Exactly-once with Spring Kafka on Spring Boot 4: demonstrations, docs and captured output

Companion suite for the ankurm.com guide. Every test runs against an in-JVM KRaft broker
(spring-kafka-test), so there is no Docker requirement. Verified on Spring Boot 4.1.0,
Spring Kafka 4.1.0, kafka-clients 4.2.1, JDK 25.0.3. results/full-run.txt is unedited
output of mvn test: 15 tests, 0 failures.

Demonstrations
  _01_idempotence   verifies idempotence from the BROKER via Admin.describeProducers rather
                    than by reading config back: a default producer registers one producer
                    state, an acks=1 producer registers zero. Also covers the ConfigException
                    you get only when idempotence is requested explicitly.
  _02_transactions  transaction markers consuming offsets, aborted records surviving in the
                    log, read_committed vs read_uncommitted, and a correction: markers do NOT
                    inflate consumer lag (a drained consumer reaches zero), they break
                    counting records by offset arithmetic (measured 100% overstatement).
  _03_read_process_write
                    the Spring loop with container-managed transactions; a listener that
                    fails after sending runs twice and is committed once.
  _04_fencing       shared transactional.id fences the zombie; a random per-startup id lets
                    both producers commit and silently defeats the guarantee entirely.
  _05_database      Kafka EOS does not extend to a database. Naive listener writes 2 rows for
                    1 message; the same listener keyed on topic-partition-offset writes 1.
  _06_performance   cost per transaction size, median of 5 interleaved rounds.

Docs
  Seven chapters: Boot 4 setup (auto-configuration moved to spring-boot-starter-kafka),
  idempotence, transactions and markers, fencing and KIP-890, the database boundary and the
  outbox pattern, performance, and corner cases.

Measured highlights
  acks=1 producer          0 broker-tracked producer states (idempotence silently off)
  5 records in one tx      endOffset 6; offset span overstates record count by 100%
  1 record  per tx            711 records/s
  100 records per tx       78,770 records/s  (~111x)

pom.xml

@@ -0,0 +1,115 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!--
+  Companion code for "Exactly-Once with Spring Kafka on Spring Boot 4" (ankurm.com).
+
+  Everything runs against an in-JVM KRaft broker started by spring-kafka-test, so there is no
+  Docker requirement and no external Kafka to install:
+
+      mvn test
+
+  A docker-compose.yml is included for readers who would rather point the same code at a real
+  broker; see docs/00-running.md.
+-->
+<project xmlns="http://maven.apache.org/POM/4.0.0"
+         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 https://maven.apache.org/xsd/maven-4.0.0.xsd">
+    <modelVersion>4.0.0</modelVersion>
+
+    <parent>
+        <groupId>org.springframework.boot</groupId>
+        <artifactId>spring-boot-starter-parent</artifactId>
+        <version>4.1.0</version>
+        <relativePath/>
+    </parent>
+
+    <groupId>com.ankurm.kafka</groupId>
+    <artifactId>spring-kafka-exactly-once-boot4</artifactId>
+    <version>1.0.0</version>
+    <name>Exactly-once with Spring Kafka on Spring Boot 4</name>
+    <description>
+        Runnable demonstrations of idempotent producers, Kafka transactions, read-process-write,
+        zombie fencing, transaction markers, and the Kafka/database boundary.
+    </description>
+
+    <properties>
+        <java.version>25</java.version>
+        <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
+    </properties>
+
+    <dependencies>
+        <dependency>
+            <groupId>org.springframework.boot</groupId>
+            <artifactId>spring-boot-starter</artifactId>
+        </dependency>
+
+        <!--
+          IMPORTANT, and a real Spring Boot 3 -> 4 migration trap.
+
+          On Boot 3 you could depend on org.springframework.kafka:spring-kafka directly and still
+          get auto-configuration, because KafkaAutoConfiguration lived in the monolithic
+          spring-boot-autoconfigure jar that every application already had.
+
+          Boot 4 split auto-configuration into per-technology modules. Kafka's now lives in
+          org.springframework.boot:spring-boot-kafka. Depend on spring-kafka alone and you get the
+          library but NO auto-configuration: no ProducerFactory, no KafkaTemplate, no
+          KafkaTransactionManager. The failure is a confusing NoSuchBeanDefinitionException for
+          KafkaTemplate at startup, which looks like a generics problem and is not.
+
+          spring-boot-starter-kafka pulls in both spring-kafka and spring-boot-kafka.
+          See docs/01-boot4-setup.md.
+        -->
+        <dependency>
+            <groupId>org.springframework.boot</groupId>
+            <artifactId>spring-boot-starter-kafka</artifactId>
+        </dependency>
+
+        <!-- Used only by the module that shows why "Kafka + DB atomically" is not a thing. -->
+        <dependency>
+            <groupId>org.springframework.boot</groupId>
+            <artifactId>spring-boot-starter-data-jpa</artifactId>
+        </dependency>
+        <dependency>
+            <groupId>com.h2database</groupId>
+            <artifactId>h2</artifactId>
+            <scope>runtime</scope>
+        </dependency>
+
+        <dependency>
+            <groupId>org.springframework.boot</groupId>
+            <artifactId>spring-boot-starter-test</artifactId>
+            <scope>test</scope>
+        </dependency>
+        <!--
+          spring-kafka-test pulls in the Kafka broker itself. On Spring Kafka 4.x this is
+          EmbeddedKafkaKraftBroker: Kafka 4.x removed ZooKeeper entirely, so the old
+          ZooKeeper-based embedded broker no longer exists.
+        -->
+        <dependency>
+            <groupId>org.springframework.kafka</groupId>
+            <artifactId>spring-kafka-test</artifactId>
+            <scope>test</scope>
+        </dependency>
+        <dependency>
+            <groupId>org.awaitility</groupId>
+            <artifactId>awaitility</artifactId>
+            <scope>test</scope>
+        </dependency>
+    </dependencies>
+
+    <build>
+        <plugins>
+            <plugin>
+                <groupId>org.apache.maven.plugins</groupId>
+                <artifactId>maven-surefire-plugin</artifactId>
+                <configuration>
+                    <!--
+                      The demos print narrated output that is meant to be read. Without this,
+                      Surefire swallows stdout unless a test fails.
+                    -->
+                    <redirectTestOutputToFile>false</redirectTestOutputToFile>
+                    <trimStackTrace>false</trimStackTrace>
+                </configuration>
+            </plugin>
+        </plugins>
+    </build>
+</project>