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
parent 22c30d4a5b
commit e478eafda3
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+=========================================================================
+Deploy 1 (EXPAND): additive column + backfill, Stage 1 code untouched
+=========================================================================
+captured: 2026-09-16T19:20:56.676919361Z
+
+
+-- schema before Deploy 1 --
+COLUMN_NAME
+-----------
+ID         
+NAME       
+EMAIL      
+CREATED_AT 
+(4 rows)
+
+-- schema after Deploy 1 (email_address added) --
+COLUMN_NAME  
+-------------
+ID           
+NAME         
+EMAIL        
+CREATED_AT   
+EMAIL_ADDRESS
+(5 rows)
+
+-- Ada's row was backfilled by the migration itself --
+NAME         | EMAIL            | EMAIL_ADDRESS   
+-------------+------------------+-----------------
+Ada Lovelace | [email protected] | [email protected]
+(1 row)
+
+-- Stage 1's original INSERT still works, unmodified, after the migration --
+NAME         | EMAIL              | EMAIL_ADDRESS
+-------------+--------------------+--------------
+Grace Hopper | [email protected] | NULL         
+(1 row)