How would you handle 10x traffic growth in your Spring Boot application?

// Original stateful code (WRONG): stores user sessions in memory
@RestController
public class UserController {
    private Map<String, User> sessions = new HashMap<>(); // Breaks with multiple instances!
    
    @PostMapping("/login")
    public String login(@RequestBody LoginRequest request, HttpSession session) {
        // HttpSession is stored in application memory, not shared across instances
        session.setAttribute("userId", user.getId());
        return "success";
    }
}

// CORRECT: Stateless with Redis for distributed sessions and caching
@RestController
public class ProductController {
    private final ProductService productService;
    
    @GetMapping("/products/{id}")
    public ProductDTO getProduct(@PathVariable Long id) {
        return productService.findById(id); // Service handles caching
    }
}

@Service
public class ProductService {
    private final ProductRepository productRepository;
    private final RedisTemplate<String, ProductDTO> redisTemplate;
    
    @Cacheable(value = "products", key = "#id") // Spring Boot caches result in Redis
    public ProductDTO findById(Long id) {
        var product = productRepository.findById(id)
            .orElseThrow(() -> new NotFoundException("Product not found"));
        return toDTO(product); // Only hits database if not in cache
    }
    
    @CacheEvict(value = "products", key = "#id") // Removes from cache when updated
    public void updateProduct(Long id, ProductRequest request) {
        var product = productRepository.findById(id).orElseThrow();
        product.setName(request.name());
        productRepository.save(product);
    }
}