# Authentication Flow - BuySellService → Auth Service

## Overview

BuySellService **does NOT validate JWT tokens locally**. Instead, it calls the auth service API to validate and authorize every request.

## Flow Diagram

```
Android App → BuySellService → Auth Service → BuySellService → Response
     |              |                |               |
     |              |                |               |
  [Token]    [Extract Token]  [Validate Token]  [User Info]
     |              |                |               |
     |              |                |               |
     └──────────────┴────────────────┴───────────────┘
```

## Detailed Flow

### 1. Request Arrives at BuySellService

**Android App sends:**
```
GET http://localhost:3200/users/user-123
Authorization: Bearer eyJhbGc...
```

### 2. BuySellService Middleware Chain

**Step 1: requestContext**
- Extracts IP, User-Agent, generates Request-ID

**Step 2: auditLogger**
- Attaches audit logger to request

**Step 3: jwtAuthenticate** ⭐ **CALLS AUTH SERVICE**
- Extracts token from `Authorization: Bearer <token>` header
- **Calls auth service**: `POST http://localhost:3000/auth/validate-token`
- **Request body**: `{ token: "eyJhbGc..." }`
- **Waits for response**

### 3. Auth Service Validates Token

**Auth Service receives:**
```json
POST /auth/validate-token
Body: { "token": "eyJhbGc..." }
```

**Auth Service validates:**
- ✅ Token signature (using JWT secret)
- ✅ Token expiry (`exp` claim)
- ✅ Issuer (`iss` claim)
- ✅ Audience (`aud` claim)
- ✅ Token version (checks database for logout-all-devices)
- ✅ User exists in database

**Auth Service responds (if valid):**
```json
{
  "valid": true,
  "payload": {
    "sub": "user-123",
    "role": "user",
    "user_type": "seller",
    "phone_number": "+919876543210",
    "token_version": 1,
    "high_assurance": false
  }
}
```

**Auth Service responds (if invalid):**
```json
{
  "valid": false,
  "error": "Invalid or expired token"
}
```

### 4. BuySellService Receives Response

**If valid:**
- Extracts user info from `payload`
- Sets `req.user = { userId: "user-123", role: "user", ... }`
- Logs successful authentication
- Continues to next middleware

**If invalid:**
- Returns `401 Unauthorized`
- Logs failed authentication
- Stops request processing

### 5. Continue with Authorization

**After authentication:**
- Rate limiting (checks limits for userId)
- Coarse-grained authorization (checks role)
- Fine-grained authorization (checks resource ownership)
- Route handler (fetches user data)
- Response returned to Android app

## Code Flow

```javascript
// Backend/middleware/jwtAuthenticate.js

async function jwtAuthenticate(req, res, next) {
  // 1. Extract token
  const token = req.headers.authorization?.slice(7); // "Bearer <token>"
  
  // 2. Call auth service
  const response = await axios.post(
    'http://localhost:3000/auth/validate-token',
    { token }
  );
  
  // 3. Check response
  if (response.data.valid === true) {
    // 4. Extract user info
    req.user = {
      userId: response.data.payload.sub,
      role: response.data.payload.role,
      // ... other fields
    };
    next(); // Continue to next middleware
  } else {
    // 5. Return 401 if invalid
    return res.status(401).json({ error: 'Unauthorized' });
  }
}
```

## Benefits

✅ **Centralized Validation**: All token validation logic in one place (auth service)  
✅ **No Secret Sharing**: BuySellService doesn't need JWT secret  
✅ **Token Version Checking**: Auth service checks database for logout-all-devices  
✅ **Consistent**: All services use same validation logic  
✅ **Easy Updates**: Update validation logic once in auth service  

## Error Handling

**Auth Service Unavailable:**
- Returns `401 Unauthorized` with message: "Authentication service unavailable"
- Request is blocked (fail closed for security)

**Token Invalid:**
- Returns `401 Unauthorized` with error details
- Request is blocked

**Timeout:**
- 5 second timeout by default (configurable via `AUTH_SERVICE_TIMEOUT`)
- Returns `401 Unauthorized` if timeout exceeded

## Configuration

**Required Environment Variables:**
```env
AUTH_SERVICE_URL=http://localhost:3000
```

**Optional:**
```env
AUTH_SERVICE_TIMEOUT=5000  # milliseconds
```

## Testing

**Test auth service endpoint directly:**
```bash
# Valid token
curl -X POST http://localhost:3000/auth/validate-token \
  -H "Content-Type: application/json" \
  -d '{"token":"your_valid_token_here"}'

# Response:
# {"valid":true,"payload":{"sub":"user-123","role":"user",...}}
```

**Test through BuySellService:**
```bash
curl -X GET http://localhost:3200/users/user-123 \
  -H "Authorization: Bearer your_valid_token_here"
```

The BuySellService will call the auth service internally to validate the token.