Real-Time Fraud Detection in Java with Kafka Streams and Vector Similarity

This content is based on a talk that my colleague Tim Kelly and I presented at DevNexus 2026 in Atlanta, one of the largest Java conferences in the world. Imagine you’re at a supermarket or shopping online. You tap your card or click “Buy Now”. In that exact moment, your bank has to make a decision: is this transaction legitimate, or is it fraud? And it has to do that in milliseconds, and get it right. If it blocks a real transaction, it frustrates the customer. If it approves a fraudulent one,