Adding custom headers in Java Kafka Streams

[miat]

I’m working on a Kafka Streams application in Java and need to figure out how to include custom headers in my stream processing. I’ve been looking through the documentation but can’t seem to find a clear example of how to properly add headers to messages as they flow through my stream topology.

Has anyone dealt with this before? I want to make sure I’m using the right approach and not missing any important details about header manipulation in KStreams.

KStream<String, UserEvent> eventStream = builder.stream("user-events");

// Need to add headers here - how?
KStream<String, UserEvent> enrichedStream = eventStream.mapValues(event -> {
    // Process the event
    return processUserEvent(event);
});

What’s the proper way to attach headers during stream processing?

[SwiftCoder42]

You’ll want to use transform instead of mapValues since headers aren’t accessible through simple value transformation methods. I hit this exact issue last year building a similar app. The trick is implementing a Transformer that gives you access to ProcessorContext, which lets you manipulate headers directly. Here’s what worked for me: java KStream<String, UserEvent> enrichedStream = eventStream.transform(() -> new Transformer<String, UserEvent, KeyValue<String, UserEvent>>() { private ProcessorContext context; @Override public void init(ProcessorContext context) { this.context = context; } @Override public KeyValue<String, UserEvent> transform(String key, UserEvent value) { UserEvent processed = processUserEvent(value); context.headers().add("custom-header", "your-value".getBytes()); return new KeyValue<>(key, processed); } @Override public void close() {} }); Header values need to be byte arrays, so convert strings accordingly. Transform gives you full control over both message content and metadata.

[Sky24]

Try using peek with transform if you need to add headers based on message content. I had a case where headers only got set for specific event types - peek let me check the values first before messing with headers. Here’s what I learned: header operations need ProcessorContext access, so any method without it won’t work. Also, watch your header serialization. If you’re using complex objects as header values, make sure downstream consumers can deserialize them. I’ve spent way too much time debugging cases where headers added fine but broke things later because of format mismatches.

[Luke_Brilliant]

you could also use transformValues if you’re not changing the key. it’s simpler than a full transform since you only deal with values, but you still get headers through context. just handle the byte array conversion right - i’ve seen people screw this up and run into encoding problems down the line.