README.md

Java Producer and Consumer with Avro

This directory includes projects demonstrating how to use the Java producer and consumer with Avro and Confluent Schema Registry

How to use these examples:

• ProducerExample.java: see Confluent Schema Registry tutorial
• ConsumerExample.java: see Confluent Schema Registry tutorial

Demo

0. Install Confluent platform locally using Docker: https://docs.confluent.io/current/quickstart/ce-docker-quickstart.html

1. Produce messages for schema v1 (missing future "region" field):

   git co schema-v1 && mvn clean package exec:java -Dexec.mainClass=io.confluent.examples.clients.basicavro.ProducerExample
   

   Note the automatically registered schema:

   curl -X GET http://localhost:8081/subjects/
   curl -X GET http://localhost:8081/subjects/transactions-value/versions
   

2. Consume messages for schema v1:

   git co schema-v1 && mvn clean package exec:java -Dexec.mainClass=io.confluent.examples.clients.basicavro.ConsumerExample
   

3. Attempt to change Producer's schema to use a new, but incompatible version (no default value for "region"):

   git co schema-v2-invalid && mvn clean package exec:java -Dexec.mainClass=io.confluent.examples.clients.basicavro.ProducerExample
   

   Note the runtime error:

   org.apache.kafka.common.errors.SerializationException: Error registering Avro schema: {"type":"record","name":"Payment","namespace":"io.confluent.examples.clients.basicavro","fields":[{"name":"id","type":"string","logicalType":"UUID"},{"name":"amount","type":"double"},{"name":"region","type":"string"}]}
   Caused by: io.confluent.kafka.schemaregistry.client.rest.exceptions.RestClientException: Schema being registered is incompatible with an earlier schema; error code: 409
   

4. Consume schema v1 messages for schema v2:

   git co schema-v2 && mvn clean package exec:java -Dexec.mainClass=io.confluent.examples.clients.basicavro.ConsumerExample
   

   Note that consumer accepts schema v1 messages, using default value.

5. Produce messages for schema 2 (contains "region" field):

   git co schema-v2 && mvn clean package exec:java -Dexec.mainClass=io.confluent.examples.clients.basicavro.ProducerExample
   

   Note that v1 consumer accepts schema v2 messages, ignoring new region field value. Note that v2 consumer accepts schema v2 messages, showing new region field value.

• To replay consumers with existing messages (of any schema version), first terminate consumer processes, wait for consume group to become "stable", and then run:

  /kafka-consumer-groups.sh --bootstrap-server localhost:9092 --reset-offsets --to-earliest --all-topics --execute --group transactions-v1
  /kafka-consumer-groups.sh --bootstrap-server localhost:9092 --reset-offsets --to-earliest --all-topics --execute --group transactions-v2
  

• To run through demo again, first reset Kafka state (topic and schemas):
  • kafka-topics.sh --zookeeper localhost:2181/ --delete --topic transaction
  • curl -X DELETE http://localhost:8081/subjects/transactions-value
• To modify and test other schema compatibility level checking by Confluent Schema Registry:

  $ curl -X GET http://localhost:8081/config
  {"compatibilityLevel":"BACKWARD"}
  $ $ curl -X PUT -H "Content-Type: application/json" -d '{"compatibility":"FULL_TRANSITIVE"}' http://localhost:8081/config
  

  See:
  • https://docs.confluent.io/current/schema-registry/develop/api.html#compatibility
  • https://docs.confluent.io/current/schema-registry/avro.html#compatibility-types
  • https://docs.confluent.io/current/schema-registry/avro.html#order-of-upgrading-clients
• To view Avro-encoded messages using Kafka CLI tools:

  docker exec -it schema-registry bash
  kafka-avro-console-consumer --bootstrap-server broker:29092 --offset earliest --topic transactions --partition 0
  

  (The Docker container schema-registry has the kafka-avro-console-consumer CLI installed)