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

• Type: Wish
• Status: Open
• Priority: Minor
• Resolution: Unresolved
• Component/s: connect
• Assignee: Unassigned
• Reporter: Ben Ellis
• Created: 23/Sep/21 14:02
• Updated: 23/Sep/21 14:06

Description

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.