Schema Validation enables the broker to verify that data produced to a Kafka topic is using a valid schema ID in Schema Registry that is registered according to the subject naming strategy.
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 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.RecordNameStrategyio.confluent.kafka.serializers.subject.TopicRecordNameStrategy
Enabling Schema Validation on a topic in Confluent Control Center
You can view, enable, or disable Schema Validation on a topic in Control Center (see Manage Schemas for Topics in Control Center guide). 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/):
Click the Configuration tab on an existing topic, and click Edit settings.
Click Switch to expert mode.
In Expert mode, change the settings for confluent.value.schema.validation and confluent.key.schema.validation from false to true.
You may need to scroll down to find confluent.key.schema.validation.
Click Save changes.
Change the subject naming strategy for a topic
To change the subject naming strategy using Control Center:
On Control Center, select the topic to update, click Configuration, then click Switch to expert mode.
Search for confluent.value.subject.name.strategy and confluent.key.subject.name.strategy.
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.RecordNameStrategyio.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.
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.
Start Confluent Platform using the following command:
confluent local services 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.
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.
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:
Keep this session of the producer running.
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.
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.
Return to the producer session, and type a second message at the > prompt.
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), restart the producer, then type and resend the same or another similarly formatted message,
the message will go through. (For example, produce 3,my third record.)
Tip
As an alternative to restarting the producer in this last step (after switching schema validation back to false), you can simply type or copy-paste the
suggested message 3,my third record on the blank line after the error and hit return. The consumer will pick it up, and you will then get the producer
prompt back. However, this is an unintuitive workflow because there is no prompt showing after the error, and you may be tempted to hit return first,
which will shut down the producer.
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.
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.