Schema Registry Transfer SMT

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.

Comcast/MirrorTool-for-Kafka-Connect - Code was tested with this first, and verified that the topic-renaming logic of this connector worked fine with this SMT.
Salesforce/mirus
Confluent Replicator - While this already can copy the schema, we observed it is only possible via the AvroConverter, which must first parse the entire message into a Kafka Connect Struct object. Thus, the class here is considered a "shallow" copier — it only inspects the first 5 bytes of the keys and values for the schema ids.
KIP-382 (MirrorMaker 2.0) - Still open at the time of writing.

Installation

Edit the Kafka Connect worker properties file on each worker to include a new directory. For example, /opt/kafka-connect/plugins

plugin.path=/usr/share/java,/opt/kafka-connect/plugins

Build this project

./mvnw clean package

Copy the JAR from target to all Kafka Connect workers under a directory set by plugin.path
(Re)start Kafka Connect processes

Usage

Standalone Kafka Connect configuration section

# 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

# 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

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).basic.auth.user.info		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

transforms=TopicRename,AvroSchemaTransfer

transforms.TopicRename.type=org.apache.kafka.connect.transforms.RegexRouter
transforms.TopicRename.regex=(.*)
transforms.TopicRename.replacement=replica.$1

transforms.AvroSchemaTransfer.type=...