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
2026-09-16 19:22:48 +00:00
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+package com.ankurm.expandcontract;
+
+import com.ankurm.expandcontract.customer.CustomerService;
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.io.TempDir;
+
+import java.nio.file.Path;
+import java.sql.Connection;
+import java.sql.DriverManager;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+/**
+ * Deploy 2 (MIGRATE WRITES): Stage 2 code writes every create and update to both
+ * columns. This is the deploy that makes Deploy 3's read switch safe later - if this
+ * one is wrong, nothing downstream can be trusted. See docs/04-the-dual-write.md.
+ */
+class DualWriteConsistencyTest {
+
+    @Test
+    void createAndUpdatePopulateBothColumnsIdentically(@TempDir Path tmp) throws Exception {
+        Path db = tmp.resolve("dual-write");
+        TestSupport.migrateTo(db, "2");
+        Transcript t = Transcript.start("04-dual-write-consistency",
+                "Deploy 2 (MIGRATE WRITES): Stage 2 writes land in both columns");
+
+        CustomerService stage2 = new CustomerService(TestSupport.jdbcClient(db), 2);
+        long id = stage2.create("Margaret Hamilton", "[email protected]");
+
+        try (Connection conn = DriverManager.getConnection(TestSupport.jdbcUrl(db), "sa", "")) {
+            t.section("after create()");
+            String row = DbDump.table(conn, "select name, email, email_address from customers where id = " + id);
+            t.line(row);
+            assertThat(row).contains("[email protected]");
+            assertThat(row.indexOf("[email protected]")).isNotEqualTo(row.lastIndexOf("[email protected]"));
+        }
+
+        stage2.updateEmail(id, "[email protected]");
+
+        try (Connection conn = DriverManager.getConnection(TestSupport.jdbcUrl(db), "sa", "")) {
+            t.section("after updateEmail() - the old value is gone from BOTH columns, not just one");
+            String row = DbDump.table(conn, "select name, email, email_address from customers where id = " + id);
+            t.line(row);
+            assertThat(row).contains("[email protected]").doesNotContain("[email protected]");
+        }
+
+        t.write();
+    }
+}