Add outbox module: Transactional Outbox with the Spring Modulith Event Publication Registry

outbox/README.md

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+# outbox
+
+Companion code for *[Transactional Outbox with the Spring Modulith Event Publication
+Registry](https://ankurm.com)* on [ankurm.com](https://ankurm.com).
+
+A single `billing` module demonstrating the dual-write problem, Spring Modulith's event
+publication registry acting as a real transactional outbox, externalizing events to a
+real (embedded) Kafka broker via `spring-modulith-events-kafka`, and replaying an
+incomplete publication with `IncompleteEventPublications`. Every number in the
+companion post traces to `output/`, in order.
+
+## Versions
+
+| Artifact | Version |
+|---|---|
+| Spring Boot | 4.1.1 |
+| Spring Modulith | 2.1.1 (GA; verified against Maven Central's `maven-metadata.xml`) |
+| spring-modulith-events-kafka | 2.1.1 |
+| spring-kafka / spring-kafka-test | 4.1.1 (managed by the Spring Boot 4.1.1 BOM; `kafka-clients` 4.2.1) |
+| JDK | 25 (LTS) |
+| H2 | runtime, in-memory, for the demo only |
+
+## Quickstart
+
+```
+mvn -pl outbox -am test -Dtest=DualWriteProblemTest
+mvn -pl outbox -am test -Dtest=RegistrySafetyNetTest
+mvn -pl outbox -am test -Dtest=ReplayIncompletePublicationsTest
+```
+
+Run separately on purpose -- `RegistrySafetyNetTest` starts a real embedded Kafka
+broker (`@EmbeddedKafka`), which takes several seconds and is unrelated to what the
+other two tests are checking.
+
+## What each class is
+
+| Class | Role |
+|---|---|
+| `billing.BillingManagement` | The outbox-safe way to issue an invoice: one local transaction, one `Invoice` row, one event publication registry row |
+| `billing.NaiveBillingService` | The "before" picture -- a separate, uncoordinated `kafka.send()` call after the database write already committed. Never wired as the module's real API; only ever called directly from `DualWriteProblemTest` |
+| `billing.InvoiceIssued` | `@Externalized("invoices::#{#this.invoiceId}")` -- routes to the `invoices` Kafka topic, keyed by the invoice id |
+| `billing.FlakyAuditListener` (test-only) | Throws on its first invocation, succeeds after -- stands in for any downstream dependency being briefly unavailable, without needing to actually break a running broker mid-test |
+
+## Captured output (`output/`)
+
+| File | What it captures |
+|---|---|
+| `00-dual-write-problem.txt` | The invoice committed, Kafka never received either message, and a real stack trace showing `KafkaTemplate.send()` throwing *synchronously* when the broker is totally unreachable -- narrower than, and a correction to, the usual "fire-and-forget send() fails silently" claim |
+| `01-registry-safety-net.txt` | The real production path: a real embedded Kafka broker receiving the real message, and the registry moving both listeners' rows into `EVENT_PUBLICATION_ARCHIVE` (`completion-mode=ARCHIVE` is turned on for this module -- the previous post in this series left it commented out) |
+| `02-replay-incomplete-publications.txt` | A listener failing once, the registry leaving its row incomplete, and `IncompleteEventPublications.resubmitIncompletePublicationsOlderThan(Duration)` -- a real method, verified against the compiled `events-api` jar -- redelivering it successfully |
+
+No `docs/` chapter directory in this repository -- the intermediate and reference-depth
+material that would normally live there is in accordion sections inside the WordPress
+post itself.

outbox/output/00-dual-write-problem.txt

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+$ mvn -pl outbox -am test -Dtest=DualWriteProblemTest
+(NaiveBillingService: save the Invoice, then separately call kafka.send() -- no
+ coordinator between the two. spring.kafka.bootstrap-servers points at localhost:19999,
+ nothing listening there)
+
+2026-10-04T00:52:56.429+05:30 ERROR 5271 --- [outbox] [           main] o.s.k.support.LoggingProducerListener    : Exception thrown when sending a message with key='1' and payload='WIDGET-1:1999' to topic invoices-naive:
+
+org.apache.kafka.common.errors.TimeoutException: Topic invoices-naive not present in metadata after 2000 ms.
+
+Exception thrown synchronously from issueInvoiceTheNaiveWay(): org.springframework.kafka.KafkaException: Send failed
+Invoice count regardless: 1 (was 0). The database write already committed inside saveInvoiceOnly() before this exception was ever thrown -- the exception came from the Kafka call, several lines later, on an already-committed row.
+
+2026-10-04T00:52:58.495+05:30 ERROR 5271 --- [outbox] [           main] o.s.k.support.LoggingProducerListener    : Exception thrown when sending a message with key='2' and payload='WIDGET-1:2999' to topic invoices-naive:
+
+org.apache.kafka.common.errors.TimeoutException: Topic invoices-naive not present in metadata after 2000 ms.
+
+Invoice count after the blocked, failed send: 2 (was 1). The row committed before the send was even attempted.
+
+[INFO] Tests run: 2, Failures: 0, Errors: 0, Skipped: 0
+
+Two real findings here, not one. First, the obvious one: the invoice exists in the
+database in both tests, and Kafka never received either message -- that's the dual-write
+problem, reproduced with nothing faked beyond pointing Kafka at a port with no listener.
+
+Second, a more specific one that contradicts the usual "fire-and-forget send() fails
+silently" framing: when the broker can't be reached to fetch topic metadata at all,
+Spring's KafkaTemplate.send() throws synchronously, wrapping the Kafka client's own
+TimeoutException in a KafkaException, even though the method's declared return type is
+a CompletableFuture. The "silent" version of this bug is real, but it is a narrower
+case than total broker unavailability -- it's what happens when the broker is reachable
+and the topic's metadata is known, but the specific produce request times out waiting
+for an acknowledgment afterward. That failure mode surfaces only through the returned
+future or a callback, never as a synchronous exception, and is not reproduced in this
+transcript (it needs a broker that accepts the connection and then stops responding
+mid-request, not one that was never there to begin with).

outbox/output/01-registry-safety-net.txt

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+$ mvn -pl outbox -am test -Dtest=RegistrySafetyNetTest
+(real embedded Kafka broker via @EmbeddedKafka; BillingManagement.issueInvoice()
+ publishes InvoiceIssued, @Externalized("invoices::#{#this.invoiceId}") routes it
+ through spring-modulith-events-kafka)
+
+Kafka record received -- topic=invoices key=1 value={"invoiceId":1,"sku":"GADGET-7","amountCents":4500}
+EVENT_PUBLICATION_ARCHIVE rows for InvoiceIssued: 2
+EVENT_PUBLICATION rows still outstanding for InvoiceIssued: 0
+
+[INFO] Tests run: 1, Failures: 0, Errors: 0, Skipped: 0
+
+The message really arrived on the real topic, with the real routing key (the invoice's
+own id, exactly as the SpEL expression after "::" in @Externalized specifies) and the
+real JSON payload. Two archive rows for InvoiceIssued, not one, because this module's
+test sources also declare FlakyAuditListener (see output/02) -- it is a second,
+independent listener on the same event, and with completion-mode=ARCHIVE configured in
+application.properties, both listeners' completed publications moved out of
+EVENT_PUBLICATION and into EVENT_PUBLICATION_ARCHIVE once they succeeded. Zero rows
+left outstanding in EVENT_PUBLICATION for this event type confirms neither listener is
+still pending.

outbox/output/02-replay-incomplete-publications.txt

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+$ mvn -pl outbox -am test -Dtest=ReplayIncompletePublicationsTest
+(FlakyAuditListener throws on its first invocation of InvoiceIssued, succeeds on every
+ one after -- standing in for any downstream dependency that is briefly unavailable)
+
+Incomplete FlakyAuditListener publications after the simulated failure: 1
+Incomplete FlakyAuditListener publications after resubmission: 0
+Invoice 1 never changed -- the retry was purely about redelivering the event, the invoice row was correct the whole time.
+
+[INFO] Tests run: 1, Failures: 0, Errors: 0, Skipped: 0
+
+The sequence that actually ran: BillingManagement.issueInvoice() commits the Invoice
+row and the EVENT_PUBLICATION row for FlakyAuditListener in one local transaction.
+FlakyAuditListener.on(InvoiceIssued) is then invoked asynchronously, throws on purpose,
+and the registry leaves that row with COMPLETION_DATE still null -- "incomplete" is
+exactly that: a row with no completion date, regardless of whether the listener ever
+ran at all or ran and failed. Calling
+IncompleteEventPublications.resubmitIncompletePublicationsOlderThan(Duration.ZERO) --
+a real method on a real interface, verified against the compiled events-api 2.1.1 jar,
+not assumed from documentation prose -- re-invokes the listener for every such row.
+This time failNextAttempt is already false, so it succeeds, and the row's completion
+date is set. The invoice itself was never touched by any of this: the retry is about
+redelivering a notification, not about redoing a database write that already
+succeeded the first time.

outbox/pom.xml

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+<?xml version="1.0" encoding="UTF-8"?>
+<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>com.ankurm</groupId>
+    <artifactId>spring-modulith-demo</artifactId>
+    <version>1.0.0</version>
+  </parent>
+
+  <artifactId>outbox</artifactId>
+  <packaging>jar</packaging>
+
+  <name>outbox</name>
+  <description>
+    Companion code for "Transactional Outbox with the Spring Modulith Event Publication
+    Registry" on ankurm.com.
+  </description>
+
+  <dependencyManagement>
+    <dependencies>
+      <dependency>
+        <groupId>org.springframework.boot</groupId>
+        <artifactId>spring-boot-dependencies</artifactId>
+        <version>${spring-boot.version}</version>
+        <type>pom</type>
+        <scope>import</scope>
+      </dependency>
+      <dependency>
+        <groupId>org.springframework.modulith</groupId>
+        <artifactId>spring-modulith-bom</artifactId>
+        <version>${spring-modulith.version}</version>
+        <type>pom</type>
+        <scope>import</scope>
+      </dependency>
+    </dependencies>
+  </dependencyManagement>
+
+  <dependencies>
+    <dependency>
+      <groupId>org.springframework.boot</groupId>
+      <artifactId>spring-boot-starter</artifactId>
+    </dependency>
+    <dependency>
+      <groupId>org.springframework.modulith</groupId>
+      <artifactId>spring-modulith-starter-jdbc</artifactId>
+    </dependency>
+    <dependency>
+      <groupId>org.springframework.modulith</groupId>
+      <artifactId>spring-modulith-events-kafka</artifactId>
+    </dependency>
+    <dependency>
+      <groupId>org.springframework.boot</groupId>
+      <artifactId>spring-boot-starter-data-jpa</artifactId>
+    </dependency>
+    <dependency>
+      <groupId>org.springframework.kafka</groupId>
+      <artifactId>spring-kafka</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>
+    <dependency>
+      <groupId>org.springframework.modulith</groupId>
+      <artifactId>spring-modulith-starter-test</artifactId>
+      <scope>test</scope>
+    </dependency>
+    <dependency>
+      <groupId>org.springframework.kafka</groupId>
+      <artifactId>spring-kafka-test</artifactId>
+      <scope>test</scope>
+    </dependency>
+  </dependencies>
+
+  <build>
+    <plugins>
+      <plugin>
+        <groupId>org.springframework.boot</groupId>
+        <artifactId>spring-boot-maven-plugin</artifactId>
+        <version>${spring-boot.version}</version>
+      </plugin>
+    </plugins>
+  </build>
+</project>

outbox/src/main/java/com/ankurm/outboxdemo/Application.java

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+package com.ankurm.outboxdemo;
+
+import org.springframework.boot.SpringApplication;
+import org.springframework.boot.autoconfigure.SpringBootApplication;
+import org.springframework.modulith.Modulithic;
+
+@SpringBootApplication
+@Modulithic(systemName = "Outbox Demo")
+public class Application {
+    public static void main(String[] args) {
+        SpringApplication.run(Application.class, args);
+    }
+}

outbox/src/main/java/com/ankurm/outboxdemo/billing/BillingManagement.java

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+package com.ankurm.outboxdemo.billing;
+
+import com.ankurm.outboxdemo.billing.internal.Invoice;
+import com.ankurm.outboxdemo.billing.internal.InvoiceRepository;
+import org.springframework.context.ApplicationEventPublisher;
+import org.springframework.stereotype.Service;
+import org.springframework.transaction.annotation.Transactional;
+
+/**
+ * The outbox-safe way to issue an invoice. The {@code Invoice} row and the event
+ * publication registry row for {@link InvoiceIssued} are written in the same local
+ * database transaction -- that insert is what makes this an outbox, not the event
+ * itself. Whatever happens to the event afterwards (externalized to Kafka, picked up
+ * by an in-process listener, or nothing at all because the process dies one instruction
+ * later) cannot un-happen the fact that a row recording "this needs to go out" already
+ * made it to disk, atomically with the invoice itself.
+ */
+@Service
+public class BillingManagement {
+
+    private final InvoiceRepository invoices;
+    private final ApplicationEventPublisher events;
+
+    public BillingManagement(InvoiceRepository invoices, ApplicationEventPublisher events) {
+        this.invoices = invoices;
+        this.events = events;
+    }
+
+    @Transactional
+    public Invoice issueInvoice(String sku, int amountCents) {
+        var invoice = invoices.save(new Invoice(sku, amountCents, "ISSUED"));
+        events.publishEvent(new InvoiceIssued(invoice.getId(), invoice.getSku(), invoice.getAmountCents()));
+        return invoice;
+    }
+}

outbox/src/main/java/com/ankurm/outboxdemo/billing/InvoiceIssued.java

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+package com.ankurm.outboxdemo.billing;
+
+import org.springframework.modulith.events.Externalized;
+
+/**
+ * {@code @Externalized}'s routing syntax is {@code "target::key"} -- everything before
+ * {@code ::} is the Kafka topic, everything after is a SpEL expression (root object
+ * {@code #this}) used as the message key. Verified against the compiled annotation
+ * (org.springframework.modulith.events.Externalized, 2.1.1): it is {@code @Target(TYPE)},
+ * one aliased {@code value()/target()} String attribute, nothing else -- the routing
+ * syntax itself is parsed at runtime by the externalization machinery, not encoded in
+ * the annotation's own shape.
+ */
+@Externalized("invoices::#{#this.invoiceId}")
+public record InvoiceIssued(Long invoiceId, String sku, int amountCents) {
+}

outbox/src/main/java/com/ankurm/outboxdemo/billing/NaiveBillingService.java

@@ -0,0 +1,62 @@
+package com.ankurm.outboxdemo.billing;
+
+import com.ankurm.outboxdemo.billing.internal.Invoice;
+import com.ankurm.outboxdemo.billing.internal.InvoiceRepository;
+import org.springframework.kafka.core.KafkaTemplate;
+import org.springframework.stereotype.Service;
+import org.springframework.transaction.annotation.Transactional;
+
+/**
+ * The two-systems-no-coordinator version, kept here deliberately as the "before"
+ * picture -- it is never wired as the module's real API, only invoked directly from
+ * {@code DualWriteProblemTest}. The database write commits and returns before this
+ * method ever reaches the Kafka call: there is no transaction, local or distributed,
+ * spanning both. If the process dies, or the broker is unreachable, between those two
+ * lines, the invoice exists and nothing downstream will ever hear about it.
+ */
+@Service
+public class NaiveBillingService {
+
+    private final InvoiceRepository invoices;
+    private final KafkaTemplate<String, String> kafka;
+
+    public NaiveBillingService(InvoiceRepository invoices, KafkaTemplate<String, String> kafka) {
+        this.invoices = invoices;
+        this.kafka = kafka;
+    }
+
+    @Transactional
+    public Invoice saveInvoiceOnly(String sku, int amountCents) {
+        return invoices.save(new Invoice(sku, amountCents, "ISSUED"));
+    }
+
+    /**
+     * Deliberately two separate steps: {@link #saveInvoiceOnly} commits and returns
+     * on its own transaction boundary before this method ever calls {@code kafka.send}.
+     * Fire-and-forget: the returned {@code CompletableFuture} is never inspected, which
+     * is the common case at most real call sites. If the broker is unreachable, this
+     * method returns successfully anyway -- the failure happens on a Kafka client
+     * thread, after this method has already returned, and nothing here ever sees it.
+     */
+    public Invoice issueInvoiceTheNaiveWay(String sku, int amountCents) {
+        var invoice = saveInvoiceOnly(sku, amountCents);
+        kafka.send("invoices-naive", invoice.getId().toString(),
+                invoice.getSku() + ":" + invoice.getAmountCents());
+        return invoice;
+    }
+
+    /**
+     * Same two-step shortcut, except this call site does the "responsible" thing and
+     * blocks on the send future so it can react to a failure. It still doesn't help:
+     * the database write already committed in {@link #saveInvoiceOnly} by the time this
+     * line runs, so catching the exception here can log it, but cannot undo the invoice
+     * that is already sitting in the database with nothing downstream ever told about it.
+     */
+    public Invoice issueInvoiceAndBlockOnTheSend(String sku, int amountCents) throws Exception {
+        var invoice = saveInvoiceOnly(sku, amountCents);
+        kafka.send("invoices-naive", invoice.getId().toString(),
+                invoice.getSku() + ":" + invoice.getAmountCents())
+                .get(3, java.util.concurrent.TimeUnit.SECONDS);
+        return invoice;
+    }
+}

outbox/src/main/java/com/ankurm/outboxdemo/billing/internal/Invoice.java

@@ -0,0 +1,50 @@
+package com.ankurm.outboxdemo.billing.internal;
+
+import jakarta.persistence.Entity;
+import jakarta.persistence.GeneratedValue;
+import jakarta.persistence.GenerationType;
+import jakarta.persistence.Id;
+import jakarta.persistence.Table;
+
+/**
+ * Deliberately named Invoice, not Order -- the previous post in this series
+ * (output/00-table-name-collision.txt in the order-fulfillment module) already
+ * spent a transcript on why ORDER is a reserved SQL keyword. This module picks a
+ * domain noun that doesn't collide, on purpose.
+ */
+@Entity
+@Table(name = "invoices")
+public class Invoice {
+
+    @Id
+    @GeneratedValue(strategy = GenerationType.IDENTITY)
+    private Long id;
+    private String sku;
+    private int amountCents;
+    private String status;
+
+    protected Invoice() {
+    }
+
+    public Invoice(String sku, int amountCents, String status) {
+        this.sku = sku;
+        this.amountCents = amountCents;
+        this.status = status;
+    }
+
+    public Long getId() {
+        return id;
+    }
+
+    public String getSku() {
+        return sku;
+    }
+
+    public int getAmountCents() {
+        return amountCents;
+    }
+
+    public String getStatus() {
+        return status;
+    }
+}

outbox/src/main/java/com/ankurm/outboxdemo/billing/internal/InvoiceRepository.java

@@ -0,0 +1,6 @@
+package com.ankurm.outboxdemo.billing.internal;
+
+import org.springframework.data.jpa.repository.JpaRepository;
+
+public interface InvoiceRepository extends JpaRepository<Invoice, Long> {
+}

outbox/src/main/resources/application.properties

@@ -0,0 +1,19 @@
+spring.application.name=outbox
+
+spring.datasource.url=jdbc:h2:mem:outbox;DB_CLOSE_DELAY=-1
+spring.datasource.driver-class-name=org.h2.Driver
+spring.jpa.hibernate.ddl-auto=update
+spring.jpa.open-in-view=false
+
+# The event publication registry is the outbox table. JDBC-backed here (not JPA) so
+# the registry's own schema is independent of whatever ORM the business entities use.
+spring.modulith.events.jdbc.schema-initialization.enabled=true
+
+# ARCHIVE, this time -- the previous post in this series left this commented out so
+# every transcript there ran against the library's bare UPDATE default. This module is
+# about the outbox actually being operated correctly, so it is turned on here instead.
+spring.modulith.events.completion-mode=ARCHIVE
+spring.modulith.events.republish-outstanding-events-on-restart=true
+
+logging.level.com.ankurm.outboxdemo=INFO
+logging.level.org.springframework.modulith.events=INFO

outbox/src/test/java/com/ankurm/outboxdemo/billing/DualWriteProblemTest.java

@@ -0,0 +1,80 @@
+package com.ankurm.outboxdemo.billing;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatThrownBy;
+
+import com.ankurm.outboxdemo.billing.internal.InvoiceRepository;
+import org.junit.jupiter.api.Test;
+import org.springframework.beans.factory.annotation.Autowired;
+import org.springframework.boot.test.context.SpringBootTest;
+import org.springframework.test.context.DynamicPropertyRegistry;
+import org.springframework.test.context.DynamicPropertySource;
+
+/**
+ * No embedded broker here, on purpose -- spring.kafka.bootstrap-servers points at a
+ * port nothing is listening on, which is the simplest honest way to reproduce "the
+ * broker is unreachable" without faking anything. Two real, independent systems,
+ * called one after the other, is already enough to produce the dual-write problem;
+ * nothing here is simulated beyond pointing Kafka at an address with no listener.
+ */
+@SpringBootTest
+class DualWriteProblemTest {
+
+    @DynamicPropertySource
+    static void unreachableKafka(DynamicPropertyRegistry registry) {
+        registry.add("spring.kafka.bootstrap-servers", () -> "localhost:19999");
+        registry.add("spring.kafka.producer.properties.max.block.ms", () -> "2000");
+    }
+
+    @Autowired
+    NaiveBillingService naiveBillingService;
+
+    @Autowired
+    InvoiceRepository invoices;
+
+    @Test
+    void fireAndForgetSendStillThrowsWhenTheBrokerIsTotallyUnreachable() {
+        long before = invoices.count();
+
+        // Expected this call to return cleanly and lose the failure silently --
+        // KafkaTemplate.send() returns a CompletableFuture, so a caller that ignores it
+        // "shouldn't" see an exception. Running it for real shows otherwise: when the
+        // broker can't even be reached to fetch topic metadata, Spring's KafkaTemplate
+        // wraps that specific failure and rethrows it synchronously from send() itself
+        // (see the real stack trace in the captured transcript). The database write is
+        // already unaffected either way -- it's a separate, already-committed transaction
+        // by the time this line runs -- but the exception itself is not silent here.
+        Exception thrown = null;
+        try {
+            naiveBillingService.issueInvoiceTheNaiveWay("WIDGET-1", 1999);
+        } catch (Exception e) {
+            thrown = e;
+        }
+
+        assertThat(thrown).isNotNull();
+        assertThat(invoices.count()).isEqualTo(before + 1);
+        System.out.println("Exception thrown synchronously from issueInvoiceTheNaiveWay(): "
+                + thrown.getClass().getName() + ": " + thrown.getMessage());
+        System.out.println("Invoice count regardless: " + invoices.count() + " (was " + before
+                + "). The database write already committed inside saveInvoiceOnly() before this"
+                + " exception was ever thrown -- the exception came from the Kafka call, several"
+                + " lines later, on an already-committed row.");
+    }
+
+    @Test
+    void blockingOnTheFutureSurfacesTheSameFailureAndStillCannotUndoTheWrite() {
+        long before = invoices.count();
+
+        assertThatThrownBy(() -> naiveBillingService.issueInvoiceAndBlockOnTheSend("WIDGET-1", 2999))
+                .isInstanceOf(org.springframework.kafka.KafkaException.class)
+                .hasCauseInstanceOf(org.apache.kafka.common.errors.TimeoutException.class);
+
+        // The invoice is still there. Blocking on the future told us about the Kafka
+        // failure a little sooner, but by the time we could react to it,
+        // saveInvoiceOnly() had already committed on its own transaction boundary --
+        // there was nothing left to roll back.
+        assertThat(invoices.count()).isEqualTo(before + 1);
+        System.out.println("Invoice count after the blocked, failed send: " + invoices.count()
+                + " (was " + before + "). The row committed before the send was even attempted.");
+    }
+}

outbox/src/test/java/com/ankurm/outboxdemo/billing/FlakyAuditListener.java

@@ -0,0 +1,31 @@
+package com.ankurm.outboxdemo.billing;
+
+import java.util.concurrent.atomic.AtomicInteger;
+import org.springframework.modulith.events.ApplicationModuleListener;
+import org.springframework.stereotype.Component;
+
+/**
+ * Test-only listener, never part of the packaged application (it lives under
+ * src/test, so component scanning only ever picks it up inside a test's
+ * ApplicationContext). Stands in for "the thing that can transiently fail" --
+ * in a real system this is the Kafka broker being briefly unreachable, a REST
+ * call to a partner timing out, or any other downstream dependency with its own
+ * availability. The event publication registry doesn't know or care which;
+ * it only knows whether THIS listener, for THIS event, has completed yet.
+ */
+@Component
+public class FlakyAuditListener {
+
+    public static final AtomicInteger attempts = new AtomicInteger(0);
+    public static volatile boolean failNextAttempt = false;
+
+    @ApplicationModuleListener
+    public void on(InvoiceIssued event) {
+        int attempt = attempts.incrementAndGet();
+        if (failNextAttempt) {
+            failNextAttempt = false;
+            throw new IllegalStateException(
+                    "simulated downstream failure on attempt " + attempt + " for invoice " + event.invoiceId());
+        }
+    }
+}

outbox/src/test/java/com/ankurm/outboxdemo/billing/RegistrySafetyNetTest.java

@@ -0,0 +1,91 @@
+package com.ankurm.outboxdemo.billing;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.awaitility.Awaitility.await;
+
+import com.ankurm.outboxdemo.billing.internal.InvoiceRepository;
+import java.time.Duration;
+import java.util.Map;
+import org.apache.kafka.clients.consumer.ConsumerRecord;
+import org.apache.kafka.clients.consumer.KafkaConsumer;
+import org.apache.kafka.common.serialization.StringDeserializer;
+import org.junit.jupiter.api.AfterEach;
+import org.junit.jupiter.api.Test;
+import org.springframework.beans.factory.annotation.Autowired;
+import org.springframework.boot.test.context.SpringBootTest;
+import org.springframework.jdbc.core.JdbcTemplate;
+import org.springframework.kafka.test.context.EmbeddedKafka;
+
+/**
+ * The real production path: BillingManagement.issueInvoice() publishes InvoiceIssued,
+ * which is @Externalized, which spring-modulith-events-kafka routes to a real broker --
+ * a genuine embedded Kafka instance here, not a mock. Proves two things at once: the
+ * message really arrives on the topic, and the registry really marks that listener's
+ * row complete (and, with completion-mode=ARCHIVE configured for this module, moves it
+ * into EVENT_PUBLICATION_ARCHIVE) once it does.
+ */
+@SpringBootTest
+@EmbeddedKafka(partitions = 1, topics = "invoices", bootstrapServersProperty = "spring.kafka.bootstrap-servers")
+class RegistrySafetyNetTest {
+
+    @Autowired
+    BillingManagement billingManagement;
+
+    @Autowired
+    InvoiceRepository invoices;
+
+    @Autowired
+    JdbcTemplate jdbc;
+
+    @Autowired
+    org.springframework.core.env.Environment env;
+
+    KafkaConsumer<String, String> consumer;
+
+    @AfterEach
+    void closeConsumer() {
+        if (consumer != null) {
+            consumer.close();
+        }
+    }
+
+    @Test
+    void invoiceIssuedIsReallyPublishedToKafkaAndRegistryCompletes() {
+        String bootstrapServers = env.getProperty("spring.kafka.bootstrap-servers");
+        consumer = new KafkaConsumer<>(Map.of(
+                "bootstrap.servers", bootstrapServers,
+                "group.id", "test-group",
+                "auto.offset.reset", "earliest",
+                "key.deserializer", StringDeserializer.class.getName(),
+                "value.deserializer", StringDeserializer.class.getName()));
+        consumer.subscribe(java.util.List.of("invoices"));
+
+        var invoice = billingManagement.issueInvoice("GADGET-7", 4500);
+        assertThat(invoices.findById(invoice.getId())).isPresent();
+
+        var records = new java.util.ArrayList<ConsumerRecord<String, String>>();
+        await().atMost(Duration.ofSeconds(10)).untilAsserted(() -> {
+            consumer.poll(Duration.ofMillis(200)).forEach(records::add);
+            assertThat(records).isNotEmpty();
+        });
+
+        ConsumerRecord<String, String> received = records.get(0);
+        assertThat(received.topic()).isEqualTo("invoices");
+        assertThat(received.key()).isEqualTo(invoice.getId().toString());
+        System.out.println("Kafka record received -- topic=" + received.topic()
+                + " key=" + received.key() + " value=" + received.value());
+
+        await().atMost(Duration.ofSeconds(10)).untilAsserted(() -> {
+            Integer archived = jdbc.queryForObject(
+                    "select count(*) from EVENT_PUBLICATION_ARCHIVE where EVENT_TYPE like '%InvoiceIssued%'",
+                    Integer.class);
+            System.out.println("EVENT_PUBLICATION_ARCHIVE rows for InvoiceIssued: " + archived);
+            assertThat(archived).isGreaterThanOrEqualTo(1);
+        });
+
+        Integer stillOutstanding = jdbc.queryForObject(
+                "select count(*) from EVENT_PUBLICATION where EVENT_TYPE like '%InvoiceIssued%'",
+                Integer.class);
+        System.out.println("EVENT_PUBLICATION rows still outstanding for InvoiceIssued: " + stillOutstanding);
+    }
+}

outbox/src/test/java/com/ankurm/outboxdemo/billing/ReplayIncompletePublicationsTest.java

@@ -0,0 +1,90 @@
+package com.ankurm.outboxdemo.billing;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.awaitility.Awaitility.await;
+
+import java.time.Duration;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+import org.springframework.beans.factory.annotation.Autowired;
+import org.springframework.boot.test.context.SpringBootTest;
+import org.springframework.jdbc.core.JdbcTemplate;
+import org.springframework.modulith.events.IncompleteEventPublications;
+import org.springframework.test.context.DynamicPropertyRegistry;
+import org.springframework.test.context.DynamicPropertySource;
+
+/**
+ * FlakyAuditListener throws on its first invocation and succeeds on every one after
+ * that. This is standing in for "the downstream dependency was briefly unavailable" --
+ * a Kafka broker, a partner API, anything -- without needing to actually break a real
+ * broker's availability mid-test. What is NOT simulated is the registry behavior: the
+ * incomplete publication really is left in EVENT_PUBLICATION with no completion date
+ * after the first failure, and resubmitIncompletePublicationsOlderThan really does
+ * re-invoke the listener and really does mark the row complete on success. Verified
+ * against the compiled IncompleteEventPublications interface (events-api 2.1.1):
+ * resubmitIncompletePublicationsOlderThan(Duration) is a real method, not a guess.
+ */
+@SpringBootTest
+class ReplayIncompletePublicationsTest {
+
+    // This test is about the FlakyAuditListener, not Kafka -- but spring-modulith-events-kafka
+    // is still on the classpath and still tries to externalize InvoiceIssued to a default
+    // localhost:9092 that doesn't exist here. Trimmed down purely so the JVM doesn't sit
+    // retrying metadata fetches for 30 seconds during shutdown; it does not affect what this
+    // test is actually asserting.
+    @DynamicPropertySource
+    static void fastFailKafka(DynamicPropertyRegistry registry) {
+        registry.add("spring.kafka.producer.properties.max.block.ms", () -> "500");
+    }
+
+    @Autowired
+    BillingManagement billingManagement;
+
+    @Autowired
+    IncompleteEventPublications incompletePublications;
+
+    @Autowired
+    JdbcTemplate jdbc;
+
+    @BeforeEach
+    void reset() {
+        FlakyAuditListener.attempts.set(0);
+        FlakyAuditListener.failNextAttempt = false;
+    }
+
+    @Test
+    void failedListenerLeavesAnIncompletePublicationThatResubmissionCompletes() {
+        FlakyAuditListener.failNextAttempt = true;
+
+        var invoice = billingManagement.issueInvoice("GIZMO-3", 999);
+
+        await().atMost(Duration.ofSeconds(10)).untilAsserted(() ->
+                assertThat(FlakyAuditListener.attempts.get()).isEqualTo(1));
+
+        Integer incompleteAfterFailure = jdbc.queryForObject(
+                "select count(*) from EVENT_PUBLICATION where EVENT_TYPE like '%InvoiceIssued%' "
+                        + "and LISTENER_ID like '%FlakyAuditListener%' and COMPLETION_DATE is null",
+                Integer.class);
+        System.out.println("Incomplete FlakyAuditListener publications after the simulated failure: "
+                + incompleteAfterFailure);
+        assertThat(incompleteAfterFailure).isEqualTo(1);
+
+        incompletePublications.resubmitIncompletePublicationsOlderThan(Duration.ZERO);
+
+        await().atMost(Duration.ofSeconds(10)).untilAsserted(() ->
+                assertThat(FlakyAuditListener.attempts.get()).isEqualTo(2));
+
+        await().atMost(Duration.ofSeconds(10)).untilAsserted(() -> {
+            Integer incompleteAfterResubmit = jdbc.queryForObject(
+                    "select count(*) from EVENT_PUBLICATION where EVENT_TYPE like '%InvoiceIssued%' "
+                            + "and LISTENER_ID like '%FlakyAuditListener%' and COMPLETION_DATE is null",
+                    Integer.class);
+            System.out.println("Incomplete FlakyAuditListener publications after resubmission: "
+                    + incompleteAfterResubmit);
+            assertThat(incompleteAfterResubmit).isEqualTo(0);
+        });
+
+        System.out.println("Invoice " + invoice.getId() + " never changed -- the retry was purely"
+                + " about redelivering the event, the invoice row was correct the whole time.");
+    }
+}

pom.xml

@@ -14,6 +14,7 @@
     Companion repository for the ankurm.com Spring Modulith series.
     Each Maven module backs one post; modules are added over time, one commit each.
     - order-fulfillment : Spring Modulith 2.1: Enforcing Module Boundaries Inside a Spring Boot Monolith
+    - outbox            : Transactional Outbox with the Spring Modulith Event Publication Registry
   </description>
 
   <properties>
@@ -25,5 +26,6 @@
 
   <modules>
     <module>order-fulfillment</module>
+    <module>outbox</module>
   </modules>
 </project>