asmhatreandClaude Sonnet 5
e478eafda3
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
73 lines
3.6 KiB
Java
73 lines
3.6 KiB
Java
package com.ankurm.expandcontract;
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import org.junit.jupiter.api.Test;
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import org.junit.jupiter.api.io.TempDir;
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import org.springframework.jdbc.core.simple.JdbcClient;
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import java.nio.file.Path;
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import java.sql.Connection;
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import java.sql.DriverManager;
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import static org.assertj.core.api.Assertions.assertThat;
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/**
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* Deploy 1 (EXPAND) in isolation: migrate to V2 and confirm two things a real rollout
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* depends on. First, that every pre-existing row got backfilled in the same migration.
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* Second - the part that is easy to get wrong - that Stage 1 code, completely unaware
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* the new column exists, still inserts rows exactly as it always has. If this second
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* assertion ever failed, the migration would not be additive and expand-contract would
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* not apply to it. See docs/02-the-expand-migration.md.
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*/
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class ExpandMigrationBackwardCompatibleTest {
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@Test
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void backfillsExistingRowsAndStaysCompatibleWithStage1Code(@TempDir Path tmp) throws Exception {
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Path db = tmp.resolve("expand");
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Transcript t = Transcript.start("02-expand-backward-compatible",
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"Deploy 1 (EXPAND): additive column + backfill, Stage 1 code untouched");
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// Seed one row the way Stage 1 always has, before the expand migration exists.
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TestSupport.migrateTo(db, "1");
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JdbcClient preExpand = TestSupport.jdbcClient(db);
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preExpand.sql("INSERT INTO customers(name, email) VALUES (?, ?)")
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.param("Ada Lovelace").param("[email protected]").update();
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t.section("schema before Deploy 1");
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try (Connection conn = DriverManager.getConnection(TestSupport.jdbcUrl(db), "sa", "")) {
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t.line(DbDump.table(conn, "select column_name from information_schema.columns "
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+ "where table_name = 'CUSTOMERS' order by ordinal_position"));
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}
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// Deploy 1: the expand migration runs against the live database. No app restart.
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TestSupport.migrateTo(db, "2");
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t.section("schema after Deploy 1 (email_address added)");
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try (Connection conn = DriverManager.getConnection(TestSupport.jdbcUrl(db), "sa", "")) {
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t.line(DbDump.table(conn, "select column_name from information_schema.columns "
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+ "where table_name = 'CUSTOMERS' order by ordinal_position"));
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t.section("Ada's row was backfilled by the migration itself");
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String row = DbDump.table(conn, "select name, email, email_address from customers where name = 'Ada Lovelace'");
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t.line(row);
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// Backfilled: both columns hold the same value for a row that predates dual-write code.
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assertThat(row).contains("[email protected]");
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assertThat(row.indexOf("[email protected]")).isNotEqualTo(row.lastIndexOf("[email protected]"));
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}
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// Stage 1 code has not been redeployed and does not know email_address exists.
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// Its insert statement is byte-for-byte what it was before Deploy 1.
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JdbcClient postExpand = TestSupport.jdbcClient(db);
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postExpand.sql("INSERT INTO customers(name, email) VALUES (?, ?)")
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.param("Grace Hopper").param("[email protected]").update();
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t.section("Stage 1's original INSERT still works, unmodified, after the migration");
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try (Connection conn = DriverManager.getConnection(TestSupport.jdbcUrl(db), "sa", "")) {
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String row = DbDump.table(conn, "select name, email, email_address from customers where name = 'Grace Hopper'");
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t.line(row);
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assertThat(row).contains("[email protected]").contains("NULL");
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}
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t.write();
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}
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}
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