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Data Serialization

Changing Data Serialization Format from Delimited (CSV) to Avro

Data comes in different formats. Oftentimes, you have to convert delimited data (otherwise known as CSV or comma-separated data) to Avro each time you want to use it in Avro format. KSQL provides a powerful way for developers to reserialize the data in a Kafka topic by defining new streams with the new topic’s desired serialization, populated in real time by streaming events of the original topic.

The fantastic thing here is that these are streaming transformations, so not only does all existing data on the topic get converted, but so does every single message that subsequently arrives on the source topic.


1. Create a Kafka topic to receive messages with DELIMITED data:

$ kafka-topics --create --zookeeper localhost:2181 --replication-factor 1 --partitions 1 --topic LegacyData

2. Create the KSQL stream for DELIMITED data:

ksql> CREATE STREAM source_delimited (column1 STRING, column2 STRING) WITH (KAFKA_TOPIC='LegacyData', VALUE_FORMAT='DELIMITED');

3. Create the KSQL stream for the converted Avro data, which automatically converts the delimited source data to Avro:

ksql> CREATE STREAM target_avro WITH (KAFKA_TOPIC='LegacyDataAvro',VALUE_FORMAT='AVRO') AS SELECT * FROM source_delimited;

4. Create a KSQL SELECT query to test the conversion:

ksql> select * from target_avro LIMIT 1;

5. Send a test message with DELIMITED data to the source topic and validate that the target topic receives the data in Avro format (check the query from step 4):

$ echo "hello,world" | kafka-console-producer --broker-list localhost:9092 --topic LegacyData

6. Validate that the target topic was created with the value format set to Avro:

ksql> describe target_avro;
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