Zero-Downtime Database Migrations

// Python 3.10+ Dual-Write & Feature Flag Cutover
class UserRepository:
    def __init__(self, db, feature_flags):
        self.db = db
        self.flags = feature_flags

    def update_user_name(self, user_id: int, full_name: str):
        first_name, last_name = full_name.split(" ", 1)

        # Phase 2 & 3: Dual Write (Write to both Old and New columns)
        sql = "UPDATE users SET name = %s, first_name = %s, last_name = %s WHERE id = %s"
        self.db.execute(sql, (full_name, first_name, last_name, user_id))

    def get_user_name(self, user_id: int) -> str:
        # Phase 4 & 5: Controlled Read Cutover via Feature Flag
        if self.flags.is_enabled("read_new_name_schema"):
            row = self.db.query("SELECT first_name, last_name FROM users WHERE id = %s", (user_id,))
            return f"{row.first_name} {row.last_name}"
        else:
            row = self.db.query("SELECT name FROM users WHERE id = %s", (user_id,))
            return row.name

// C++20 Dual-Write Repository
#include <string>

class UserRepository {
    Database* db;
    FeatureFlags* flags;
public:
    void updateName(int userId, const std::string& fullName, const std::string& first, const std::string& last) {
        // Dual write to old and new columns
        db->execute("UPDATE users SET name=?, first_name=?, last_name=? WHERE id=?", fullName, first, last, userId);
    }

    std::string getName(int userId) {
        if (flags->isEnabled("read_new_name_schema")) {
            auto row = db->query("SELECT first_name, last_name FROM users WHERE id=?", userId);
            return row.first + " " + row.last;
        }
        return db->query("SELECT name FROM users WHERE id=?", userId).name;
    }
};

// Java 17+ Dual-Write Repository
public class UserRepository {
    private final JdbcTemplate db;
    private final FeatureFlagService flags;

    public void updateUserName(int userId, String fullName) {
        String[] parts = fullName.split(" ", 2);
        // Dual write to both old and new schema columns
        db.update("UPDATE users SET name = ?, first_name = ?, last_name = ? WHERE id = ?",
            fullName, parts[0], parts[1], userId);
    }

    public String getUserName(int userId) {
        if (flags.isEnabled("read_new_name_schema")) {
            return db.queryForObject("SELECT CONCAT(first_name, ' ', last_name) FROM users WHERE id = ?", String.class, userId);
        }
        return db.queryForObject("SELECT name FROM users WHERE id = ?", String.class, userId);
    }
}

// Go 1.20+ Dual-Write Controller
package main

import "strings"

func (r *UserRepo) UpdateName(userId int, fullName string) error {
    parts := strings.SplitN(fullName, " ", 2)
    // Write to both Old and New schema
    _, err := r.db.Exec("UPDATE users SET name=$1, first_name=$2, last_name=$3 WHERE id=$4",
        fullName, parts[0], parts[1], userId)
    return err
}

// TypeScript Dual-Write Service
export class UserService {
    constructor(private db: Database, private flags: FeatureFlags) {}

    async updateName(userId: number, fullName: string): Promise<void> {
        const [first, last] = fullName.split(' ');
        // Dual-write in a single transaction or query
        await this.db.query(
            'UPDATE users SET name = $1, first_name = $2, last_name = $3 WHERE id = $4',
            [fullName, first, last, userId]
        );
    }
}