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

db-migrations-expand-contract/scripts/run-load-generator.sh

@@ -0,0 +1,22 @@
+#!/usr/bin/env bash
+# Starts the load generator against both replica ports for a fixed duration and
+# returns immediately - it runs in the background for the rest of run-all.sh.
+# Usage: run-load-generator.sh <durationSeconds>
+set -euo pipefail
+DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+source "$DIR/env.sh"
+
+DURATION="${1:-170}"
+rm -f "$EC_PHASE_FILE" "$EC_LOAD_SUMMARY"
+set_phase "00-baseline-soak"
+
+nohup java -cp "$MODULE_DIR/target/classes:$(cat "$CP_FILE")" \
+    com.ankurm.expandcontract.loadgen.LoadGenerator \
+    --ports="$PORT_A,$PORT_B" \
+    --durationSeconds="$DURATION" \
+    --phaseFile="$EC_PHASE_FILE" \
+    --outFile="$EC_LOAD_SUMMARY" \
+    --threads=8 \
+    > "$EC_LOG_DIR/load-generator.log" 2>&1 < /dev/null &
+echo $! > "$EC_PID_DIR/loadgen.pid"
+echo "load generator started (pid $(cat "$EC_PID_DIR/loadgen.pid")), running for ${DURATION}s"