java - Is it possible to access message headers with Kafka Streams?

Question

With the addition of Headers to the records (ProducerRecord & ConsumerRecord) in Kafka 0.11, is it possible to get these headers when processing a topic with Kafka Streams? When calling methods like map on a KStream it provides arguments of the key and the value of the record but no way I can see to access the headers. It would be nice if we could just map over the ConsumerRecords.

ex.

KStreamBuilder kStreamBuilder = new KStreamBuilder();
KStream<String, String> stream = kStreamBuilder.stream("some-topic");
stream
    .map((key, value) ->  ... ) // can I get access to headers in methods like map, filter, aggregate, etc?
    ... 

something like this would work:

KStreamBuilder kStreamBuilder = new KStreamBuilder();
KStream<String, String> stream = kStreamBuilder.stream("some-topic");
stream
    .map((record) -> {
        record.headers();
        record.key();
        record.value();
    })
    ...

Answer 1

Records headers are accessible since versions 2.0.0 (cf. KIP-244 for details).

You can access record metadata via the Processor API (ie, via transform(), transformValues(), or process()), by the given "context" object (cf. https://docs.confluent.io/current/streams/developer-guide/processor-api.html#accessing-processor-context).

Update

As of 2.7.0 release, the Processor API was improved (cf. KIP-478), adding a new type-safe api.Processor class with process(Record) instead of process(K, V) method. For this case, headers (and record metadata) are accessible via the Record class).

This new feature is not yet available in "PAPI method of the DSL though (eg. KStream#process(), KStream#transform() and siblings).

+++++

Prior to 2.0, the context only exposes topic, partition, offset, and timestamp---but not headers that are in fact dropped by Streams on read in those older versions.

Metadata is not available at DSL level though. However, there is also work in progress to extend the DSL via KIP-159.