KStream-KStream Outer Join and Left Join — Flexible Stream Correlations in Real Time

We have two streams: streamA and streamB. We need to join them by key, but not all events exist in both streams.

Join Types Comparison:

KStream<String, String> streamA = builder.stream("streamA");
KStream<String, String> streamB = builder.stream("streamB");

KStream<String, String> joined = streamA.leftJoin(
    streamB,
    (valueA, valueB) -> valueA + ":" + (valueB == null ? "NO_MATCH" : valueB),
    JoinWindows.of(Duration.ofMinutes(5))
);

joined.to("joined-topic");

What happens: If streamA has a key but streamB doesn’t, it returns valueA:NO_MATCH. If both have the key, it returns the combined value. Outer Join additionally includes records from streamB with no match in streamA.

Real-World Example — Correlating Orders and Payments in an E-commerce System:

• orders (key: order_id, value: order details)
• payments (key: order_id, value: payment details)

Inner Join

• Shows only paid orders.
• Perfect for analytics.

Left Join

• Shows all orders, even without payment.
• Great for CRM and real-time dashboards.

Outer Join

• Gives a full picture: both unmatched orders and payments.
• Useful for troubleshooting, investigations, and monitoring failures.