Flink 1.14.0 消费 kafka 数据自定义反序列化类

在最近发布的 Flink 1.14.0 版本中对 Source 接口进行了重构,细节可以参考 FLIP-27: Refactor Source Interface

重构之后 API 层面的改动还是非常大的,那在使用新的 API 消费 kafka 数据的时候如何自定义序列化类呢?

Kafka Source

KafkaSource<String> source = KafkaSource.<String>builder()
    .setBootstrapServers(brokers)
    .setTopics("input-topic")
    .setGroupId("my-group")
    .setStartingOffsets(OffsetsInitializer.earliest())
    .setValueOnlyDeserializer(new SimpleStringSchema())
    .build();

env.fromSource(source, WatermarkStrategy.noWatermarks(), "Kafka Source");

KafkaSourceBuilder 类提供了两个方法来反序列数据,分别是 setDeserializer 和 setValueOnlyDeserializer 从名字上就应该可以看出这两者的区别,前者是反序列化完整的 ConsumerRecord,后者只反序列化 ConsumerRecord 的 value.然后我们来看一下底层的源码

KafkaSourceBuilder 源码

/**
 * Sets the {@link KafkaRecordDeserializationSchema deserializer} of the {@link
 * org.apache.kafka.clients.consumer.ConsumerRecord ConsumerRecord} for KafkaSource.
 *
 * @param recordDeserializer the deserializer for Kafka {@link
 *     org.apache.kafka.clients.consumer.ConsumerRecord ConsumerRecord}.
 * @return this KafkaSourceBuilder.
 */
public KafkaSourceBuilder<OUT> setDeserializer(
        KafkaRecordDeserializationSchema<OUT> recordDeserializer) {
    this.deserializationSchema = recordDeserializer;
    return this;
}

/**
 * Sets the {@link KafkaRecordDeserializationSchema deserializer} of the {@link
 * org.apache.kafka.clients.consumer.ConsumerRecord ConsumerRecord} for KafkaSource. The given
 * {@link DeserializationSchema} will be used to deserialize the value of ConsumerRecord. The
 * other information (e.g. key) in a ConsumerRecord will be ignored.
 *
 * @param deserializationSchema the {@link DeserializationSchema} to use for deserialization.
 * @return this KafkaSourceBuilder.
 */
public KafkaSourceBuilder<OUT> setValueOnlyDeserializer(
        DeserializationSchema<OUT> deserializationSchema) {
    this.deserializationSchema =
            KafkaRecordDeserializationSchema.valueOnly(deserializationSchema);
    return this;
}

可以看到这两个方法实际上是一样的,虽然两个方法的参数不同,setDeserializer 方法参数类型是 KafkaRecordDeserializationSchema 而 setValueOnlyDeserializer 方法的参数类型是 DeserializationSchema 那这两种参数类型有什么区别和联系呢?下面会进一步解释, 但是这两个方法最后返回的都是 KafkaRecordDeserializationSchema 对象,我们继续来看 KafkaRecordDeserializationSchema 的源码

先来看一下 DeserializationSchema 的部分源码

DeserializationSchema 源码

@Public
public interface DeserializationSchema<T> extends Serializable, ResultTypeQueryable<T> {
   
    @PublicEvolving
    default void open(InitializationContext context) throws Exception {}
   
    T deserialize(byte[] message) throws IOException;

    @PublicEvolving
    default void deserialize(byte[] message, Collector<T> out) throws IOException {
        T deserialize = deserialize(message);
        if (deserialize != null) {
            out.collect(deserialize);
        }
    }
    
    boolean isEndOfStream(T nextElement);
}

KafkaRecordDeserializationSchema 源码

/** An interface for the deserialization of Kafka records. */
public interface KafkaRecordDeserializationSchema<T> extends Serializable, ResultTypeQueryable<T> {
    
    @PublicEvolving
    default void open(DeserializationSchema.InitializationContext context) throws Exception {}
 
    @PublicEvolving
    void deserialize(ConsumerRecord<byte[], byte[]> record, Collector<T> out) throws IOException;
  
    
    static <V> KafkaRecordDeserializationSchema<V> of(
            KafkaDeserializationSchema<V> kafkaDeserializationSchema) {
        return new KafkaDeserializationSchemaWrapper<>(kafkaDeserializationSchema);
    }

    
    static <V> KafkaRecordDeserializationSchema<V> valueOnly(
            DeserializationSchema<V> valueDeserializationSchema) {
        return new KafkaValueOnlyDeserializationSchemaWrapper<>(valueDeserializationSchema);
    }

    
    static <V> KafkaRecordDeserializationSchema<V> valueOnly(
            Class<? extends Deserializer<V>> valueDeserializerClass) {
        return new KafkaValueOnlyDeserializerWrapper<>(
                valueDeserializerClass, Collections.emptyMap());
    }


    static <V, D extends Configurable & Deserializer<V>>
            KafkaRecordDeserializationSchema<V> valueOnly(
                    Class<D> valueDeserializerClass, Map<String, String> config) {
        return new KafkaValueOnlyDeserializerWrapper<>(valueDeserializerClass, config);
    }
}

顾名思义,这两个都是反序列接口,并且都继承了 Serializable, ResultTypeQueryable 这两个接口。不同点是,deserialize 方法的参数不一样,KafkaDeserializationSchema 接口很明显是为反序列化 kafka 数据而生的。DeserializationSchema 接口可以反序列化任意二进制数据,更加具有通用性。所以这两个是同一级接口

如果你想要获取 kafka 的元数据信息选择实现 KafkaDeserializationSchema 接口就可以了,KafkaDeserializationSchema 接口还有 4 个静态方法,其中的 of 方法就是用来反序列化 ConsumerRecord 的,剩下的 3 个 valueOnly 是用来反序列化 kafka 消息中的 value 的.

到这里就非常清楚了,如果我们要自定义序列化类,实现 DeserializationSchema 和 KafkaRecordDeserializationSchema 任何一个都是可以的.下面就以 KafkaRecordDeserializationSchema 接口为例,实现一个简单的反序列化类.

MyKafkaDeserialization 自定义序列化类

package flink.stream.deserialization;

import bean.Jason;
import com.alibaba.fastjson.JSON;
import org.apache.flink.api.common.typeinfo.TypeInformation;
import org.apache.flink.streaming.connectors.kafka.KafkaDeserializationSchema;
import org.apache.kafka.clients.consumer.ConsumerRecord;
import org.apache.log4j.Logger;

public class MyKafkaDeserialization implements KafkaDeserializationSchema<Jason> {
    private static final Logger log = Logger.getLogger(MyKafkaDeserialization.class);
    private final String encoding = "UTF8";
    private boolean includeTopic;
    private boolean includeTimestamp;

    public MyKafkaDeserialization(boolean includeTopic, boolean includeTimestamp) {
        this.includeTopic = includeTopic;
        this.includeTimestamp = includeTimestamp;
    }

    @Override
    public TypeInformation<Jason> getProducedType() {
        return TypeInformation.of(Jason.class);
    }

    @Override
    public boolean isEndOfStream(Jason nextElement) {
        return false;
    }

    @Override
    public Jason deserialize(ConsumerRecord<byte[], byte[]> consumerRecord) throws Exception {
        if (consumerRecord != null) {
            try {
                String value = new String(consumerRecord.value(), encoding);
                Jason jason = JSON.parseObject(value, Jason.class);
                if (includeTopic) jason.setTopic(consumerRecord.topic());
                if (includeTimestamp) jason.setTimestamp(consumerRecord.timestamp());
                return jason;
            } catch (Exception e) {
                log.error("deserialize failed : " + e.getMessage());
            }
        }
        return null;
    }
}

整个实现是非常简单的,这样就可以把消费到的数据反序列化成自己想要的格式,虽然 Flink 1.14.0 重构了 Source 接口,但是反序列化接口几乎没变,只不过在原有的基础上增加了几个方法而已.

使用

KafkaSource<Jason> source = KafkaSource.<Jason>builder()
        .setProperty("security.protocol", "SASL_PLAINTEXT")
        .setProperty("sasl.mechanism", "PLAIN")
        .setProperty("sasl.jaas.config", "org.apache.kafka.common.security.plain.PlainLoginModule required username=\"" + username + "\" password=\"" + password + "\";")
        // discover new partitions per 10 seconds
        .setProperty("partition.discovery.interval.ms", "10000")
        .setBootstrapServers(broker)
        .setTopics(topic)
        .setGroupId(group_id)
        .setStartingOffsets(OffsetsInitializer.earliest())
        .setDeserializer(KafkaRecordDeserializationSchema.of(new MyKafkaDeserialization(true, true)))
        // 只反序列化 value
        .setValueOnlyDeserializer(new MyDeSerializer())

setDeserializer 和 setValueOnlyDeserializer 只用设置一个即可.

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