# Token Validation in Mesh SDK

## Overview

The Mesh SDK implements a robust token validation system that ensures authentication tokens are valid and usable while gracefully handling various edge cases. This document explains the token validation process in detail.

## Validation Process

The SDK uses a two-tier validation approach:

1. **Local Validation**
   - Checks token structure (verifies it's a properly formatted JWT with three parts)
   - Validates the token's expiration time
   - Doesn't require network requests to the server
   - Used as first-pass validation and fallback when server validation is unavailable

2. **Backend Validation**
   - Sends the token to the server's `/auth/validate` endpoint for validation
   - The server performs comprehensive validation, including signature verification
   - Can detect revoked tokens or other issues not visible to local validation
   - Used when available to provide enhanced security

## Implementation Details

The validation process follows this sequence:

1. Check if token exists and has basic format
2. Perform local validation to check expiration
3. Attempt backend validation if available
4. Fall back to local validation if backend validation fails or is unavailable

```python
# Simplified flow of the validation process:
def validate_token():
    # Step 1: Check token existence and format
    if not token or not has_valid_format(token):
        return False
        
    # Step 2: Local validation (expiration check)
    if not is_token_locally_valid(token):
        return False
        
    # Step 3: Try backend validation if possible
    try:
        if backend_validation_available():
            return validate_with_backend(token)
    except:
        # Step 4: Fall back to local validation
        return True  # We already passed local validation
        
    return True  # Local validation passed
```

## Edge Cases

The validation system handles several edge cases:

1. **Missing Backend Validation Endpoint**:
   - If the server does not have the `/auth/validate` endpoint (404 response)
   - The SDK gracefully falls back to local validation
   - This ensures compatibility with different server configurations

2. **Network Issues**:
   - Connection failures when trying to reach the validation endpoint
   - Timeout handling to prevent blocking operations
   - Retry logic to handle transient failures

3. **Token Revocation**:
   - When backend validation fails with 401/403 status codes
   - The SDK attempts token refresh if a refresh token is available
   - This handles cases where tokens were revoked server-side

## Configuration

The validation behavior can be configured through:

- `auto_refresh` parameter (default: True) - Controls whether to automatically refresh expired tokens
- Timeout settings for network requests to the validation endpoint

## Troubleshooting

Common validation issues and solutions:

1. **Local Validation Passes but Backend Fails**:
   - Token might be revoked on the server side
   - Auth0 configuration might have changed
   - Try clearing tokens and re-authenticating

2. **Repeated Authentication Prompts**:
   - Could indicate issues with token storage
   - Check keychain access and permissions
   - Verify network connectivity to validation endpoints

3. **Missing Validation Endpoint (404)**:
   - This is expected with some server configurations
   - The SDK handles this gracefully with local validation
   - No action needed as this is handled automatically
