How to add a custom StateStore to the Kafka Stream

For one of my Kafka streams apps, I need to use the features of both DSL and Processor API. My streaming app flow is

source -> selectKey -> filter -> aggregate (on a window) -> sink

After aggregation I need to send a SINGLE aggregated message to the sink. So I define my topology as below

KStreamBuilder builder = new KStreamBuilder();
KStream<String, String> source = builder.stream(source_stream);
source.selectKey(new MyKeyValueMapper())
      .filterNot((k,v) -> k.equals("UnknownGroup"))
      .process(() -> new MyProcessor());

I define a custom StateStore and register it with my processor as below

public class MyProcessor implements Processor<String, String> {

    private ProcessorContext context = null;
    Serde<HashMapStore> invSerde = Serdes.serdeFrom(invJsonSerializer, invJsonDeserializer);


    KeyValueStore<String, HashMapStore> invStore = (KeyValueStore) Stores.create("invStore")
        .withKeys(Serdes.String())
        .withValues(invSerde)
        .persistent()
        .build()
        .get();

    public MyProcessor() {
    }

    @Override
    public void init(ProcessorContext context) {
        this.context = context;
        this.context.register(invStore, false, null); // register the store
        this.context.schedule(10 * 60 * 1000L);
    }

    @Override
    public void process(String partitionKey, String message) {
        try {
            MessageModel smb = new MessageModel(message);
            HashMapStore oldStore = invStore.get(partitionKey);
            if (oldStore == null) {
                oldStore = new HashMapStore();
            }
            oldStore.addSmb(smb);
            invStore.put(partitionKey, oldStore);
        } catch (Exception e) {
           e.printStackTrace();
        }
    }

    @Override
    public void punctuate(long timestamp) {
       // processes all the messages in the state store and sends single aggregate message
    }


    @Override
    public void close() {
        invStore.close();
    }
}

When I run the app, I get java.lang.NullPointerException

Exception in thread "StreamThread-18" java.lang.NullPointerException at org.apache.kafka.streams.state.internals.MeteredKeyValueStore.flush(MeteredKeyValueStore.java:167) at org.apache.kafka.streams.processor.internals.ProcessorStateManager.flush(ProcessorStateManager.java:332) at org.apache.kafka.streams.processor.internals.StreamTask.commit(StreamTask.java:252) at org.apache.kafka.streams.processor.internals.StreamThread.commitOne(StreamThread.java:446) at org.apache.kafka.streams.processor.internals.StreamThread.commitAll(StreamThread.java:434) at org.apache.kafka.streams.processor.internals.StreamThread.maybeCommit(StreamThread.java:422) at org.apache.kafka.streams.processor.internals.StreamThread.runLoop(StreamThread.java:340) at org.apache.kafka.streams.processor.internals.StreamThread.run(StreamThread.java:218)

Any idea what's going wrong here?

You need to register you store outside of you processor using StreamsBuilder (or KStreamBuilder in older releases). First you create the store, than you registers it to StreamsBuilder (KStreamBuilder), and when you add the processor you provide the store name to connect the processor and the store.

StreamsBuilder builder = new StreamsBuilder();

// create store
StoreBuilder storeBuilder = Stores.keyValueStoreBuilder(
    Stores.persistentKeyValueStore("invStore"),
    Serdes.String(),
    invSerde));
// register store
builder.addStateStore(storeBuilder);

KStream<String, String> source = builder.stream(source_stream);
source.selectKey(new MyKeyValueMapper())
        .filterNot((k,v) -> k.equals("UnknownGroup"))
        .process(() -> new MyProcessor(), "invStore"); // connect store to processor by providing store name


// older API:

KStreamBuilder builder = new KStreamBuilder();

// create store
StateStoreSupplier storeSupplier = (KeyValueStore)Stores.create("invStore")
    .withKeys(Serdes.String())
    .withValues(invSerde)
    .persistent()
    .build();
// register store
builder.addStateStore(storeSupplier);

KStream<String, String> source = builder.stream(source_stream);
source.selectKey(new MyKeyValueMapper())
        .filterNot((k,v) -> k.equals("UnknownGroup"))
        .process(() -> new MyProcessor(), "invStore"); // connect store to processor by providing store name