I'm working on an example by Stepane Mareek regarding KafkaStreams.
Basically, the code takes transactions and calculates the AccountBalance. Unfortunately I get a ClassCastException and i do not understand why or whats happening.
The Exception:
Exception in thread "bank-balance-dz-application-8f38e2f1-fc8e-4bb9-bcb7-82958aa39aff-StreamThread-1" org.apache.kafka.streams.errors.StreamsException: A serializer (org.apache.kafka.common.serialization.ByteArraySerializer) is not compatible to the actual value type (value type: com.fasterxml.jackson.databind.node.ObjectNode). Change the default Serdes in StreamConfig or provide correct Serdes via method parameters.
at org.apache.kafka.streams.state.StateSerdes.rawValue(StateSerdes.java:204)
at org.apache.kafka.streams.state.internals.MeteredKeyValueStore.lambda$put$3(MeteredKeyValueStore.java:144)
at org.apache.kafka.streams.processor.internals.metrics.StreamsMetricsImpl.maybeMeasureLatency(StreamsMetricsImpl.java:836)
at org.apache.kafka.streams.state.internals.MeteredKeyValueStore.put(MeteredKeyValueStore.java:144)
at org.apache.kafka.streams.processor.internals.AbstractReadWriteDecorator$KeyValueStoreReadWriteDecorator.put(AbstractReadWriteDecorator.java:100)
at org.apache.kafka.streams.kstream.internals.KStreamAggregate$KStreamAggregateProcessor.process(KStreamAggregate.java:108)
at org.apache.kafka.streams.processor.internals.ProcessorNode.lambda$process$2(ProcessorNode.java:142)
at org.apache.kafka.streams.processor.internals.metrics.StreamsMetricsImpl.maybeMeasureLatency(StreamsMetricsImpl.java:836)
at org.apache.kafka.streams.processor.internals.ProcessorNode.process(ProcessorNode.java:142)
at org.apache.kafka.streams.processor.internals.ProcessorContextImpl.forward(ProcessorContextImpl.java:236)
at org.apache.kafka.streams.processor.internals.ProcessorContextImpl.forward(ProcessorContextImpl.java:216)
at org.apache.kafka.streams.processor.internals.ProcessorContextImpl.forward(ProcessorContextImpl.java:168)
at org.apache.kafka.streams.processor.internals.SourceNode.process(SourceNode.java:96)
at org.apache.kafka.streams.processor.internals.StreamTask.lambda$process$1(StreamTask.java:679)
at org.apache.kafka.streams.processor.internals.metrics.StreamsMetricsImpl.maybeMeasureLatency(StreamsMetricsImpl.java:836)
at org.apache.kafka.streams.processor.internals.StreamTask.process(StreamTask.java:679)
at org.apache.kafka.streams.processor.internals.TaskManager.process(TaskManager.java:1033)
at org.apache.kafka.streams.processor.internals.StreamThread.runOnce(StreamThread.java:690)
at org.apache.kafka.streams.processor.internals.StreamThread.runLoop(StreamThread.java:551)
at org.apache.kafka.streams.processor.internals.StreamThread.run(StreamThread.java:510)
Caused by: java.lang.ClassCastException: class com.fasterxml.jackson.databind.node.ObjectNode cannot be cast to class [B (com.fasterxml.jackson.databind.node.ObjectNode is in unnamed module of loader 'app'; [B is in module java.base of loader 'bootstrap')
at org.apache.kafka.common.serialization.ByteArraySerializer.serialize(ByteArraySerializer.java:19)
at org.apache.kafka.streams.state.internals.ValueAndTimestampSerializer.serialize(ValueAndTimestampSerializer.java:104)
at org.apache.kafka.streams.state.internals.ValueAndTimestampSerializer.serialize(ValueAndTimestampSerializer.java:95)
at org.apache.kafka.streams.state.internals.ValueAndTimestampSerializer.serialize(ValueAndTimestampSerializer.java:27)
at org.apache.kafka.streams.state.StateSerdes.rawValue(StateSerdes.java:192)
In debug it tries to use a "valueAndTimestampSerializer?"(Picture in Link)
So I try to explain the Code, hopefully my unterstanding ist right.
- Banktransaction takes the key and value of the topic and try to parse the key to String and the value to JsonNode
- Then I group the entries by key and aggregate the values. (Do I have to pass the Serdes again? If i don't pass them would they be serialized as String?)
- I produce the table to the topic "bank-balance-exactly-once" (Do i have to do the ".toStream()"?)
"My" Code:
import com.fasterxml.jackson.databind.JsonNode;
import com.fasterxml.jackson.databind.node.JsonNodeFactory;
import com.fasterxml.jackson.databind.node.ObjectNode;
import org.apache.kafka.clients.consumer.ConsumerConfig;
import org.apache.kafka.common.serialization.Deserializer;
import org.apache.kafka.common.serialization.Serde;
import org.apache.kafka.common.serialization.Serdes;
import org.apache.kafka.common.serialization.Serializer;
import org.apache.kafka.connect.json.JsonDeserializer;
import org.apache.kafka.connect.json.JsonSerializer;
import org.apache.kafka.streams.KafkaStreams;
import org.apache.kafka.streams.StreamsBuilder;
import org.apache.kafka.streams.StreamsConfig;
import org.apache.kafka.streams.kstream.*;
import java.time.Instant;
import java.util.Properties;
public class BankBalanceStream {
public static void main(String[] args) {
Properties properties = new Properties();
properties.put(StreamsConfig.APPLICATION_ID_CONFIG, "bank-balance-application");
properties.put(StreamsConfig.BOOTSTRAP_SERVERS_CONFIG, "localhost:9092");
properties.put(ConsumerConfig.AUTO_OFFSET_RESET_CONFIG, "earliest");
properties.put(StreamsConfig.PROCESSING_GUARANTEE_CONFIG, "0");
properties.put(StreamsConfig.PROCESSING_GUARANTEE_CONFIG, StreamsConfig.EXACTLY_ONCE);
final Serializer<JsonNode> jsonNodeSerializer = new JsonSerializer();
final Deserializer<JsonNode> jsonNodeDeserializer = new JsonDeserializer();
final Serde<JsonNode> jsonSerde = Serdes.serdeFrom(jsonNodeSerializer, jsonNodeDeserializer);
StreamsBuilder builder = new StreamsBuilder();
KStream<String, JsonNode> bankTransactions = builder.stream("bank-transactions", Consumed.with(Serdes.String(), jsonSerde));
ObjectNode initialBalance = JsonNodeFactory.instance.objectNode();
initialBalance.put("count", 0);
initialBalance.put("balance", 0);
initialBalance.put("time", Instant.ofEpochMilli(0L).toString());
KTable<String, JsonNode> bankBalance = bankTransactions
.groupByKey(Grouped.with(Serdes.String(), jsonSerde))
.aggregate(
() -> initialBalance,
(key, transaction, balance) -> newBalance(transaction, balance)
);
bankBalance.toStream().to("bank-balance-exactly-once", Produced.with(Serdes.String(), jsonSerde));
KafkaStreams stream = new KafkaStreams(builder.build(), properties);
stream.cleanUp();
stream.start();
System.out.println(stream.toString());
Runtime.getRuntime().addShutdownHook(new Thread(stream::close));
}
private static JsonNode newBalance(JsonNode transaction, JsonNode balance) {
ObjectNode newBalance = JsonNodeFactory.instance.objectNode();
newBalance.put("count", balance.get("count").asInt() + 1);
newBalance.put("balance", balance.get("balance").asInt() + transaction.get("amount").asInt());
Long balanceEpoch = Instant.parse(balance.get("time").asText()).toEpochMilli();
Long transactionEpoch = Instant.parse(transaction.get("time").asText()).toEpochMilli();
Instant newBalanceInstant = Instant.ofEpochMilli(Math.max(balanceEpoch, transactionEpoch));
newBalance.put("time", newBalanceInstant.toString());
return newBalance;
}
}
