# MongoDB to MySQL Migration Summary

## Overview
This document has been successfully migrated from **MongoDB/Mongoose** to **MySQL/Sequelize** architecture.

---

## Major Changes

### 1. **Technology Stack Update**
- **Changed from**: MongoDB with Mongoose ODM
- **Changed to**: MySQL 8.0+ with Sequelize ORM
- **Stack name**: Updated from "MEAN Stack" to "MySQL + Express + Angular + Node"

### 2. **Database Schema Transformation**

#### ID Strategy
- **MongoDB**: ObjectId (12-byte identifier)
- **MySQL**: UUID (CHAR(36) or BINARY(16))

#### Data Types Mapping
| MongoDB | MySQL |
|---------|-------|
| ObjectId | CHAR(36) UUID |
| String | VARCHAR/TEXT |
| Number | INT/DECIMAL |
| Boolean | BOOLEAN |
| Date | TIMESTAMP |
| Mixed | JSON |
| Array | JSON or Junction Tables |
| Embedded Documents | JSON or Separate Tables |

#### Schema Design Approach
- **Normalized tables** for core entities (users, questions, categories)
- **Junction tables** for many-to-many relationships (bookmarks, achievements, quiz questions)
- **JSON columns** for flexible data (question options, keywords, tags, feature restrictions)
- **Proper foreign keys** with cascading deletes/updates

### 3. **Database Tables Created**

1. **users** - User accounts and statistics
2. **categories** - Question categories
3. **questions** - Question bank with guest visibility controls
4. **quiz_sessions** - Active and completed quiz sessions
5. **quiz_session_questions** - Junction table for session questions
6. **quiz_answers** - Individual question answers
7. **guest_sessions** - Guest user sessions
8. **guest_settings** - Guest access configuration
9. **guest_settings_categories** - Junction table for guest-accessible categories
10. **achievements** - Achievement definitions
11. **user_achievements** - User earned achievements
12. **user_bookmarks** - User bookmarked questions

### 4. **Key Features**

#### Indexes Added
- Primary key indexes (UUID)
- Foreign key indexes for relationships
- Composite indexes for common queries
- Full-text search indexes on question content
- Performance indexes on frequently queried columns

#### MySQL-Specific Optimizations
- InnoDB storage engine
- UTF8MB4 character set for emoji support
- Connection pooling configuration
- Query optimization examples
- Backup and restore strategies

### 5. **Sequelize Models**

All models include:
- UUID primary keys
- Proper associations (hasMany, belongsTo, belongsToMany)
- Field name mapping (camelCase to snake_case)
- Timestamps (created_at, updated_at)
- Validation rules
- Indexes defined at model level

### 6. **Configuration Updates**

#### Environment Variables
```bash
# Old (MongoDB)
MONGODB_URI=mongodb://localhost:27017/interview_quiz

# New (MySQL)
DB_HOST=localhost
DB_PORT=3306
DB_NAME=interview_quiz_db
DB_USER=root
DB_PASSWORD=your_password
DB_DIALECT=mysql
```

#### Dependencies to Update
```bash
# Remove
npm uninstall mongoose

# Install
npm install sequelize mysql2
npm install -g sequelize-cli
```

### 7. **Migration Strategy**

#### Development Setup
1. Install MySQL 8.0+
2. Create database with UTF8MB4
3. Run Sequelize migrations
4. Seed initial data
5. Start application

#### Migration Commands
```bash
# Initialize Sequelize
npx sequelize-cli init

# Create migration
npx sequelize-cli migration:generate --name migration-name

# Run migrations
npx sequelize-cli db:migrate

# Rollback migration
npx sequelize-cli db:migrate:undo

# Seed database
npx sequelize-cli db:seed:all
```

### 8. **Security Enhancements**

- **SQL Injection Prevention**: Sequelize parameterized queries
- **Prepared Statements**: All queries use prepared statements
- **Connection Security**: SSL/TLS support for production
- **Role-based Access**: Maintained through user roles table

### 9. **Performance Considerations**

#### Query Optimization
- Use of composite indexes
- Efficient JOIN operations
- Connection pooling (max 10 connections)
- Query result caching with Redis
- Lazy loading for associations

#### Database Configuration
- InnoDB buffer pool tuning
- Query cache optimization (MySQL 5.7)
- Binary logging for replication
- Slow query log monitoring

### 10. **Deployment Updates**

#### Development
- Local MySQL instance
- Sequelize migrations for schema management

#### Staging
- Managed MySQL (AWS RDS, Azure Database, PlanetScale)
- Automated migration deployment

#### Production
- Read replicas for scaling
- Automated backups (daily snapshots)
- Point-in-time recovery
- Connection pooling with ProxySQL (optional)

---

## What Stayed the Same

✅ **API Endpoints** - All endpoints remain unchanged  
✅ **Frontend Code** - No changes required to Angular app  
✅ **JWT Authentication** - Token strategy unchanged  
✅ **Business Logic** - Core features remain identical  
✅ **User Stories** - All acceptance criteria maintained  

---

## Breaking Changes

⚠️ **Database Migration Required** - Cannot directly migrate MongoDB data to MySQL without transformation  
⚠️ **ORM Changes** - All Mongoose code must be rewritten to Sequelize  
⚠️ **Array Queries** - MongoDB array operations need to be rewritten for JSON columns or junction tables  
⚠️ **Aggregation Pipelines** - Complex MongoDB aggregations need to be rewritten as SQL JOINs and GROUP BY  

---

## Data Migration Steps

If migrating existing MongoDB data:

1. **Export MongoDB Data**
   ```bash
   mongoexport --db interview_quiz --collection users --out users.json
   mongoexport --db interview_quiz --collection questions --out questions.json
   # ... export all collections
   ```

2. **Transform Data**
   - Convert ObjectIds to UUIDs
   - Flatten embedded documents
   - Split arrays into junction tables
   - Map MongoDB types to MySQL types

3. **Import to MySQL**
   ```bash
   # Use custom Node.js script to import transformed data
   node scripts/import-from-mongodb.js
   ```

---

## Testing Checklist

- [ ] All database tables created successfully
- [ ] Foreign key constraints working
- [ ] Indexes created and optimized
- [ ] User registration and login working
- [ ] Guest session management working
- [ ] Quiz session creation and completion
- [ ] Question CRUD operations (admin)
- [ ] Bookmark functionality
- [ ] Achievement system
- [ ] Full-text search on questions
- [ ] Dashboard statistics queries
- [ ] Performance testing (1000+ concurrent users)
- [ ] Backup and restore procedures
- [ ] Migration rollback testing

---

## Additional Resources

### Sequelize Documentation
- [Sequelize Official Docs](https://sequelize.org/docs/v6/)
- [Migrations Guide](https://sequelize.org/docs/v6/other-topics/migrations/)
- [Associations](https://sequelize.org/docs/v6/core-concepts/assocs/)

### MySQL Documentation
- [MySQL 8.0 Reference](https://dev.mysql.com/doc/refman/8.0/en/)
- [InnoDB Storage Engine](https://dev.mysql.com/doc/refman/8.0/en/innodb-storage-engine.html)
- [Performance Tuning](https://dev.mysql.com/doc/refman/8.0/en/optimization.html)

---

## Version Information

- **Document Version**: 2.0 - MySQL Edition
- **Migration Date**: November 2025
- **Database**: MySQL 8.0+
- **ORM**: Sequelize 6.x
- **Node.js**: 18+

---

**Migration completed successfully!** 🎉