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
60 changed files with 3561 additions and 0 deletions
@@ -0,0 +1,72 @@
+package com.ankurm.expandcontract;
+
+import java.sql.Connection;
+import java.sql.ResultSet;
+import java.sql.ResultSetMetaData;
+import java.sql.SQLException;
+import java.sql.Statement;
+import java.util.ArrayList;
+import java.util.List;
+
+/** Renders a JDBC query as a plain-text table for transcripts - no ORM, no formatting library. */
+public final class DbDump {
+
+    private DbDump() {
+    }
+
+    public static String table(Connection conn, String sql) {
+        try (Statement st = conn.createStatement(); ResultSet rs = st.executeQuery(sql)) {
+            ResultSetMetaData meta = rs.getMetaData();
+            int cols = meta.getColumnCount();
+            List<String> headers = new ArrayList<>();
+            for (int i = 1; i <= cols; i++) {
+                headers.add(meta.getColumnLabel(i));
+            }
+            List<List<String>> rows = new ArrayList<>();
+            while (rs.next()) {
+                List<String> row = new ArrayList<>();
+                for (int i = 1; i <= cols; i++) {
+                    Object v = rs.getObject(i);
+                    row.add(v == null ? "NULL" : v.toString());
+                }
+                rows.add(row);
+            }
+            int[] widths = new int[cols];
+            for (int i = 0; i < cols; i++) {
+                widths[i] = headers.get(i).length();
+            }
+            for (List<String> row : rows) {
+                for (int i = 0; i < cols; i++) {
+                    widths[i] = Math.max(widths[i], row.get(i).length());
+                }
+            }
+            StringBuilder sb = new StringBuilder();
+            appendRow(sb, headers, widths);
+            StringBuilder sep = new StringBuilder();
+            for (int i = 0; i < cols; i++) {
+                sep.append("-".repeat(widths[i])).append(i < cols - 1 ? "-+-" : "");
+            }
+            sb.append(sep).append(System.lineSeparator());
+            for (List<String> row : rows) {
+                appendRow(sb, row, widths);
+            }
+            sb.append("(").append(rows.size()).append(" row").append(rows.size() == 1 ? "" : "s").append(")");
+            return sb.toString();
+        }
+        catch (SQLException ex) {
+            return "query failed: " + ex.getMessage();
+        }
+    }
+
+    private static void appendRow(StringBuilder sb, List<String> values, int[] widths) {
+        for (int i = 0; i < values.size(); i++) {
+            sb.append(pad(values.get(i), widths[i]));
+            sb.append(i < values.size() - 1 ? " | " : "");
+        }
+        sb.append(System.lineSeparator());
+    }
+
+    private static String pad(String s, int width) {
+        return s + " ".repeat(Math.max(0, width - s.length()));
+    }
+}