Kafka Connect FAQ¶

How do I change the output data format of a SinkConnector?¶

The format that is written to the output system is dependent on the SinkConnector itself. The SinkConnector will translate records into the output data format from the format produced by the converter specified in key.converter and value.converter. To see the supported output formats, refer to the specific connector documentation.

Why does a connector configuration update trigger a task rebalance?¶

Not all connector configuration updates will trigger a task rebalance, but most will. This is because the trigger for the task rebalance is actually a need to reconfigure tasks in a way that Connect cannot safely do without a rebalance. There are two basic reasons for a rebalance to happen:

The total number of connectors or tasks has changed, requiring reassignments to happen within the active workers.
Configuration of tasks change in a way that Connect cannot be sure if tasks will need to coordinate to ensure correct behavior. For example, you want to avoid skipping or repeating messages if partitions are reassigned.

Why should I use distributed mode instead of standalone?¶

Confluent recommends you use distributed mode for most production use cases. For details, see here.

Do I need to write custom code to use Kafka Connect?¶

The Connector Hub has many Connectors that cover many use cases such that most users will not need to write any code. If your use case is not covered, it may require you to extend one of these Connectors or write a new one. If you find that to be the case, see the developer guide.

Is Schema Registry a required service to run Kafka Connect?¶

No, it is not required, but it is recommended if you plan to use Avro for a data format. This is because it can help you with serialization and schema evolution as described here.

How can I access Producer and Consumer JMX metrics from Connect workers?¶

Add the following properties to the Connect worker config file. The ConfluentMetricsReporter metrics reporter will pull out producer and consumer JMX metrics from each worker JVM and write them into an Apache Kafka® topic (_confluent-connect-metrics).

metric.reporters=io.confluent.metrics.reporter.ConfluentMetricsReporter
confluent.metrics.reporter.bootstrap.servers=metrics-kafka:9092
confluent.metrics.reporter.topic=_confluent-connect-metrics
confluent.metrics.reporter.topic.replicas=1
confluent.metrics.reporter.whitelist=.*
confluent.metrics.reporter.publish.ms=60000

The messages in _confluent-connect-metrics can be consumed using the following command.

kafka-console-consumer --topic _confluent-connect-metrics \
                       --bootstrap-server metrics-kafka:9092 \
                       --formatter io.confluent.metrics.reporter.ConfluentMetricsFormatter

How can I use plain JSON data with Connect?¶

When using plain JSON data with Connect, users will see the following kind of error message:

org.apache.kafka.connect.errors.DataException: JsonDeserializer with schemas.enable requires "schema" and "payload" fields and may not contain additional fields

You will need to set the schemas.enable parameters for the converter to false as described here.

Does source connector X support output format Y?¶

Source connectors read data from a system and use a converter to change data from the connect data format being used to byte array. For source connectors, this means that any format for which a converter exists can be used to write data to Kafka. More details about converters are here.

Tip

Confluent has an article that goes into greater detail about this subject. See Kafka Connect Deep Dive – Converters and Serialization Explained.

Why is CPU usage high for my Connect worker when no connectors have been deployed?¶

This is because upon startup the worker will read all files on the CLASSPATH to search for available connector plugins. Avoid having large portions of the file system on the worker’s CLASSPATH to avoid this problem.

Can connect sink connectors read data written by other clients, e.g. a custom client?¶

This depends on how the data was written. The data must be written so that it is compatible with the converter specified in key.converter and value.converter. For example, applications writing Avro data using the converter provided in Schema Registry will produce compatible data to be read by a sink connector using the same converter.

After testing a connector in standalone mode, restarting it doesn’t write the data again?¶

Standalone connectors store their offsets in a local file specified by offset.storage.file.filename. Connectors are designed to not reprocess data typically, so you would want to remove the file specified by this configuration to force the connector to reprocess the data.

Can I use a newer version of Connect with older brokers?¶

Connect follows the same compatibility rules as the Java clients for Kafka. As of Confluent Platform 3.2.0 and Kafka 0.10.2, in general it is possible to mix older and newer versions of both Kafka brokers and Kafka Connect workers. However, newer features in Kafka Connect (such as support for headers) will not work unless Connect is operating with a broker that also supports those features.