What is the difference between implementing Deserializer and Serde in Kafka Consumer API?

Onur Tokat :

I try to simulate Gwen (Chen) Shapira's kafka-clickstream-enrich kafka-stream project on GitHub (https://github.com/onurtokat/kafka-clickstream-enrich). When I consume a topic using consumer class using Deserializers, I encounter an error. The customized Serde class have serializer and deserializer. But, I try to understand why custom serde is used for deserializer, then consumer API gives error as it is not an instance of org.apache.kafka.common.serialization.Deserializer

The topic can be consumed using KTable with Serdes.Integer() Serializer and new ProfileSerde() Deserializer like below.

KTable<Integer, UserProfile> profiles = builder.table(Constants.USER_PROFILE_TOPIC,
                Consumed.with(Serdes.Integer(), new ProfileSerde()),
                Materialized.as("profile-store"));

Customized Serde is defined as;

static public final class ProfileSerde extends WrapperSerde<UserProfile> {
        public ProfileSerde() {
            super(new JsonSerializer<UserProfile>(), new JsonDeserializer<UserProfile>(UserProfile.class));
        }
    }

And generic Serde is customized liked below;

package com.onurtokat.serde;

import org.apache.kafka.common.serialization.Deserializer;
import org.apache.kafka.common.serialization.Serde;
import org.apache.kafka.common.serialization.Serializer;

import java.util.Map;


public class WrapperSerde<T> implements Serde<T> {

    final private Serializer<T> serializer;
    final private Deserializer<T> deserializer;

    public WrapperSerde(Serializer<T> serializer, Deserializer<T> deserializer) {
        this.serializer = serializer;
        this.deserializer = deserializer;
    }

    @Override
    public void configure(Map<String, ?> configs, boolean isKey) {
        serializer.configure(configs, isKey);
        deserializer.configure(configs, isKey);
    }

    @Override
    public void close() {
        serializer.close();
        deserializer.close();
    }

    @Override
    public Serializer<T> serializer() {
        return serializer;
    }

    @Override
    public Deserializer<T> deserializer() {
        return deserializer;
    }
}

My Consumer is so simple and can be seen below;

package com.onurtokat.consumers;

import com.onurtokat.ClickstreamEnrichment;
import com.onurtokat.Constants;
import com.onurtokat.model.UserProfile;
import org.apache.kafka.clients.consumer.ConsumerConfig;
import org.apache.kafka.clients.consumer.ConsumerRecord;
import org.apache.kafka.clients.consumer.ConsumerRecords;
import org.apache.kafka.clients.consumer.KafkaConsumer;
import org.apache.kafka.common.serialization.IntegerDeserializer;

import java.time.Duration;
import java.util.Arrays;
import java.util.Properties;

public class ConsumeProfileData {

    public static void main(String[] args) {
        //prepare config
        Properties config = new Properties();
        config.put(ConsumerConfig.BOOTSTRAP_SERVERS_CONFIG, "localhost:9092");
        config.put(ConsumerConfig.KEY_DESERIALIZER_CLASS_CONFIG, IntegerDeserializer.class.getName());
        config.put(ConsumerConfig.VALUE_DESERIALIZER_CLASS_CONFIG, ClickstreamEnrichment.ProfileSerde.class);

        KafkaConsumer<Integer, UserProfile> consumerProfileTopic = new KafkaConsumer<>(config);
        consumerProfileTopic.subscribe(Arrays.asList(Constants.USER_PROFILE_TOPIC));
        while (true) {
            ConsumerRecords<Integer, UserProfile> records = consumerProfileTopic.poll(Duration.ofMillis(100));
            for (ConsumerRecord<Integer, UserProfile> record : records) {
                System.out.println(record.key() + " " + record.value());
            }
        }
    }
}

The error when I try to consume topic with my consumer is;

log4j:WARN No appenders could be found for logger (org.apache.kafka.clients.consumer.ConsumerConfig).
log4j:WARN Please initialize the log4j system properly.
log4j:WARN See http://logging.apache.org/log4j/1.2/faq.html#noconfig for more info.
Exception in thread "main" org.apache.kafka.common.KafkaException: Failed to construct kafka consumer
    at org.apache.kafka.clients.consumer.KafkaConsumer.<init>(KafkaConsumer.java:811)
    at org.apache.kafka.clients.consumer.KafkaConsumer.<init>(KafkaConsumer.java:659)
    at org.apache.kafka.clients.consumer.KafkaConsumer.<init>(KafkaConsumer.java:639)
    at com.onurtokat.consumers.ConsumeProfileData.main(ConsumeProfileData.java:25)
Caused by: org.apache.kafka.common.KafkaException: com.onurtokat.ClickstreamEnrichment$ProfileSerde is not an instance of org.apache.kafka.common.serialization.Deserializer
    at org.apache.kafka.common.config.AbstractConfig.getConfiguredInstance(AbstractConfig.java:304)
    at org.apache.kafka.clients.consumer.KafkaConsumer.<init>(KafkaConsumer.java:712)
    ... 3 more

Michael G. Noll :

The difference is:

• Serdes are used by Kafka's Streams API (aka Kafka Streams). A Serde is a wrapper for a pair of (1) serializer and (2) deserializer for the same data type. That is, a Serde<T> has a Serializer<T> and a Deserializer<T>. The first code snippet you posted (with e.g. a KTable) is a Kafka Streams code snippet, that's why it needs a Serde. Kafka Streams needs a Serde because it both produces messages (for which it needs a Serializer) and reads messages (for which it needs a Deserializer).
• Deserializers are used by Kafka's consumer API (aka consumer client) for reading messages. Your last code snippet (with e.g. a KafkaConsumer) is using the consumer client and therefore needs a Deserializer, not a Serde.
• Serializers are used by Kafka's producer API (aka producer client) for writing messages.

Regarding:

Caused by: org.apache.kafka.common.KafkaException: com.onurtokat.ClickstreamEnrichment$ProfileSerde is not an instance of org.apache.kafka.common.serialization.Deserializer
    at org.apache.kafka.common.config.AbstractConfig.getConfiguredInstance(AbstractConfig.java:304)
    at org.apache.kafka.clients.consumer.KafkaConsumer.<init>(KafkaConsumer.java:712)
    ... 3 more

Your Kafka consumer client code was given a Serde where it was expecting a Deserializer.

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