How to Deserialising Kafka AVRO messages using Apa

问题:

The main goal is the aggregate two Kafka topics, one compacted slow moving data and the other fast moving data which is received every second.

I have been able to consume messages in simple scenarios such as a KV (Long,String) using something like:

PCollection<KV<Long,String>> input = p.apply(KafkaIO.<Long, 
String>read()
.withKeyDeserializer(LongDeserializer.class)
.withValueDeserializer(StringDeserializer.class)

PCollection<String> output = input.apply(Values.<String>create());

But this doesn’t seem to be the approach when you need to deserialise from AVRO. I have a KV(STRING, AVRO) which I need to consume.

I attempted generating the Java Classes from the AVRO schema and then including them in the “apply” for example:

PCollection<MyClass> output = input.apply(Values.<MyClass>create());

But this didn’t seem to be the correct approach.

Is there any documentation/examples anyone could point me to, so I could get an understanding as to how you would work with Kafka AVRO and Beam. Any help would be much appreciated.

I have updated my code:

import io.confluent.kafka.serializers.KafkaAvroDeserializer;
import org.apache.beam.sdk.Pipeline;
import org.apache.beam.sdk.coders.AvroCoder;
import org.apache.beam.sdk.io.kafka.KafkaIO;
import org.apache.beam.sdk.options.PipelineOptions;
import org.apache.beam.sdk.options.PipelineOptionsFactory;
import org.apache.beam.sdk.values.KV;
import org.apache.beam.sdk.values.PCollection;
import org.apache.kafka.common.serialization.LongDeserializer;

public class Main {

public static void main(String[] args) {

    PipelineOptions options = PipelineOptionsFactory.create();

    Pipeline p = Pipeline.create(options);

    PCollection<KV<Long, Myclass>> input = p.apply(KafkaIO.<Long, String>read()
            .withKeyDeserializer(LongDeserializer.class)
            .withValueDeserializerAndCoder(KafkaAvroDeserializer.class, AvroCoder.of(Myclass.class))
    );

    p.run();

}
}
#######################################################
import org.apache.beam.sdk.coders.AvroCoder;
import org.apache.beam.sdk.coders.DefaultCoder;

@DefaultCoder(AvroCoder.class)
public class Myclass{
String name;
String age;

Myclass(){}
Myclass(String n, String a) {
    this.name= n;
    this.age= a;
}
}

But i now get the following error incompatible types: java.lang.Class < io.confluent.kafka.serializers.KafkaAvroDeserializer > cannot be converted to java.lang.Class < ? extends org.apache.kafka.common.serialization.Deserializer < java.lang.String > >

I must be importing the incorrect serializers?

回答1:

You can use KafkaAvroDeserializer as following:

PCollection<KV<Long,MyClass>> input = p.apply(KafkaIO.<Long, String>read()
.withKeyDeserializer(LongDeserializer.class)
  .withValueDeserializerAndCoder(KafkaAvroDeserializer.class, AvroCoder.of(MyClass.class))

Where MyClass is the POJO class generated Avro Schema.

Make sure your POJO class has annotation AvroCoder as in below example :

@DefaultCoder(AvroCoder.class)
   public class MyClass{
      String name;
      String age;

      MyClass(){}
      MyClass(String n, String a) {
         this.name= n;
         this.age= a;
      }
  }

回答2:

I have faced the same issue. Found the solution in this mail-archives. http://mail-archives.apache.org/mod_mbox/beam-user/201710.mbox/%3CCAMsy_NiVrT_9_xfxOtK1inHxb=[e-mail]%3E

In your case, you need to defined your own KafkaAvroDeserializer like as follows.

public class MyClassKafkaAvroDeserializer extends
  AbstractKafkaAvroDeserializer implements Deserializer<MyClass> {

  @Override
  public void configure(Map<String, ?> configs, boolean isKey) {
      configure(new KafkaAvroDeserializerConfig(configs));
  }

  @Override
  public MyClass deserialize(String s, byte[] bytes) {
      return (MyClass) this.deserialize(bytes);
  }

  @Override
  public void close() {} }

Then specify your KafkaAvroDeserializer as ValueDeserializer.

p.apply(KafkaIO.<Long, MyClass>read()
 .withKeyDeserializer(LongDeserializer.class)
 .withValueDeserializer(MyClassKafkaAvroDeserializer.class) );

回答3:

Change KafkaIO.<Long, String>read() to KafkaIO.<Long, Object>read().

If you look into the implementation of KafkaAvroDeserializer, it implements Deserializer:

public class KafkaAvroDeserializer extends AbstractKafkaAvroDeserializer implements Deserializer<Object>

回答4:

Yohei's answer is good, but I also found this to work

import io.confluent.kafka.streams.serdes.avro.SpecificAvroDeserializer;

...

public static class CustomKafkaAvroDeserializer extends SpecificAvroDeserializer<MyCustomClass> {}

...
.withValueDeserializerAndCoder(CustomKafkaAvroDeserializer.class, AvroCoder.of(MyCustomClass.class))
...

where MyCustomClass is code gen'd with Avro tools.