Status

Current state: "Under Discussion"

Discussion thread: here

JIRA: Unable to render Jira issues macro, execution error.

Please keep the discussion on the mailing list rather than commenting on the wiki (wiki discussions get unwieldy fast).

Motivation

There's currently an incompatibility between Sink connectors overriding the SinkTask.preCommit method (for asynchronous processing) and SMTs that mutate the topic field.

The problem was present since the preCommit method inception and is rooted in a mismatch between the topic/partition that is passed to open/preCommit (the original topic and partition before applying any transformations) and the topic partition that is present in the SinkRecord that the SinkTask.put method receives (after transformations are applied). Since that's all the information the connector has to implement any kind of internal offset tracking, the topic/partitions it can return in preCommit will correspond to the transformed topic, when the framework actually expects it to be the original topic.

In Unable to render Jira issues macro, execution error. we fixed the problem for connectors that don't override  SinkTask.preCommit. For the others, we acknowledge that "broader API changes are required". 

Public Interfaces

org.apache.kafka.connect.sink.SinkRecord

Add two new fields, with their corresponding getters and constructor:

    private final String originalTopic;
    private final Integer originalKafkaPartition;
...
    public SinkRecord(String topic, int partition, Schema keySchema, Object key, Schema valueSchema, Object value, long kafkaOffset,
                      Long timestamp, TimestampType timestampType, Iterable<Header> headers, String originalTopic, int originalKafkaPartition) {
...
    /**
     * @return topic corresponding to the Kafka record before transformations were applied. This is
     * necessary for internal offset tracking, to be compatible with SMTs that mutate the topic name.
     */
    public String originalTopic() {
        return originalTopic;
    }

    /**
     * @return topic partition corresponding to the Kafka record before transformations were applied. This is
     * necessary for internal offset tracking, to be compatible with SMTs that mutate the topic name.
     */
    public String originalKafkaPartition() {
        return originalKafkaPartition;
    }

Proposed Changes

Expose the original topic/partition in SinkRecord and ask Sink Connectors to use that information for offset tracking purposes.

Compatibility, Deprecation, and Migration Plan

Backwards Compatibility

This proposal is backward compatible such that existing sink connector implementations will continue to work as before.

Forward Compatibility

To ensure that new connectors using this new method and interface can still be deployed on older versions of Kafka Connect, the developer should use a try catch block to catch the NoSuchMethodError or NoClassDefFoundError thrown by worker with an older version of AK, but it should be clearly documented that they would not be compatible with topic-mutating SMTs in those environments.

Rejected Alternatives

1. Address the problem entirely within the framework, doing some kind of mapping from the transformed topic back to the original topic.

   1. This would only work in the cases where there’s no overlap between the transformed topic names, but would break for the rest of the transformations (e.g. static transformation, topic = “a”).

   2. Even if we wanted to limit the support to those cases, it would require considerable bookkeeping to add a validation to verify that the transformation chain adheres to that expectation (and fail fast if it doesn’t).

2. Expose the entire original record instead of only topic/partition (e.g. originalSinkRecord)

   • We should not expose the original value/key, transformations might be editing them for security reasons.

3. Create a method in the SinkTaskContext to get this information, instead of updating SinkRecord (e.g. SinkTaskContext.getOriginalTopic(SinkRecord sr) / SinkTaskContext.getOriginalKafkaPartition(SinkRecord sr)

   • Requires extra bookkeeping without concrete value.

4. Update SinkTask.put in any way to pass the new information outside SinkRecord (e.g. a Map or a derived class)

   • Much more disruptive change without considerable pros