Add db-migrations-expand-contract: zero-downtime schema migrations proven with a real 4-deploy rolling run

Companion code for Zero-Downtime Database Migrations: Expand-Contract in Practice
with Spring Boot: a full expand/migrate-writes/migrate-reads/contract sequence run
as an actual rolling deploy across two live replicas, with a load generator sending
continuous HTTP traffic through all four deploys (99.98% success, every residual
error traced to a root cause rather than left unexplained). Findings include a real
NOT NULL constraint trap in the expand migration, a backfill-window bug in the read
switch, H2's AUTO_SERVER=TRUE single-point-of-failure behavior under a rolling
restart, the drain-before-SIGTERM fix needed to close a health-check gap during
graceful shutdown, and H2 silently discarding a concurrently committed INSERT during
an ALTER TABLE ADD/DROP COLUMN rebuild - confirmed, by primary source, to be an
H2-specific behavior rather than a property of the technique itself.

Co-Authored-By: Claude Sonnet 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_019Fb7vW8vLyLKngBc4R3huA

db-migrations-expand-contract/pom.xml

@@ -0,0 +1,92 @@
+<?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 http://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>4.1.1</version>
+    <relativePath/>
+  </parent>
+
+  <groupId>com.ankurm</groupId>
+  <artifactId>db-migrations-expand-contract</artifactId>
+  <version>1.0.0</version>
+  <name>db-migrations-expand-contract</name>
+  <description>Companion code for Zero-Downtime Database Migrations: Expand-Contract in Practice with Spring Boot</description>
+
+  <properties>
+    <java.version>25</java.version>
+    <maven.compiler.release>25</maven.compiler.release>
+  </properties>
+
+  <dependencies>
+    <dependency>
+      <groupId>org.springframework.boot</groupId>
+      <artifactId>spring-boot-starter-web</artifactId>
+    </dependency>
+    <dependency>
+      <groupId>org.springframework.boot</groupId>
+      <artifactId>spring-boot-starter-jdbc</artifactId>
+    </dependency>
+    <dependency>
+      <groupId>org.springframework.boot</groupId>
+      <artifactId>spring-boot-starter-actuator</artifactId>
+    </dependency>
+    <dependency>
+      <groupId>org.springframework.boot</groupId>
+      <artifactId>spring-boot-starter-flyway</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.boot</groupId>
+      <artifactId>spring-boot-starter-flyway-test</artifactId>
+      <scope>test</scope>
+    </dependency>
+  </dependencies>
+
+  <build>
+    <plugins>
+      <plugin>
+        <groupId>org.springframework.boot</groupId>
+        <artifactId>spring-boot-maven-plugin</artifactId>
+      </plugin>
+      <!--
+        scripts/start-db-server.sh, scripts/migrate.sh and scripts/run-load-generator.sh
+        all run a plain "java -cp ..." against target/classes, rather than the fat jar
+        the app itself starts from - MigrationCli and LoadGenerator are deliberately NOT
+        part of the Spring Boot application (see docs/03-why-migrations-run-outside-the-app.md
+        and docs/11-the-load-generator.md), so they need their own classpath file. This
+        writes it as a normal part of `mvn package`, so a fresh clone never needs a
+        separate manual step.
+      -->
+      <plugin>
+        <groupId>org.apache.maven.plugins</groupId>
+        <artifactId>maven-dependency-plugin</artifactId>
+        <executions>
+          <execution>
+            <id>write-runtime-classpath</id>
+            <phase>prepare-package</phase>
+            <goals>
+              <goal>build-classpath</goal>
+            </goals>
+            <configuration>
+              <outputFile>${project.basedir}/cp.txt</outputFile>
+            </configuration>
+          </execution>
+        </executions>
+      </plugin>
+    </plugins>
+  </build>
+</project>