[jira] [Created] (KAFKA-13320) Suggestion: SMT support for null key/value should be documented

["Ben Ellis (Jira)" <ji...@apache.org>]

Ben Ellis created KAFKA-13320:
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             Summary: Suggestion: SMT support for null key/value should be documented
                 Key: KAFKA-13320
                 URL: https://issues.apache.org/jira/browse/KAFKA-13320
             Project: Kafka
          Issue Type: Wish
          Components: KafkaConnect
            Reporter: Ben Ellis


While working with a JDBC Sink Connector, I noticed that some SMT choke on a tombstone (null value) while others handle tombstones fine.

For example:

```
"transforms": "flattenKey,valueToJSON,wrapValue,addTimestamp", "transforms.flattenKey.type": "org.apache.kafka.connect.transforms.Flatten$Key", "transforms.flattenKey.delimiter": "_", "transforms.valueToJSON.type": "com.github.jcustenborder.kafka.connect.transform.common.ToJSON$Value", "transforms.valueToJSON.schemas.enable": "false", "transforms.valueToJSON.predicate": "tombstone", "transforms.valueToJSON.negate": true, "transforms.wrapValue.type":"org.apache.kafka.connect.transforms.HoistField$Value", "transforms.wrapValue.field":"matrix", "transforms.wrapValue.predicate": "tombstone", "transforms.wrapValue.negate": true, "transforms.addTimestamp.type": "org.apache.kafka.connect.transforms.InsertField$Value", "transforms.addTimestamp.timestamp.field": "message_timestamp", "predicates": "tombstone", "predicates.tombstone.type": "org.apache.kafka.connect.transforms.predicates.RecordIsTombstone"

```

To avoid the cryptic error “java.lang.ClassCastException: class java.util.HashMap cannot be cast to class org.apache.kafka.connect.data.Struct” when processing a tombstone record, I had to add a negated predicate of RecordIsTombstone for ToJSON (community SMT) and HoistField, but did not need to add that to InsertField.

Digging in the source, I find that InsertField handles the case where key or value is null:
https://github.com/a0x8o/kafka/blob/f8237749f6ad34c09154f807e53273be64e1261e/connect/transforms/src/main/java/org/apache/kafka/connect/transforms/InsertField.java#L130

^ Thanks to this, there's no need to add a predicate to skip InsertField$Value when value is null.

It would help if the docs listed how the individual SMTs behave when dealing with a null key/value.

Of course we can always find this out by trial and error or by studying the source code.
But if we were to make a best practice of describing how an SMT handles null key/value, that would have two benefits:
1) Save developers time when working with the official (shipped with Kafka) SMT
2) Inspire developers who write their own SMT to likewise document how they handle null key/value

Perhaps a standard way of dealing with nulls ("no-op if key/value is null") could be promoted, and SMT authors would only need to document their behavior when it differs.
​



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