

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+package com.baeldung.spring.kafka.topicsandpartitions;
+
+import org.springframework.kafka.core.KafkaTemplate;
+import org.springframework.stereotype.Service;
+
+import java.util.Random;
+
+@Service
+public class ThermostatService {
+
+    private final KafkaTemplate<String, Double> kafkaTemplate;
+
+    public ThermostatService(KafkaTemplate<String, Double> kafkaTemplate) {
+        this.kafkaTemplate = kafkaTemplate;
+    }
+
+    public void measureCelsiusAndPublish(int numMeasurements) {
+        new Random().doubles(25, 35)
+                .limit(numMeasurements)
+                .forEach(tmp -> {
+                    kafkaTemplate.send("celcius-scale-topic", tmp);
+                });
+    }
+}

spring-kafka-2/src/test/java/com/baeldung/spring/kafka/topicsandpartitions/KafkaTopicsAndPartitionsIntegrationTest.java

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+package com.baeldung.spring.kafka.topicsandpartitions;
+
+import org.junit.ClassRule;
+import org.junit.jupiter.api.Test;
+import org.springframework.beans.factory.annotation.Autowired;
+import org.springframework.boot.test.context.SpringBootTest;
+import org.springframework.kafka.test.EmbeddedKafkaBroker;
+import org.springframework.kafka.test.context.EmbeddedKafka;
+
+import java.util.concurrent.TimeUnit;
+
+@SpringBootTest(classes = ThermostatApplicationKafkaApp.class)
+@EmbeddedKafka(partitions = 2, brokerProperties = {"listeners=PLAINTEXT://localhost:9092", "port=9092"})
+public class KafkaTopicsAndPartitionsIntegrationTest {
+    @ClassRule
+    public static EmbeddedKafkaBroker embeddedKafka = new EmbeddedKafkaBroker(1, true, "multitype");
+
+    @Autowired
+    private ThermostatService service;
+
+    @Autowired
+    private TemperatureConsumer consumer;
+
+    @Test
+    public void givenTopic_andConsumerGroup_whenConsumersListenToEvents_thenConsumeItCorrectly() throws Exception {
+        service.measureCelsiusAndPublish(10000);
+        consumer.getLatch().await(1, TimeUnit.SECONDS);
+        System.out.println(consumer.consumedRecords);
+    }
+}