Confluent Cloud Schema Registry Tutorial

Use Confluent Cloud Schema Registry to enable client applications to read and write Avro data and check schema compatibility as schemas evolve. The following steps walk through the full workflow.

Tip

Try out the embedded Confluent Cloud interactive tutorials Want to jump right in? Sign up or sign in to Confluent Cloud, and try out the guided workflows directly in Confluent Cloud.

What the tutorial covers

This tutorial covers defining an Avro schema, writing and reading Avro data with Java producers and consumers, and checking schema compatibility as schemas evolve.

This tutorial runs on Confluent Cloud Schema Registry. If you have a local Confluent Platform install, consult the Confluent Schema Registry tutorial for on-premises deployments at On-Premises Schema Registry Tutorial.

Schema Registry terms: topic, schema, and subject

What is a topic versus a schema versus a subject?

An Apache Kafka® topic contains messages, and each message is a key-value pair. The producer can serialize the message key, the message value, or both, as Avro, JSON Schema, or Protobuf.
A schema defines the structure of the data format. The Kafka topic name can be independent of the schema name.
The schema subject is a Schema Registry-defined scope in which schemas can evolve. The name of the subject depends on the configured subject name strategy, which, by default, derives the subject name from the topic name.

You can change the subject name strategy on a per-topic basis. As a practical example, consider a retail business streaming transactions in a Kafka topic called transactions. A producer is writing data with a schema Payment to that Kafka topic transactions. If the producer is serializing the message value as Avro, then Schema Registry has a subject called transactions-value.

If the producer is also serializing the message key as Avro, Schema Registry would have a subject called transactions-key. For simplicity, this tutorial considers only the message value. The subject transactions-value defines the scope in which schemas for that subject can evolve. Schema Registry checks compatibility within this scope. The Schema Registry subject transactions-value contains at least one schema called Payment.

In this scenario, if developers evolve the schema Payment and produce new messages to the topic transactions, Schema Registry checks that those newly evolved schemas are compatible with older schemas in the subject transactions-value. If compatible, Schema Registry adds the new schemas to the subject.

Set up your Confluent Cloud environment and tools

Prerequisites

Before proceeding with this tutorial, you can optionally review a summary of the Schema Registry concepts in Schema Registry Key Concepts.

Prerequisite setup includes:

An initialized Confluent Cloud cluster. If you don’t have one yet, Quick Start for Confluent Cloud walks you through creating one.

On your local machine:

Confluent CLI v1.7.0 or later.
Java 1.8 or 1.11 to run the Java client.
Maven to compile the client Java code.
Current versions of slf4j-log4j12 and confluent-log4j libraries. Otherwise, you might get a missing dependencies error when attempting to run mvn clean compile package in the steps to Run the producer and Run the consumer.
jq tool to nicely format the results from querying the Confluent Cloud Schema Registry REST endpoint.

Environment setup

Run this tutorial in a new Confluent Cloud environment so it doesn’t interfere with your other work.
Tip

You can use the Example: Create Fully Managed Services on Confluent Cloud to provision a new Confluent Cloud stack, which includes:
- A new environment
- A new service account
- A new Kafka cluster and associated credentials
- Confluent Cloud Schema Registry and associated credentials
- Access control lists (ACLs) with wildcard for the service account
Follow the Usage instructions for ccloud-stack to log in to Confluent Cloud and create a ccloud-stack.

If you used ccloud-stack, it also generates a configuration file with all the Confluent Cloud and Confluent Cloud Schema Registry connection information. Verify that the auto-generated file examples/ccloud/ccloud-stack/stack-configs/java-service-account-<account>.config resembles the following. If you set up your environment manually, create a configuration file with the equivalent values instead:

# ------------------------------
# ENVIRONMENT ID: <ENVIRONMENT ID>
# SERVICE ACCOUNT ID: <SERVICE ACCOUNT ID>
# KAFKA CLUSTER ID: <KAFKA CLUSTER ID>
# SCHEMA REGISTRY CLUSTER ID: <SCHEMA REGISTRY CLUSTER ID>
# ------------------------------
ssl.endpoint.identification.algorithm=https
security.protocol=SASL_SSL
sasl.mechanism=PLAIN
bootstrap.servers=<BROKER ENDPOINT>
sasl.jaas.config=org.apache.kafka.common.security.plain.PlainLoginModule required username='<API KEY>' password='<API SECRET>';
basic.auth.credentials.source=USER_INFO
schema.registry.basic.auth.user.info=<SR API KEY>:<SR API SECRET>
schema.registry.url=<SR ENDPOINT>

Save this configuration file to $HOME/.confluent/java.config.
Export the variables to your shell. Substitute values for <SR API KEY>, <SR API SECRET>, and <SR ENDPOINT> so you can copy and paste the commands in the rest of the tutorial.
```
export SCHEMA_REGISTRY_BASIC_AUTH_USER_INFO=<SR API KEY>:<SR API SECRET>
export SCHEMA_REGISTRY_URL=<SR ENDPOINT>
```
Tip

To get the Schema Registry REST endpoint, do one of the following:
- From Confluent Cloud, navigate to an environment with a Schema Registry enabled to view the value given for Endpoint under Stream Governance API.
- From the Confluent CLI, run confluent schema-registry cluster describe to view the value given for Endpoint URL.
Clone the Confluent examples repo from GitHub and work in the clients/avro/ subdirectory, which provides the sample code you compile and run in this tutorial.
```
git clone https://github.com/confluentinc/examples.git
```
```
cd examples/clients/avro
```
```
git checkout master
```

Create the `transactions` topic

For the exercises in this tutorial, you produce to and consume from a topic called transactions. Create this topic in Confluent Cloud Console.

Navigate to the Cloud Console at https://confluent.cloud. Click your environment and Kafka cluster.
Select Topics, and then click Create topic.
Name the topic transactions, and then click Create with defaults.

The new topic appears.

Schema definition

The first thing developers need to do is agree on a basic schema for data. Client applications form a contract:

Producers write data in a schema
Consumers read that data

Consider the original Payment schema Payment.avsc. To view the schema, run this command:

cat src/main/resources/avro/io/confluent/examples/clients/basicavro/Payment.avsc

Observe the schema definition:

{
 "namespace": "io.confluent.examples.clients.basicavro",
 "type": "record",
 "name": "Payment",
 "fields": [
     {"name": "id", "type": "string"},
     {"name": "amount", "type": "double"}
 ]
}

Here is a breakdown of what this schema defines:

namespace: a fully qualified name that avoids schema naming conflicts
type: Avro data type, for example, record, enum, union, array, map, or fixed
name: unique schema name in this namespace
fields: one or more simple or complex data types for a record. The first field in this record, id, is type string. The second, amount, is type double.

With the schema in place, set up the client applications that produce and consume this data.

Client applications writing Avro

Maven

This tutorial uses Maven to configure the project and dependencies. Java applications that have Kafka producers or consumers using Avro require pom.xml files to include, among other things:

Confluent Maven repository
Confluent Maven plugin repository
Dependencies org.apache.avro.avro and io.confluent.kafka-avro-serializer to serialize data as Avro
Plugin avro-maven-plugin to generate Java class files from the source schema

The pom.xml file may also include:

Plugin kafka-schema-registry-maven-plugin to check compatibility of evolving schemas

For a full pom.xml example, refer to this pom.xml.

Configuring Avro

Kafka applications using Avro data and Schema Registry need to specify at least two configuration parameters:

Avro serializer or deserializer
Properties to connect to Schema Registry

There are two basic types of Avro records that your application can use:

a specific code-generated class, or
a generic record

The examples in this tutorial demonstrate how to use the specific Payment class. Using a specific code-generated class requires you to define and compile a Java class for your schema, but it easier to work with in your code.

However, in other scenarios where you need to work dynamically with data of any type and do not have Java classes for your record types, use GenericRecord.

Confluent Platform also provides a serializer and deserializer for writing and reading data in “reflection Avro” format. To learn more, see Reflection Based Avro Serializer and Deserializer.

Java producers

Within the client application, Java producers need to configure the Avro serializer for the Kafka value (or Kafka key) and URL to Schema Registry. Then the producer can write records where the Kafka value is of Payment class.

Example producer code

When constructing the producer, configure the message value class to use the application’s code-generated Payment class. For example:

...
import io.confluent.kafka.serializers.KafkaAvroSerializer;
...
props.put(ProducerConfig.KEY_SERIALIZER_CLASS_CONFIG, StringSerializer.class);
props.put(ProducerConfig.VALUE_SERIALIZER_CLASS_CONFIG, KafkaAvroSerializer.class);
...
KafkaProducer<String, Payment> producer = new KafkaProducer<String, Payment>(props));
final Payment payment = new Payment(orderId, 1000.00d);
final ProducerRecord<String, Payment> record = new ProducerRecord<String, Payment>(TOPIC, payment.getId().toString(), payment);
producer.send(record);
...

Because the pom.xml includes avro-maven-plugin, the Payment class is automatically generated during compile.

In this example, the connection information to the Kafka brokers and Schema Registry is provided by the configuration file that is passed into the code, but if you want to specify the connection information directly in the client application, see this java template.

For a full Java producer example, refer to the producer example.

Run the producer

Run the following commands in a shell from examples/clients/avro.

To run this producer, first compile the project:
```
mvn clean compile package
```
If you get a missing dependencies error, see the Prerequisites for the required slf4j-log4j12 and confluent-log4j library versions.
From Cloud Console, select the cluster, and then click Topics.

Next, click the transactions topic and go to the Messages tab.

You should see no messages because no messages have been produced to this topic yet.

Run ProducerExample, which produces Avro-formatted messages to the transactions topic. Pass in the path to the file you created earlier, $HOME/.confluent/java.config.

mvn exec:java -Dexec.mainClass=io.confluent.examples.clients.basicavro.ProducerExample \
  -Dexec.args="$HOME/.confluent/java.config"

The command takes a moment to run. When it completes, you should see:

...
Successfully produced 10 messages to a topic called transactions
[INFO] ------------------------------------------------------------------------
[INFO] BUILD SUCCESS
[INFO] ------------------------------------------------------------------------
...

To see messages in Cloud Console, inspect the transactions topic, which dynamically shows the newly arriving data.

From Cloud Console, select the cluster, then go to Topics > transactions > Messages.

Tip

If you don’t see any data, rerun the producer and verify it completed successfully, and then look at Cloud Console again. The messages don’t persist in the Cloud Console, so you need to view them soon after you run the producer.

Java consumers

Within the client application, Java consumers need to configure the Avro deserializer for the Kafka value (or Kafka key) and URL to Schema Registry. Then the consumer can read records where the Kafka value is of Payment class.

Example consumer code

By default, each record is deserialized into an Avro GenericRecord, but in this tutorial the record should be deserialized using the application’s code-generated Payment class. Therefore, configure the deserializer to use Avro SpecificRecord, i.e., SPECIFIC_AVRO_READER_CONFIG should be set to true. For example:

...
import io.confluent.kafka.serializers.KafkaAvroDeserializer;
...
props.put(ConsumerConfig.KEY_DESERIALIZER_CLASS_CONFIG, StringDeserializer.class);
props.put(ConsumerConfig.VALUE_DESERIALIZER_CLASS_CONFIG, KafkaAvroDeserializer.class);
props.put(KafkaAvroDeserializerConfig.SPECIFIC_AVRO_READER_CONFIG, true);
...
KafkaConsumer<String, Payment> consumer = new KafkaConsumer<>(props));
consumer.subscribe(Collections.singletonList(TOPIC));
while (true) {
  ConsumerRecords<String, Payment> records = consumer.poll(100);
  for (ConsumerRecord<String, Payment> record : records) {
    String key = record.key();
    Payment value = record.value();
  }
}
...

Because the pom.xml includes avro-maven-plugin, the Payment class is automatically generated during compile.

In this example, the connection information to the Kafka brokers and Schema Registry is provided by the configuration file that is passed into the code, but if you want to specify the connection information directly in the client application, see this java template.

For a full Java consumer example, refer to the consumer example.

Run the consumer

To run this consumer, first compile the project.
```
mvn clean compile package
```
The BUILD SUCCESS message indicates the project built, and the command prompt becomes available again.

Then run ConsumerExample (assuming you already ran the ProducerExample above). Pass in the path to the file you created earlier, $HOME/.confluent/java.config.

mvn exec:java -Dexec.mainClass=io.confluent.examples.clients.basicavro.ConsumerExample \
  -Dexec.args="$HOME/.confluent/java.config"

You should see:

...
key = id0, value = {"id": "id0", "amount": 1000.0}
key = id1, value = {"id": "id1", "amount": 1000.0}
key = id2, value = {"id": "id2", "amount": 1000.0}
key = id3, value = {"id": "id3", "amount": 1000.0}
key = id4, value = {"id": "id4", "amount": 1000.0}
key = id5, value = {"id": "id5", "amount": 1000.0}
key = id6, value = {"id": "id6", "amount": 1000.0}
key = id7, value = {"id": "id7", "amount": 1000.0}
key = id8, value = {"id": "id8", "amount": 1000.0}
key = id9, value = {"id": "id9", "amount": 1000.0}
...

Press Ctrl+C to stop.

Other Kafka clients

The objective of this tutorial is to learn about Avro and Schema Registry centralized schema management and compatibility checks. To keep examples simple, this tutorial focuses on Java producers and consumers, but other Kafka clients work in similar ways. For examples of other Kafka clients interoperating with Avro and Schema Registry:

Centralized schema management

Viewing schemas in Schema Registry

View the latest schema registered for a topic from Cloud Console’s Schema tab.

From Cloud Console, select the cluster, and then click Topics.
Click the transactions topic and go to the Schema tab to retrieve the latest schema from Confluent Cloud Schema Registry for this topic:

The schema is identical to the schema file defined for Java client applications.

Schema IDs in messages

Integration with Schema Registry means producers don’t need to write the entire Avro schema into each Kafka message. Instead, producers write the schema ID into the message. The producers writing the messages and the consumers reading the messages must be using the same Schema Registry to get the same mapping between a schema and schema ID.

Schema ID caching for producers and consumers
Step	Producer	Consumer
Sends/reads data	Sends the new schema (`Payment`) to Confluent Cloud Schema Registry	Reads data containing the Avro schema ID (`100001`)
Contacts Confluent Cloud Schema Registry	Confluent Cloud Schema Registry registers the schema to subject `transactions-value` and returns schema ID `100001`	Sends a schema request; Confluent Cloud Schema Registry retrieves the schema for ID `100001` and returns it
Caches mapping	Caches the schema-to-ID mapping; contacts Confluent Cloud Schema Registry only on the first write	Caches the schema-to-ID mapping; contacts Confluent Cloud Schema Registry only on the first read

Using curl to interact with Schema Registry

You can also use curl commands to connect directly to the REST endpoint in Confluent Cloud Schema Registry to view subjects and associated schemas.

To view all the subjects registered in Confluent Cloud Schema Registry, use the following command.
```
curl --silent -X GET -u $SCHEMA_REGISTRY_BASIC_AUTH_USER_INFO $SCHEMA_REGISTRY_URL/subjects | jq .
```
Here is the expected output of the above command:
```
[
  "transactions-value"
]
```
In this example, the Kafka topic transactions has messages whose value (that is, payload) is Avro, and by default the Confluent Cloud Schema Registry subject name is transactions-value.
To view the latest schema for this subject in more detail:
```
curl --silent -X GET -u $SCHEMA_REGISTRY_BASIC_AUTH_USER_INFO $SCHEMA_REGISTRY_URL/subjects/transactions-value/versions/latest | jq .
```
Here is the expected output of the above command:
```
{
  "subject": "transactions-value",
  "version": 1,
  "id": 100001,
  "schema": "{\"type\":\"record\",\"name\":\"Payment\",\"namespace\":\"io.confluent.examples.clients.basicavro\",\"fields\":[{\"name\":\"id\",\"type\":\"string\"},{\"name\":\"amount\",\"type\":\"double\"}]}"
}
```
Here is a breakdown of what this version of the schema defines:
- subject: the scope in which schemas for the messages in the topic transactions can evolve
- version: the schema version for this subject, which starts at 1 for each subject
- id: the globally unique schema version ID, unique across all schemas in all subjects
- schema: the structure that defines the schema format
Notice that in the output to the preceding curl command, the schema is escaped JSON. The double quotes are preceded by backslashes.

Based on the schema ID, you can also retrieve the associated schema by querying Confluent Cloud Schema Registry REST endpoint as follows:

curl --silent -X GET -u $SCHEMA_REGISTRY_BASIC_AUTH_USER_INFO $SCHEMA_REGISTRY_URL/schemas/ids/100001 | jq .

Here is the expected output:

{
  "schema": "{\"type\":\"record\",\"name\":\"Payment\",\"namespace\":\"io.confluent.examples.clients.basicavro\",\"fields\":[{\"name\":\"id\",\"type\":\"string\"},{\"name\":\"amount\",\"type\":\"double\"}]}"
}

Auto schema registration

By default, client applications automatically register new schemas. If they produce new messages to a new topic, then they will automatically try to register new schemas. This is convenient in development environments, but in production environments it’s recommended that client applications do not automatically register new schemas. Best practice is to register schemas outside of the client application to control when schemas are registered with Schema Registry and how they evolve.

Within the application, you can disable automatic schema registration by setting the configuration parameter auto.register.schemas=false, as shown in the following example.

props.put(AbstractKafkaAvroSerDeConfig.AUTO_REGISTER_SCHEMAS, false);

Tip

If you want to enable use.latest.version for producers, you must disable auto schema registration by setting auto.register.schemas to false, and use.latest.version to true. (The opposite of their defaults.) The option auto.register.schemas must be set to false in order for use.latest.version to work.

Setting auto.register.schemas to false disables auto-registration of the event type, so that it does not override the latest schema in the subject. Setting use.latest.version to true causes the serializer to look up the latest schema version in the subject and use that for serialization. If use.latest.version is set to false (which is the default), the serializer will look for the event type in the subject and fail to find it.

You can also set use.latest.version on consumers, which causes the consumer to look up the latest schema version in the subject and use that as the target schema during deserialization.
See also, Schema Registry Configuration Options for Kafka Connect.
The configuration option auto.register.schemas is a Confluent Platform feature; not available in Apache Kafka®.

To manually register the schema outside of the application, you can use Cloud Console.

First, create a new topic called test in the same way that you created a new topic called transactions earlier in the tutorial. Then from the Schema tab, click Set a schema to define the new schema. Specify values for:

namespace: a fully qualified name that avoids schema naming conflicts
type: Avro data type, one of record, enum, union, array, map, fixed
name: unique schema name in this namespace
fields: one or more simple or complex data types for a record. The first field in this record is called id, and it is of type string. The second field in this record is called amount, and it is of type double.

If you were to define the same schema as used earlier, you would enter the following in the schema editor:

{
  "type": "record",
  "name": "Payment",
  "namespace": "io.confluent.examples.clients.basicavro",
  "fields": [
    {
      "name": "id",
      "type": "string"
    },
    {
      "name": "amount",
      "type": "double"
    }
  ]
}

If you prefer to connect directly to the REST endpoint in Schema Registry, run the following command to define a schema for a new subject for the topic test. This test topic and its test-value subject are only a throwaway example for this command, distinct from the transactions topic used throughout the rest of this tutorial.

curl -X POST -H "Content-Type: application/vnd.schemaregistry.v1+json" \
  --data '{"schema": "{\"type\":\"record\",\"name\":\"Payment\",\"namespace\":\"io.confluent.examples.clients.basicavro\",\"fields\":[{\"name\":\"id\",\"type\":\"string\"},{\"name\":\"amount\",\"type\":\"double\"}]}"}' \
  -u $SCHEMA_REGISTRY_BASIC_AUTH_USER_INFO \
  $SCHEMA_REGISTRY_URL/subjects/test-value/versions

This sample output creates a schema with an ID of 100001:

{"id":100001}

Schema evolution and compatibility

Evolving schemas

So far in this tutorial, you have seen the benefit of Schema Registry as being centralized schema management that enables client applications to register and retrieve globally unique schema ids. The main value of Schema Registry, however, is in enabling schema evolution. Similar to how APIs evolve and need to be compatible for all applications that rely on old and new versions of the API, schemas also evolve and likewise need to be compatible for all applications that rely on old and new versions of a schema. This schema evolution is a natural behavior of how applications and data develop over time.

Schema Registry allows for schema evolution and provides compatibility checks to ensure that the contract between producers and consumers is not broken. This allows producers and consumers to update independently and evolve their schemas independently, with assurances that they can read new and legacy data. This is especially important in Kafka because producers and consumers are decoupled applications that are sometimes developed by different teams.

Transitive compatibility checking is important once you have more than two versions of a schema for a given subject. If compatibility is configured as transitive, then it checks compatibility of a new schema against all previously registered schemas; otherwise, it checks compatibility of a new schema only against the latest schema.

For example, if there are three schemas for a subject that change in order X-2, X-1, and X then:

transitive: ensures compatibility between X-2 <==> X-1 and X-1 <==> X and X-2 <==> X
non-transitive: ensures compatibility between X-2 <==> X-1 and X-1 <==> X, but not necessarily X-2 <==> X

Refer to an example of schema changes which are incrementally compatible, but not transitively so.

The Confluent Schema Registry default compatibility type BACKWARD is non-transitive, which means that it’s not BACKWARD_TRANSITIVE. As a result, new schemas are checked for compatibility only against the latest schema.

These are the compatibility types:

BACKWARD: (default) consumers using the new schema can read data written by producers using the latest registered schema
BACKWARD_TRANSITIVE: consumers using the new schema can read data written by producers using all previously registered schemas
FORWARD: consumers using the latest registered schema can read data written by producers using the new schema
FORWARD_TRANSITIVE: consumers using all previously registered schemas can read data written by producers using the new schema
FULL: the new schema is forward and backward compatible with the latest registered schema
FULL_TRANSITIVE: the new schema is forward and backward compatible with all previously registered schemas
NONE: schema compatibility checks are disabled

Refer to Schema Evolution and Compatibility for a more in-depth explanation on the compatibility types.

Failing compatibility checks

Schema Registry checks compatibility as schemas evolve to uphold the producer-consumer contract. Without Schema Registry checking compatibility, your applications could break on schema changes.

In the Payment schema example, assume the business now tracks more information for each payment. For example, a field region that represents the place of sale. Consider the Payment2a schema which includes this extra field region:

cat src/main/resources/avro/io/confluent/examples/clients/basicavro/Payment2a.avsc

{
 "namespace": "io.confluent.examples.clients.basicavro",
 "type": "record",
 "name": "Payment",
 "fields": [
     {"name": "id", "type": "string"},
     {"name": "amount", "type": "double"},
     {"name": "region", "type": "string"}
 ]
}

This schema is not backward compatible. A consumer using the new schema can’t read data written by producers using the older schema, because the older data lacks the region field.

Before proceeding with any schema change, check whether the default Schema Registry backward compatibility type holds, meaning whether a consumer using the new schema can read data written with the older schema.

Confluent offers a Schema Registry Maven Plugin, which you can use to check compatibility in development or integrate into your continuous integration/continuous delivery (CI/CD) pipeline.

The sample pom.xml includes this plugin to enable compatibility checks.

...
<properties>
  <schemaRegistryUrl>http://localhost:8081</schemaRegistryUrl>
  <schemaRegistryBasicAuthUserInfo></schemaRegistryBasicAuthUserInfo>
</properties>
...
<build>
  <plugins>
  ...
    <plugin>
        <groupId>io.confluent</groupId>
        <artifactId>kafka-schema-registry-maven-plugin</artifactId>
        <version>${confluent.version}</version>
        <configuration>
            <schemaRegistryUrls>
                <param>${schemaRegistryUrl}</param>
            </schemaRegistryUrls>
            <userInfoConfig>${schemaRegistryBasicAuthUserInfo}</userInfoConfig>
            <subjects>
                <transactions-value>src/main/resources/avro/io/confluent/examples/clients/basicavro/Payment2a.avsc</transactions-value>
            </subjects>
        </configuration>
        <goals>
            <goal>test-compatibility</goal>
        </goals>
    </plugin>
...
  </plugins>
</build>

This configuration checks compatibility of the new Payment2a schema for the transactions-value subject in Schema Registry.

Run the compatibility check.

mvn io.confluent:kafka-schema-registry-maven-plugin:test-compatibility \
    "-DschemaRegistryUrl=$SCHEMA_REGISTRY_URL" \
    "-DschemaRegistryBasicAuthUserInfo=$SCHEMA_REGISTRY_BASIC_AUTH_USER_INFO" \
    "-DschemaLocal=src/main/resources/avro/io/confluent/examples/clients/basicavro/Payment2a.avsc"

Verify that the compatibility check fails, which causes this error message:

...
[ERROR] Schema examples/clients/avro/src/main/resources/avro/io/confluent/examples/clients/basicavro/Payment2a.avsc is not compatible with subject(transactions-value)
...

Try to register the new schema Payment2a manually to Schema Registry, which is a useful way for non-Java clients to check compatibility from the command line:

curl -X POST -H "Content-Type: application/vnd.schemaregistry.v1+json" \
  --data '{"schema": "{\"type\":\"record\",\"name\":\"Payment\",\"namespace\":\"io.confluent.examples.clients.basicavro\",\"fields\":[{\"name\":\"id\",\"type\":\"string\"},{\"name\":\"amount\",\"type\":\"double\"},{\"name\":\"region\",\"type\":\"string\"}]}"}' \
  -u $SCHEMA_REGISTRY_BASIC_AUTH_USER_INFO \
  $SCHEMA_REGISTRY_URL/subjects/transactions-value/versions

Verify that Confluent Cloud Schema Registry rejects the schema with an error message that it is incompatible:
```
{"error_code":409,"message":"Schema being registered is incompatible with an earlier schema"}
```

Passing compatibility checks

To maintain backward compatibility, a new schema must assume default values for the new field when it’s missing.

Consider an updated Payment2b schema that has a default value for region. To view the schema, run this command:

cat src/main/resources/avro/io/confluent/examples/clients/basicavro/Payment2b.avsc

You should see the following output.

{
 "namespace": "io.confluent.examples.clients.basicavro",
 "type": "record",
 "name": "Payment",
 "fields": [
     {"name": "id", "type": "string"},
     {"name": "amount", "type": "double"},
     {"name": "region", "type": "string", "default": ""}
 ]
}

From the Confluent Cloud Console, click the transactions topic and go to the Schema tab to retrieve the transactions topic’s latest schema from Schema Registry.
Click Edit Schema.
Add the new field region again with the default value, and then click Save:
```
{
 "name": "region",
 "type": "string",
 "default": ""
}
```
Verify that Confluent Cloud Schema Registry accepts the new schema.

Note

If you get error messages about invalid Avro, check syntax. For example, check quotes and colons, enclosing brackets, comma-separated from the previous field, and so on.
Review the registered schema versions. The Schema Registry subject transactions-value for the topic transactions has two schemas:
- Version 1 is Payment.avsc.
- Version 2 is Payment2b.avsc, which adds the region field with a default empty value.
To compare the two versions in Cloud Console, on the Schema tab for the topic transactions, click Version history, and then select Turn on version diff:
At the command line, go back to the Schema Registry Maven Plugin. Update the pom.xml to refer to Payment2b.avsc instead of Payment2a.avsc.

Re-run the compatibility check and verify that it passes:

mvn io.confluent:kafka-schema-registry-maven-plugin:test-compatibility

Verify the schema passed the compatibility check with this message:

...
[INFO] Schema examples/clients/avro/src/main/resources/avro/io/confluent/examples/clients/basicavro/Payment2b.avsc is compatible with subject(transactions-value)
...

If you prefer to connect directly to the REST endpoint in Schema Registry, then to register the new schema Payment2b, run the following command:

curl -X POST -H "Content-Type: application/vnd.schemaregistry.v1+json" \
  --data '{"schema": "{\"type\":\"record\",\"name\":\"Payment\",\"namespace\":\"io.confluent.examples.clients.basicavro\",\"fields\":[{\"name\":\"id\",\"type\":\"string\"},{\"name\":\"amount\",\"type\":\"double\"},{\"name\":\"region\",\"type\":\"string\",\"default\":\"\"}]}"}' \
  -u $SCHEMA_REGISTRY_BASIC_AUTH_USER_INFO \
  $SCHEMA_REGISTRY_URL/subjects/transactions-value/versions

If successful, the preceding curl command returns the id of the newly registered schema:

{"id":100002}

View the latest subject for transactions-value in Confluent Cloud Schema Registry:

curl --silent -X GET -u $SCHEMA_REGISTRY_BASIC_AUTH_USER_INFO $SCHEMA_REGISTRY_URL/subjects/transactions-value/versions/latest | jq .

This command returns the latest Confluent Cloud Schema Registry subject for the transactions-value topic, including version number, id, and a description of the schema in JSON:

{
  "subject": "transactions-value",
  "version": 2,
  "id": 100002,
  "schema": "{\"type\":\"record\",\"name\":\"Payment\",\"namespace\":\"io.confluent.examples.clients.basicavro\",\"fields\":[{\"name\":\"id\",\"type\":\"string\"},{\"name\":\"amount\",\"type\":\"double\"},{\"name\":\"region\",\"type\":\"string\",\"default\":\"\"}]}"
}

Notice the changes:

version: changed from 1 to 2
id: changed from 100001 to 100002
schema: updated with the new field region that has a default value

Changing compatibility type

The default compatibility type is backward, but you may change it globally or per subject.

To change the compatibility type per subject from the UI, click the transactions topic and go to the Schema tab to retrieve the transactions topic’s latest schema from Schema Registry. Click Edit Schema and then click Compatibility Mode.

Notice that the compatibility for this topic is set to the default backward, but you may change this as needed.

If you prefer to connect directly to the REST endpoint in Confluent Cloud Schema Registry, then change the compatibility type for the topic transactions. Use transactions-value for the subject. Run the following example command:

curl -X PUT -H "Content-Type: application/vnd.schemaregistry.v1+json" \
       --data '{"compatibility": "BACKWARD_TRANSITIVE"}' \
       -u $SCHEMA_REGISTRY_BASIC_AUTH_USER_INFO $SCHEMA_REGISTRY_URL/config/transactions-value

Destroy the ccloud-stack

When you finish the tutorial, destroy the resources you created in Confluent Cloud.

If you used a ccloud-stack for this tutorial, call the bash script ccloud_stack_destroy.sh and pass in the properties file auto-generated when you created the ccloud-stack.

# Change directory if needed
cd <path to examples>/ccloud/ccloud-stack/

./ccloud_stack_destroy.sh stack-configs/java-service-account-<account>.config

If you didn’t use a ccloud-stack, manually delete the topic, Schema Registry subjects, and any other resources you created for this tutorial from the Cloud Console.

Important

Always verify that no resources remain in Confluent Cloud.

Next steps

Schema Registry on Confluent Cloud basics

Deep dive on working with schemas

Blog post: Why Avro For Kafka Data
Blog post: Yes, Virginia, You Really Do Need a Schema Registry
Apache Avro® official site: How to get started with Apache Avro using Java Clients
How to produce and consume (Avro) messages via console tools with Confluent Cloud (Confluent Support)

Confluent supported schema formats, and how to configure clients using Avro, Protobuf, or JSON Schema: Formats, Serializers, and Deserializers
Try it out: Schema Registry API Usage Examples, showing more curl commands over HTTP and HTTPS