# Optimization Guide

## Overview
This guide provides strategies and recommendations for optimizing the XYZ POS Server application for performance, scalability, and efficiency.

## Database Optimization

### Index Optimization

#### Current Indexes
The application already has indexes on:
- Primary keys (automatic)
- Foreign keys (customer_id, product_id, user_id, etc.)
- Status fields (sale.status, shift.status, etc.)
- Date fields (created_at, opened_at, etc.)
- Unique fields (username, invoice_no, etc.)

#### Additional Index Recommendations

1. **Composite Indexes for Common Queries**
   ```sql
   -- Sales queries by date and status
   CREATE INDEX idx_sales_date_status ON sales(created_at, status);
   
   -- Inventory queries by product and variant
   CREATE INDEX idx_inventory_product_variant ON inventory(product_id, variant_id);
   
   -- Sale items by sale and product
   CREATE INDEX idx_sale_items_sale_product ON sale_items(sale_id, product_id);
   ```

2. **Indexes for Report Queries**
   ```sql
   -- Sales reports by customer and date
   CREATE INDEX idx_sales_customer_date ON sales(customer_id, created_at);
   
   -- Sales reports by cashier and date
   CREATE INDEX idx_sales_user_date ON sales(user_id, created_at);
   ```

### Query Optimization

1. **Use Eager Loading (Includes)**
   - Always include related data in a single query instead of N+1 queries
   - Example: `Sale.findAll({ include: [{ model: SaleItem }, { model: Customer }] })`

2. **Use Raw SQL for Complex Aggregations**
   - Reports use raw SQL for better performance
   - Example: `sequelize.query('SELECT ... GROUP BY ...')`

3. **Limit Result Sets**
   - Always use pagination for list endpoints
   - Default limit: 50 items per page
   - Maximum limit: 100 items per page

4. **Select Specific Fields**
   - Use `attributes` to select only needed fields
   - Example: `Product.findAll({ attributes: ['id', 'name', 'sku'] })`

5. **Use Transactions Efficiently**
   - Group related operations in a single transaction
   - Keep transaction scope as small as possible
   - Commit transactions promptly

### Database Connection Pooling

Current configuration (Sequelize default):
- Max connections: 5
- Min connections: 0
- Idle timeout: 10000ms

**Recommendation for Production:**
```javascript
// config/database.js
pool: {
  max: 20,        // Maximum connections
  min: 5,         // Minimum connections
  acquire: 30000, // Max time to acquire connection
  idle: 10000     // Idle timeout
}
```

## API Response Time Optimization

### Current Targets
- Average response time: < 200ms
- 95th percentile: < 500ms
- 99th percentile: < 1000ms

### Optimization Strategies

1. **Caching**
   - Cache frequently accessed data (product catalogs, tax rates)
   - Use Redis for distributed caching
   - Cache invalidation on updates

2. **Response Compression**
   - Already enabled with `compression` middleware
   - Compresses responses > 1KB

3. **Pagination**
   - All list endpoints support pagination
   - Reduces response payload size

4. **Lazy Loading**
   - Only load related data when needed
   - Use separate endpoints for detailed data

## Code Optimization

### 1. Avoid N+1 Queries
```javascript
// BAD: N+1 queries
const sales = await Sale.findAll();
for (const sale of sales) {
  const items = await sale.getItems(); // N queries
}

// GOOD: Eager loading
const sales = await Sale.findAll({
  include: [{ model: SaleItem, as: 'items' }]
});
```

### 2. Batch Operations
```javascript
// BAD: Individual inserts
for (const item of items) {
  await SaleItem.create(item);
}

// GOOD: Bulk insert
await SaleItem.bulkCreate(items);
```

### 3. Use Transactions for Multiple Operations
```javascript
const transaction = await sequelize.transaction();
try {
  await operation1({ transaction });
  await operation2({ transaction });
  await transaction.commit();
} catch (error) {
  await transaction.rollback();
  throw error;
}
```

### 4. Memory Management
- Release database connections promptly
- Limit result sets
- Use streaming for large datasets

## Performance Monitoring

### Metrics to Track

1. **API Response Times**
   - Average response time
   - P95 and P99 percentiles
   - Slowest endpoints

2. **Database Performance**
   - Query execution times
   - Connection pool usage
   - Slow query log

3. **Resource Usage**
   - CPU usage
   - Memory usage
   - Database connections

### Tools

1. **Application Monitoring**
   - Use Winston logger for request logging
   - Track response times in middleware
   - Log slow queries

2. **Database Monitoring**
   - MySQL slow query log
   - EXPLAIN for query analysis
   - Index usage statistics

3. **Load Testing**
   ```bash
   # Use autocannon for load testing
   autocannon -c 10 -d 30 http://localhost:3000/api/health
   
   # Use artillery for stress testing
   artillery quick --count 50 --num 1000 http://localhost:3000/api/health
   ```

## Security Optimization

### Current Security Measures

1. **Authentication**
   - JWT tokens with expiration
   - Password hashing with bcrypt (10 rounds)

2. **Authorization**
   - Role-based access control (RBAC)
   - Endpoint-level permission checks

3. **Input Validation**
   - express-validator for all endpoints
   - SQL injection protection via Sequelize ORM
   - XSS protection via input sanitization

4. **Security Headers**
   - Helmet.js for security headers
   - CORS configuration
   - Rate limiting

### Recommendations

1. **Password Security**
   - Consider increasing bcrypt rounds to 12 for production
   - Implement password complexity requirements
   - Add password reset functionality

2. **Token Security**
   - Use refresh tokens for long sessions
   - Implement token blacklisting
   - Shorter access token expiration (15-30 minutes)

3. **Rate Limiting**
   - Current: 100 requests per 15 minutes
   - Consider stricter limits for sensitive endpoints
   - IP-based rate limiting

4. **Audit Logging**
   - Log all sensitive operations
   - Track user actions
   - Monitor for suspicious activity

## Scaling Strategies

### Vertical Scaling
- Increase server resources (CPU, RAM)
- Increase database resources
- Optimize database configuration

### Horizontal Scaling
- Load balancing across multiple servers
- Database read replicas for reports
- Distributed caching (Redis)

### Database Scaling
- Read replicas for read-heavy operations (reports)
- Sharding for very large datasets
- Archiving old data

## Checklist for Production Deployment

- [ ] Database indexes optimized
- [ ] Query performance tested
- [ ] API response times < 200ms average
- [ ] Security audit completed
- [ ] Error handling comprehensive
- [ ] Logging and monitoring setup
- [ ] Backup strategy in place
- [ ] Disaster recovery plan
- [ ] Load testing completed
- [ ] Documentation complete

