Latest Value from the KStream for a given key - apache-kafka-streams

Question

I have KTable like below
KTable<productId1,productValue1>
KTable<productId2,productValue2>
KTable<productId3,productValue2>
Then i have KStream<custId,productId>, the productId is of any of productId1, productId2, productId3. I want to have the corresponding productValue.
The response would be like KStream<custId, productValueN>
If the KStream receives productId as productId2, then the response would be KStream<custId, productValue2>
How this could be possible?
Thanks

Answer 1

Unfortunately KTables don't support KeyValueMappers: KStream/KTable joins are performed strictly by comparing keys. GlobalKTables support this, making the whole process of joining much easier (see my other answer).
To get this to work then, you need to rekey the customers topic KStream to use the product ID, so that you can perform the join with the products topic KTable.
Once you've obtained the product name, you can rekey the resulting joined stream back to the customer ID. For example:
// Key: product ID, value: product name
KTable<String, String> productsTable =
streamsBuilder.table(PRODUCTS_TOPIC, Consumed.with(stringSerde, stringSerde),
Materialized.<String, String, KeyValueStore<Bytes, byte[]>>as("productsStore")
.withKeySerde(stringSerde)
.withValueSerde(stringSerde));
streamsBuilder.stream(CUSTOMERS_TOPIC, Consumed.with(stringSerde, stringSerde))
// Key: customer ID, value: product ID
.map((customerId, productId) -> KeyValue.pair(productId, String.format("%s|%s", customerId, productId)),
Named.as("mapToProductIdAndCustomerIdProductId"))
// Key: product ID, value: customer ID | product ID
.join(productsTable, (customerIdProductId, productName) -> {
String[] parts = customerIdProductId.split("\\|");
String customerId = parts[0];
return String.format("%s|%s", customerId, productName);
},
Joined.<String, String, String>as("customersProductsInnerJoin")
.withKeySerde(stringSerde)
.withValueSerde(stringSerde))
// Key: product ID, value: customer ID | product name
.map((productId, customerIdProductName) -> {
String[] parts = customerIdProductName.split("\\|");
String customerId = parts[0];
String productName = parts[1];
return KeyValue.pair(customerId, productName);
}, Named.as("mapToCustomerIdAndProductName"))
// Key: customer ID, value: product name
.print(Printed.toSysOut());

Answer 2

You can accomplish this using a GlobalKTable for products, and then joining the customers KStream with it as follows:
Serde<String> stringSerde = new Serdes.StringSerde();
StreamsBuilder streamsBuilder = new StreamsBuilder();
GlobalKTable<String, String> productsTable =
streamsBuilder.globalTable(PRODUCTS_TOPIC, Consumed.with(stringSerde, stringSerde),
Materialized.<String, String, KeyValueStore<Bytes, byte[]>>as("products")
.withKeySerde(stringSerde)
.withValueSerde(stringSerde));
streamsBuilder.stream(CUSTOMERS_TOPIC, Consumed.with(stringSerde, stringSerde))
.join(productsTable,
(customerId, productId) -> productId,
(productName1, productName2) -> productName2,
Named.as("customersProducts"))
.print(Printed.toSysOut());