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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+==============================================================================
+Deploy 4 (CONTRACT): Stage 4 after the drop, and what breaks if you drop too soon
+==============================================================================
+captured: 2026-09-16T19:20:57.618780340Z
+
+
+-- Stage 4 read, after V3 dropped the email column --
+Customer[id=1, name=Annie Easley, [email protected]]
+
+-- Stage 4 create + read, entirely after the drop --
+Customer[id=2, name=Mary Allen Wilkes, [email protected]]