Schema Registry Transfer SMT

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A Kafka Connect Single Message Transformation (SMT) that reads the serialized wire format header of Confluent's KafkaAvroSerializer, performs a lookup against a source Confluent Schema Registry for the ID in the message, and registers that schema into a destination Registry for that topic/subject under a new ID.

To be used where it is not feasible to make the destination Schema Registry as a follower to the source Registry, or when migrating topics to a new cluster.

Requires that the Kafka Connect tasks can reach both Schema Registries.

This transform doesn't mirror the contents of the _schemas topic, so therefore each registry can be completely isolated from one another. As a side-effect of this, the subject configurations that might be applied to the /config endpoint in the source registry are not copied to the destination. In other words, you might get schema registration errors if using differing compatibility levels on the registries. Just a heads-up.

Example Kafka Connectors where this could be applied.


  1. Edit the Kafka Connect worker properties file on each worker to include a new directory. For example, /opt/kafka-connect/plugins
  1. Build this project
./mvnw clean package
  1. Copy the JAR from target to all Kafka Connect workers under a directory set by plugin.path

  2. (Re)start Kafka Connect processes


Standalone Kafka Connect configuration section

# Requires that records are entirely byte-arrays. These can go in the worker or connector configuration.

# Setup the SMT


Distributed Kafka Connect configuration section

"config" : {

    "__comment": "Requires that records are entirely byte-arrays. These can go in the worker or connector configuration.",
    "key.converter": "org.apache.kafka.connect.converters.ByteArrayConverter",
    "value.converter": "org.apache.kafka.connect.converters.ByteArrayConverter",

    "__comment": "Setup the SMT",
    "transforms": "AvroSchemaTransfer",

    "transforms.AvroSchemaTransfer.type": "cricket.jmoore.kafka.connect.transforms.SchemaRegistryTransfer",
    "transforms.AvroSchemaTransfer.src.schema.registry.url": "http://schema-registry-1:8081",
    "transforms.AvroSchemaTransfer.dest.schema.registry.url": "http://schema-registry-2:8081"

Advanced Configuration

Configuration Parameter Default Description
transfer.message.keys true Indicates whether Avro schemas from message keys in source records should be copied to the destination Registry.
include.message.headers true Indicates whether message headers from source records should be preserved after the transform.
schema.capacity 100 Capacity of schemas that can be cached in each CachedSchemaRegistryClient

Embedded Schema Registry Client Configuration

Schema Registry Transfer SMT passes some properties prefixed by either src. or dest. through to its embedded schema registry clients, after stripping away src. or dest. prefix used to disambiguate which client is to receive which configuration value.

Properties prefixed by src. are passed through to the source consumer's schema registry client. Properties prefixed by dest. are passed through to the target producer's schema registry client.

Configuration Parameter Default Description
(src|dest).basic.auth.credentials.source URL Specify how to pick credentials for Basic Auth header. Supported values are URL, USER_INFO and SASL_INHERIT
(src|dest) Specify credentials for Basic Auth in form of {username}:{password} when source is USER_INFO

Subject Renaming

Renaming of a subject can be done with the RegexRouter Transform before this one.

Example Configuration