spring-messaging-demo/kafka-streams

kafka-streams

Companion code for Kafka Streams with Spring Boot 4: Real-Time Fraud Detection with Vector Similarity on ankurm.com. The whole topology runs under TopologyTestDriver, so no Kafka broker is needed to run anything here.

Stack	Version
JDK	25 (Temurin 25.0.4.1+1)
Spring Boot	4.1.1
Spring for Apache Kafka	4.1.1
kafka-streams / kafka-streams-test-utils	4.2.1 (what Boot 4.1.1 manages)

Run it

./scripts/run-all.sh        # runs the tests, rewrites output/*.txt

What is where

File	Role
FraudTopology	The topology as plain Kafka Streams code: similarity scoring, 5-minute velocity count, blocklist KTable join
Embedder	A hand-built 6-number feature vector and cosine similarity (not a learned model)
FraudStreamsConfig	The only Spring wiring: @EnableKafkaStreams plus handing the StreamsBuilder to the topology
FraudTopologyTest	Five tests; each one writes the matching output/NN-*.txt
ContextLoadsTest	Loads the Spring context with auto-startup=false to show the wiring without a broker

Captured output

Every file in output/ is written by a test that also asserts it.

File	Shows
01-similarity-scores.txt	Scores for five sample transactions against the 0.98 threshold
02-similarity-alerts.txt	Only the fraud-shaped transaction produces an alert
03-velocity-window.txt	Six transactions in two minutes give one alert; the next window starts from one
04-blocklist-table-join.txt	Join before blocking, after blocking, and after a tombstone
05-topology-description.txt	Topology.describe() of the whole graph
06-spring-context.txt	The Spring factory bean is wired and not running
07-ambiguous-null-tombstone.txt	The real javac error for pipeInput("c5", null); source in src/broken/, regenerated by scripts/compile-broken.sh

The article carries the explanations; there is deliberately no docs/ folder.