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Server Overview (server.js)
This is the main entry point of the backend service.

Key Responsibilities
Loads environment variables.

Sets up Express app with CORS and JSON parsing.

Logs all incoming requests with timestamps.

Registers route groups:

/sse → Server-Sent Events routes

/api → REST API routes

Health check endpoint: /health

Root endpoint: /

Handles 404 and server errors.

Graceful shutdown on SIGTERM/SIGINT.

2. Database Layer (db.js)
   This uses pg’s connection pooling to interact with a PostgreSQL database.

Key Exports
query(text, params) → Executes SQL queries with logging & error handling.

getClient() → Gets a pooled DB client (for transactions).

transaction(callback) → Runs multiple DB operations atomically.

testConnection() → Verifies DB connection & logs current DB time.

closePool() → Closes pool on shutdown.
SSE Service (sseService.js)
Manages real-time client connections using Server-Sent Events.

Core Features
addClient(res, userId?, sessionId?) → Registers a new SSE connection.

removeClient(clientId) → Closes and removes a connection.

sendToClient(clientId, eventType, data) → Sends event to a specific client.

sendToUser(userId, eventType, data) → Sends to all clients of a user.

broadcast(eventType, data) → Sends event to all clients.

Heartbeat mechanism → Sends ping events every 30s to keep connections alive.

getStats() → Returns connection statistics.

cleanupStaleConnections(maxAge) → Removes clients inactive for too long.
Flow:

Validates clientId.

Optionally fetches user details from DB.

Sends event via sseService.sendToClient.
Flow:

1. Validates clientId.

2. Get User by ID
   GET /api/user/:id
   Fetches a single user record from the database.

3. Get All Users
   GET /api/users
   Returns all users from DB, ordered by id.

4. Send Notification to All Clients of a User
   POST /api/notify/user/:userId
   Broadcasts to all SSE connections for a given user.

5. Broadcast Notification to All Clients
   POST /api/notify/broadcast
   Sends an event to all connected clients.

6. Trigger Test Notification
   POST /api/test-notification
   Broadcasts a sample test notification for demo purposes.

7. Clean Up Stale Connections
   POST /api/cleanup
   Removes connections idle beyond the configured timeout (default: 5 minutes).

8. API Health Check
   GET /api/health
   Checks DB connection and returns SSE stats.