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

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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
This commit is contained in:
asmhatre
2026-09-16 19:22:48 +00:00
co-authored by Claude Sonnet 5
parent 22c30d4a5b
commit e478eafda3
60 changed files with 3561 additions and 0 deletions
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db-migrations-expand-contract/docs/output/06-not-null-trap.txt
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==========================================================================
The NOT NULL trap: expand without relaxing the old column's constraint
==========================================================================
captured: 2026-09-16T19:21:04.662650854Z
-- Stage 4 create() against the NAIVE migration (no DROP NOT NULL) --
org.springframework.dao.DataIntegrityViolationException: PreparedStatementCallback; SQL [INSERT INTO customers(name, email_address) VALUES (?, ?)]; NULL not allowed for column "EMAIL"; SQL statement:
INSERT INTO customers(name, email_address) VALUES (?, ?) [23502-240]
root cause: org.h2.jdbc.JdbcSQLIntegrityConstraintViolationException: NULL not allowed for column "EMAIL"; SQL statement:
INSERT INTO customers(name, email_address) VALUES (?, ?) [23502-240]
-- Stage 4 create() against the SHIPPED V2 migration (DROP NOT NULL included) --
Customer[id=1, name=On Time, [email protected]]