Stateful filtering/flatMapValues in Kafka Streams? - apache-kafka

Question

I'm trying to write a simple Kafka Streams application (targeting Kafka 2.2/Confluent 5.2) to transform an input topic with at-least-once semantics into an exactly-once output stream. I'd like to encode the following logic:
For each message with a given key:
Read a message timestamp from a string field in the message value
Retrieve the greatest timestamp we've previously seen for this key from a local state store
If the message timestamp is less than or equal to the timestamp in the state store, don't emit anything
If the timestamp is greater than the timestamp in the state store, or the key doesn't exist in the state store, emit the message and update the state store with the message's key/timestamp
(This is guaranteed to provide correct results based on ordering guarantees that we get from the upstream system; I'm not trying to do anything magical here.)
At first I thought I could do this with the Kafka Streams flatMapValues operator, which lets you map each input message to zero or more output messages with the same key. However, that documentation explicitly warns:
This is a stateless record-by-record operation (cf. transformValues(ValueTransformerSupplier, String...) for stateful value transformation).
That sounds promising, but the transformValues documentation doesn't make it clear how to emit zero or one output messages per input message. Unless that's what the // or null aside in the example is trying to say?
flatTransform also looked somewhat promising, but I don't need to manipulate the key, and if possible I'd like to avoid repartitioning.
Anyone know how to properly perform this kind of filtering?

Answer 1

you could use Transformer for implementing stateful operations as you described above. In order to not propagate a message downstream, you need to return null from transform method, this mentioned in Transformer java doc. And you could manage propagation via processorContext.forward(key, value). Simplified example provided below
kStream.transform(() -> new DemoTransformer(stateStoreName), stateStoreName)
public class DemoTransformer implements Transformer<String, String, KeyValue<String, String>> {
private ProcessorContext processorContext;
private String stateStoreName;
private KeyValueStore<String, String> keyValueStore;
public DemoTransformer(String stateStoreName) {
this.stateStoreName = stateStoreName;
}
#Override
public void init(ProcessorContext processorContext) {
this.processorContext = processorContext;
this.keyValueStore = (KeyValueStore) processorContext.getStateStore(stateStoreName);
}
#Override
public KeyValue<String, String> transform(String key, String value) {
String existingValue = keyValueStore.get(key);
if (/* your condition */) {
processorContext.forward(key, value);
keyValueStore.put(key, value);
}
return null;
}
#Override
public void close() {
}
}