Feat/545 546 547 548 refactor

[jellyroll-giga]

Refactor: Consolidate API responses, extract business logic, standardize logging, and optimize database queries

Overview

This PR implements comprehensive refactoring across four key areas of the backend to improve code quality, maintainability, and performance. All changes are backward compatible and follow project conventions.

Changes Implemented

#545 - Consolidate API Response Formats

• Created standardized ApiResponseDto class with consistent structure across all endpoints
• Implemented PaginatedResponseDto for paginated endpoints with metadata (page, limit, total, totalPages)
• Updated TransformInterceptor to automatically wrap responses in standardized format
• All API responses now include: data, statusCode, timestamp, and optional pagination and error fields
• Files: apps/backend/src/common/dto/api-response.dto.ts, apps/backend/src/common/interceptors/transform.interceptor.ts

#546 - Extract Business Logic to Services

• Created BusinessLogicService base class with common patterns (authorization validation, pagination utilities)
• Implemented CoursesBusinessService to handle course-specific business logic
• Moved authorization checks from controller to business service layer
• Centralized pagination validation and offset calculation
• Updated CoursesController to delegate business logic to service
• Files: apps/backend/src/common/services/business-logic.service.ts, apps/backend/src/courses/courses-business.service.ts, apps/backend/src/courses/courses.controller.ts, apps/backend/src/courses/courses.module.ts

#547 - Implement Logging Standardization

• Created LoggerFactory for creating standardized loggers with context
• Implemented StandardizedLogger with structured logging methods (info, warn, error, debug, verbose)
• Added LoggingMiddleware for automatic HTTP request/response logging with duration tracking
• Added ErrorLoggingMiddleware for error tracking and logging
• Implemented LoggingInterceptor for method-level logging with performance metrics
• Integrated request ID tracking for distributed tracing across services
• Added specialized logging methods: logRequest(), logRequestError(), logQuery(), logQueryError(), logOperation()
• Included comprehensive LOGGING_GUIDE.md with usage examples and best practices
• Files: apps/backend/src/common/logger/logger-factory.ts, apps/backend/src/common/logger/logging.middleware.ts, apps/backend/src/common/logger/logging.interceptor.ts, apps/backend/src/common/logger/LOGGING_GUIDE.md, apps/backend/src/common/logger/logger.module.ts

#548 - Optimize Database Queries

• Created QueryOptimizer utility class with methods for:
  • Eager loading relations to prevent N+1 queries
  • Pagination with skip/take
  • Sorting with ASC/DESC
  • Filtering with parameterized queries
  • Column selection to reduce data transfer
• Implemented CacheQuery and InvalidateCache decorators for query result caching
• Created DatabasePerformanceService for monitoring and analysis:
  • Slow query detection and logging
  • Database statistics (connection count, active connections)
  • Table statistics (row count, size, vacuum/analyze dates)
  • Query execution plan analysis
  • Missing index detection
• Updated CoursesService to use QueryOptimizer for optimized queries
• Included comprehensive DATABASE_OPTIMIZATION.md with patterns and best practices
• Files: apps/backend/src/common/database/query-optimizer.ts, apps/backend/src/common/database/query-cache.decorator.ts, apps/backend/src/common/database/database-performance.service.ts, apps/backend/src/common/database/DATABASE_OPTIMIZATION.md, apps/backend/src/courses/courses.service.ts

Key Benefits

• Consistency: All API responses follow the same structure
• Maintainability: Business logic separated from HTTP concerns
• Observability: Comprehensive structured logging with request tracing
• Performance: Optimized queries with N+1 prevention and caching
• Documentation: Detailed guides for logging and database optimization

Testing Recommendations

• Unit tests for ApiResponseDto structure validation
• Integration tests for business logic authorization
• Performance tests for query optimization
• Logging output verification tests

Migration Notes

• Existing code continues to work (backward compatible)
• New services can be adopted incrementally
• Logging is automatically applied via middleware/interceptors
• Query optimization can be applied to existing services gradually

Closes #545
Closes #546
Closes #547
Closes #548

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