10. What are controllers, services, repositories, middlewares and request context? TL;DR: This content explains the structured flow of an API request through backend architecture, detailing the roles of Handlers, Services, Repositories, Middlewares, and Request Context. The Gist Topic: Backend API Request Lifecycle & Architecture Core Concept: This content provides a comprehensive overview of how an API request is processed in a typical backend application, breaking down the responsibilities of different architectural layers and the utility of middlewares and request context. How it works: Handler/Controller Layer Receives HTTP request and response objects . Extracts data (query parameters, request body) from the request object. Deserializes JSON into native programming language data structures (binding) . Performs validation and optional data transformations . Calls the Service Layer with the processed data. Determines and sends the appropriate HTTP response (status code and data) back to the client . Service Layer Encapsulates core business logic, independent of HTTP specifics . Orchestrates operations, potentially calling multiple Repository methods or external APIs (e.g., sending emails). Returns processed data to the Handler. Repository Layer Solely responsible for database interactions . Takes data, constructs and executes database queries (insert, fetch, sort, filter), and returns results. Each repository method should have a single, well-defined responsibility. Middlewares Functions executed at various points in the request lifecycle (e.g., between routing and handler, or before/after service calls) . Receive request, response objects, and a next() function to pass control. Used for cross-cutting concerns like: CORS headers Authentication and authorization Rate limiting Logging and monitoring Global error handling Response compression Their execution order is crucial . Request Context A storage or state object scoped to a particular request . Accessible across middlewares and handlers throughout the request's execution. Used to store request-specific information like user ID, permissions, or a unique request ID for auditing and debugging . Key Learnings & Takeaways: Separation of Concerns: Clearly defined roles for Handlers, Services, and Repositories enhance scalability, maintainability, and testability. Robust Validation: All client-sent data (path parameters, query parameters, request body) must be validated on the server side. Middleware Efficiency: Leverage middlewares to handle recurring, non-core logic (e.g., security, logging) efficiently and avoid code duplication. Request Context for State Management: Use request context to manage request-specific state and information that needs to be accessible across different layers and middlewares. Key Topics: Request Object -> Response Object -> Handler/Controller Responsibilities -> , , , Deserialization/Binding -> Validation and Transformation -> Service Layer Responsibilities -> Repository Layer Responsibilities -> API Request Lifecycle -> Middlewares -> Middleware Parameters (request, response, next) -> CORS Middleware -> Authentication Middleware -> Rate Limiting Middleware -> Logging Middleware -> Global Error Handling Middleware -> Compression Middleware -> Middleware Ordering -> Request Context -> ,