**Refactoring to Strict Layered Architecture**

- **Routing Layer:** Keep only request parsing, response formatting, and delegation to service layer. No business logic or data access here.

  ```js
  // routes/posts.js
  const express = require("express");
  const router = express.Router();
  const postService = require("../services/postService");

  router.get("/:id", async (req, res, next) => {
    try {
      const post = await postService.getPostById(req.params.id);
      res.json(post);
    } catch (err) {
      next(err);
    }
  });

  module.exports = router;
  ```

- **Service (Business Logic) Layer:** Implement domain logic here; orchestrate data access and external dependencies. Validate input sanity minimally.

  ```js
  // services/postService.js
  const postRepo = require("../repositories/postRepository");

  async function getPostById(id) {
    if (!id.match(/^[a-f0-9]{24}$/)) throw new Error("Invalid post ID"); // minimal sanity check
    const post = await postRepo.findById(id);
    if (!post) throw new NotFoundError("Post not found");
    return post;
  }

  module.exports = { getPostById };
  ```

- **Data Access (Repository) Layer:** Encapsulate database operations behind interfaces; isolate ORM or DB client usage.

  ```js
  // repositories/postRepository.js
  const PostModel = require("../models/Post");

  async function findById(id) {
    return PostModel.findById(id).lean();
  }

  module.exports = { findById };
  ```

---

**Caching and Rate Limiting Integration**

- **Caching:** Use Redis with `express-redis-cache` or `cache-manager` for response or data caching at service/repository level.

  - Cache data reads (e.g., posts) with TTL.
  - Invalidate cache on updates.

- **Rate Limiting:** Use `express-rate-limit` middleware.

  ```js
  const rateLimit = require("express-rate-limit");

  const limiter = rateLimit({
    windowMs: 15 * 60 * 1000,
    max: 100, // requests per window per IP
    standardHeaders: true,
    legacyHeaders: false,
  });

  app.use(limiter);
  ```

---

**Centralized Error Handling Middleware**

- Define custom error classes with types (e.g., `NotFoundError`, `ValidationError`, `AuthError`, `ServerError`).

- Middleware example:

  ```js
  function errorHandler(err, req, res, next) {
    let status = 500;
    let message = "Internal Server Error";

    if (err.name === "ValidationError") {
      status = 400;
      message = err.message;
    } else if (err.name === "NotFoundError") {
      status = 404;
      message = err.message;
    } else if (err.name === "AuthError") {
      status = 401;
      message = err.message;
    }

    if (process.env.NODE_ENV !== "production") {
      return res.status(status).json({ error: message, stack: err.stack });
    }
    res.status(status).json({ error: message });
  }

  app.use(errorHandler);
  ```

---

**Minimal Internal Input Validation/Sanitization**

- Validate critical identifiers and enum-like fields for format and allowed values.

- Sanitize strings to prevent injection if data enters database or logs.

- Use libraries like `validator` for light checks without full validation overhead.

- Example:

  ```js
  const validator = require("validator");

  function sanitizeInput(input) {
    return validator.escape(input.trim());
  }
  ```

---

**Dependency Injection Frameworks for ExpressJS**

- Use `awilix` or `inversify` for container-based dependency injection.

- Example with Awilix:

  ```js
  const { createContainer, asClass } = require("awilix");
  const container = createContainer();

  container.register({
    postRepository: asClass(PostRepository).scoped(),
    postService: asClass(PostService).scoped(),
  });

  // Inject into router:
  router.use((req, res, next) => {
    req.scope = container.createScope();
    next();
  });

  router.get("/:id", async (req, res, next) => {
    const postService = req.scope.resolve("postService");
    // ...
  });
  ```

---

**Documentation Templates/Structure**

- **API Contract:**

  - Endpoint, method, URL
  - Request parameters, headers, body schema
  - Response schema and status codes
  - Error responses and codes
  - Authentication/authorization notes

- **Module Interaction:**

  - Diagram showing routing → services → repositories → database
  - Responsibilities per module
  - Data flow and dependencies

- **Deployment & Security:**

  - Authentication flow and external delegation (Authelia)
  - Validation and sanitization assumptions
  - Environment variable usage and secrets handling
  - Error handling policy and logging
  - Rate limiting and caching configuration

- Use markdown with OpenAPI or Swagger specs for API.

---

**Performance Measurement Techniques**

- Use **profiling tools** like `clinic.js`, `node --inspect` with Chrome DevTools.

- Instrument app with **metrics middleware** (e.g., `express-prometheus-middleware`).

- Use **APM tools**: NewRelic, Datadog, or open-source alternatives (e.g., Elastic APM).

- Add **request timing logs** and measure DB query times.

- Analyze cache hit/miss rates and rate limiter effectiveness.

---

**Scalability Architectural Patterns**

- **Stateless services:** Keep session and state outside the app (consistent with external Auth and Redis for cache/session).

- **Horizontal scaling:** Use load balancers; ensure no in-process state.

- **Asynchronous processing:** Offload heavy or slow tasks (e.g., email notifications) to background queues (RabbitMQ, Bull).

- **Database optimization:** Indexes, pagination, query optimization, read replicas.

- **Microservices or modular services:** If growth demands, split monolith by bounded contexts (posts, users, comments).

- **API versioning:** To maintain backward compatibility with evolving client needs.

---

This suite of strategies improves maintainability, testability, scalability, security posture, and operational visibility of the ExpressJS blogging app.
