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Schema Validation

Schema Validation gives operators a centralized location to enforce data format correctness at the topic level.

Prerequisites and Setting Schema Registry URLs

Basic requirements to run these examples are generally the same as those described for the Schema Registry Tutorial with the exception of Maven, which is not needed here. Also, Confluent Platform version 5.4.0 or later is required here.

As an additional prerequisite to enable Schema Validation on the brokers, you must specify confluent.schema.registry.url in the Kafka server.properties file ($CONFLUENT_HOME/etc/kafka/server.properties) before you start Confluent Platform.

For example:

confluent.schema.registry.url=http://schema-registry:8081

This configuration accepts a comma-separated list of URLs for Schema Registry instances. This setting is required to make Schema Validation available both from the Confluent CLI and on the Confluent Control Center.

Enabling Schema Validation on a topic from the Confluent CLI

You can enable Schema Validation on a topic when you create a topic or modify an existing topic.

Create a Topic with Schema Validation

To set Schema Validation on a topic when you create the topic, set confluent.value.schema.validation=true and confluent.key.schema.validation=true.

Value schema and key schema validation are independent of each other; you can enable either or both. (By default, schema validation is not enabled; both key and value schema validation default to false.)

For example, this command creates a topic called my-topic-sv with schema validation enabled on the value schema:

kafka-topics --create --bootstrap-server localhost:9092 --replication-factor 1 \
--partitions 1 --topic my-topic-sv \
--config confluent.value.schema.validation=true

The output of this command is:

Created topic my-topic-sv.

With this configuration, if a message is produced to the topic my-topic-sv that does not have a valid schema for the value of the message, an error is returned to the producer, and the message is discarded.

If a batch of messages is sent and at least one is invalid, then the entire batch is discarded.

Tip

To learn more about value and key schemas, see Terminology Review in the Schema Registry Tutorial.

Add Schema Validation to an Existing Topic

Create a new topic called my-first-topic:

kafka-topics --create --bootstrap-server localhost:9092 --replication-factor 1 --partitions 1 --topic my-first-topic

The output of this command is:

Created topic my-first-topic.

To change topic validation configuration on an existing topic (in this case from false to true), specify validation using the alter and --add-config flags as shown in this example:

kafka-configs --bootstrap-server localhost:9092 --alter --entity-type topics --entity-name my-first-topic --add-config confluent.value.schema.validation=true

You should get this confirmation:

Completed updating config for entity: topic 'my-first-topic'.

To disable topic validation on this topic (from true to false), rerun the above command with --add-config confluent.value.schema.validation=false.

Change the subject naming strategy for a topic

By default, Confluent Server uses the TopicNameStrategy as the naming strategy to map topics with schemas in Schema Registry.

Before Confluent Platform 5.5.0, the subject name strategy was configured on the brokers in server.properties, which required that you use the same strategy for all topics on a broker.

Starting with Confluent Platform 5.5.0, the naming strategy is associated with the topics. Therefore, you now have the option to configure a naming strategy to something other than the default on a per-topic basis for both the schema subject key and value with confluent.key.subject.name.strategy and confluent.value.subject.name.strategy.

From the Confluent CLI, use the --config option to create or modify a topic with the specified naming strategy. For example:

To create a topic that uses RecordNameStrategy for the value:

./bin/kafka-topics --create --bootstrap-server localhost:9092 \
--replication-factor 1 --partitions 1 --topic my-other-cool-topic \
--config confluent.value.schema.validation=true --config confluent.value.subject.name.strategy=io.confluent.kafka.serializers.subject.RecordNameStrategy

To modify a topic to use RecordNameStrategy as the key:

kafka-configs --bootstrap-server localhost:9092 \
--alter --entity-type topics --entity-name my-other-cool-topic \
--add-config confluent.key.subject.name.strategy=io.confluent.kafka.serializers.subject.RecordNameStrategy

The possible configurations for both confluent.value.subject.name.strategy and confluent.key.subject.name.strategy are:

• io.confluent.kafka.serializers.subject.TopicNameStrategy (the default)
• io.confluent.kafka.serializers.subject.RecordNameStrategy
• io.confluent.kafka.serializers.subject.TopicRecordNameStrategy

See also

Schemas, Subjects, and Topics.

Enabling Schema Validation on a topic in Confluent Control Center

You can view, enable, or disable Schema Validation on a topic in Control Center. Additionally, you can configure per-topic subject naming strategies.

View or Change Schema Validation settings for a topic

To view current configurations or enable Schema Validation on a topic from the Control Center (http://localhost:9021/):

1. Click the Configuration tab on an existing topic.

2. Click Switch to expert mode.

   

3. In Expert mode, change the settings for confluent.value.schema.validation and confluent.key.schema.validation from false to true.

   

4. Click Save changes.

Change the subject naming strategy for a topic

To change the subject naming strategy using Control Center:

1. On Control Center, select the topic to update, click Configuration, then click Switch to expert mode.

   

2. Modify the settings and click Save changes.

The possible configurations for both confluent.value.subject.name.strategy and confluent.key.subject.name.strategy are:

• io.confluent.kafka.serializers.subject.TopicNameStrategy (the default)
• io.confluent.kafka.serializers.subject.RecordNameStrategy
• io.confluent.kafka.serializers.subject.TopicRecordNameStrategy

To learn more, see Change the subject naming strategy for a topic, which describes how to perform the same tasks on the Confluent CLI and gives more background information on the feature, which is new in Confluent Platform 5.5.0.

Demo: Enabling Schema Validation on a Topic at the Command Line

This short demo shows the effect of enabling or disabling schema validation on a topic.

If you are just getting started with Confluent Platform and Schema Registry, you might want to first work through the Schema Registry Tutorials, then return to this demo.

The examples make use of the kafka-console-producer and kafka-console-consumer, which are located in $CONFLUENT_HOME/bin.

1. On a local install of Confluent Platform version 5.4.0 or later, modify $CONFLUENT_HOME/etc/kafka/server.properties to include the following configuration for the Schema Registry URL:

   ############################## My Schema Validation Demo Settings ################
   # Schema Registry URL
   confluent.schema.registry.url=http://localhost:8081
   

   The example above includes two lines of comments, which are optional, to keep track of the configurations in the file.

2. Start Confluent Platform using the following command:

   confluent local start
   

   Tip

   • Alternatively, you can simply run confluent local schema-registry which also starts kafka and zookeeper as dependencies. This demo does not directly reference the other services, such as Connect and Control Center. That said, you may want to run the full stack anyway to further explore, for example, how the topics and messages display on Control Center. To learn more about confluent local, see Quick Start for Apache Kafka using Confluent Platform (Local) and confluent local in the Confluent CLI command reference.
   • The confluent local commands run in the background so you can re-use this command window. Separate sessions are required for the producer and consumer.

3. Create a test topic called test-schemas without specifying the Schema Validation setting so that it defaults to false.

   kafka-topics --bootstrap-server localhost:9092 --create --partitions 1 --replication-factor 1 --topic test-schemas
   

   This creates a topic with no broker validation on records produced to the test topic, which is what you want for the first part of the demo. You can verify that the topic was created with kafka-topics --bootstrap-server localhost:9092 --list.

4. In a new command window for the producer, run this command to produce a serialized record (using the default string serializer) to the topic test-schemas.

   kafka-console-producer --broker-list localhost:9092 --topic test-schemas --property parse.key=true --property key.separator=,
   

   The command is successful because you currently have Schema Validation disabled for this topic. If broker Schema Validation had been enabled for this topic, the above command to produce to it would not be permitted.

   The output of this command is a producer command prompt (>), where you can type the messages you want to produce.

   Type your first message at the > prompt as follows:

   1,my first record
   

   Keep this session of the producer running.

5. Open a new command window for the consumer, and enter this command to read the messages:

   kafka-console-consumer --bootstrap-server localhost:9092 --from-beginning --topic test-schemas --property print.key=true
   

   The output of this command is my first record.

   Keep this session of the consumer running.

6. Now, set Schema Validation for the topic test-schemas to true.

   kafka-configs --bootstrap-server localhost:9092 --alter --entity-type topics --entity-name test-schemas --add-config confluent.value.schema.validation=true
   

   You should get a confirmation: Completed updating config for topic test-schemas.

7. Return to the producer session, and type a second message at the > prompt.

   2,my second record
   

   You will get an error because Schema Validation is enabled and the messages we are sending do not contain schema IDs: This record has failed the validation on broker

   If you subsequently disable Schema Validation (use the same command to set it to false), stop and restart the producer, then retry the producer and consumer steps to send and read messages in those open sessions, the messages will go through. (For example, produce 3,my third record.)

   The messages that were successfully produced also show on Control Center (http://localhost:9021/ in your web browser) in Topics > test-schemas > messages. You may have to select a partition or jump to a timestamp to see messages sent earlier.

   

8. Run shutdown and cleanup tasks.

   • You can stop the consumer and producer with Ctl-C in their respective command windows.
   • To stop Confluent Platform, type confluent local stop.
   • If you would like to clear out existing data (topics, schemas, and messages) before starting again with another test, type confluent local destroy.

Suggested Reading

• Blog post: Schema Validation with Confluent Platform 5.4