Let's design a web proxy. Consider the following requirements:

1. Basic Functionality: The proxy should be able to forward HTTP and HTTPS requests from clients to web servers and return the responses.
2. Caching: Implement caching to reduce latency and bandwidth usage for frequently accessed resources. How would you handle cache invalidation?
3. Security: The proxy should support HTTPS and protect client data. How would you handle SSL/TLS certificates?
4. Scalability: The proxy should be able to handle a large number of concurrent connections. What architectural choices would you make to ensure scalability?
5. Authentication: Implement basic authentication (username/password) to control access to certain websites.
6. Logging: Log requests and responses for debugging and monitoring purposes. What information should be logged, and how would you manage log files?
7. Blocking: Implement the functionality to block access to certain websites based on a blacklist.

Discuss the architecture, components, and algorithms involved in building such a web proxy. Include considerations for performance, security, and maintainability. Provide example scenarios and solutions to potential challenges.

Web Proxy Design

Let's design a web proxy that handles HTTP and HTTPS requests, implements caching, provides security features, scales to handle many concurrent connections, supports authentication, logging, and blocking of websites.

1. Requirements

• Basic Functionality: Forward HTTP and HTTPS requests.
• Caching: Reduce latency and bandwidth via caching. Implement cache invalidation.
• Security: Support HTTPS and protect client data using SSL/TLS.
• Scalability: Handle a large number of concurrent connections.
• Authentication: Control access to websites using basic authentication.
• Logging: Log requests and responses for debugging and monitoring.
• Blocking: Block access to websites based on a blacklist.

2. High-Level Design

The web proxy will consist of the following components:

• Clients: Send HTTP/HTTPS requests to the proxy.
• Proxy Server: Receives requests, handles caching, authentication, blocking, logging, and forwards requests to origin servers.
• Cache: Stores responses to reduce latency and bandwidth usage.
• Origin Servers: Web servers that host the requested resources.
• Authentication Service: Validates user credentials.
• Logging Service: Stores logs for debugging and monitoring.
• Blacklist: List of blocked websites.

+------------+
|   Client   |  
+------------+
      |
      |
+----------------+
|   Proxy Server  |
+----------------+
      |        |        |        |
      |        |        |        +------------+
      |        |        |        | Blacklist  |
      |        |        |        +------------+
      |        |        |
      |        |        +----------------------+
      |        |        | Authentication Service |
      |        |        +----------------------+
      |        |
      |        +-------+
      |        | Cache |
      |        +-------+
      |
      |
+----------------+
|  Origin Server |
+----------------+
      |
      |
+----------------+
| Logging Service|
+----------------+


## 3. Data Model

### Cache Entry

| Field          | Type     | Description                                        |
| -------------- | -------- | -------------------------------------------------- |
| URL            | VARCHAR  | URL of the requested resource                      |
| Response Headers | TEXT     | HTTP response headers                              |
| Response Body  | BLOB     | HTTP response body                                 |
| Timestamp      | DATETIME | Timestamp when the resource was cached             |
| Expiry         | DATETIME | Timestamp when the cache entry expires             |

### Log Entry

| Field         | Type      | Description                                      |
| ------------- | --------- | ------------------------------------------------ |
| Timestamp     | DATETIME  | Timestamp of the request/response                |
| Client IP     | VARCHAR   | IP address of the client                       |
| URL           | VARCHAR   | Requested URL                                    |
| HTTP Method   | VARCHAR   | HTTP method (GET, POST, etc.)                    |
| Response Code | INTEGER   | HTTP response code                               |
| Request Headers| TEXT      | HTTP request headers                             |
| Response Headers| TEXT      | HTTP response headers                            |
| User ID        | VARCHAR   | User ID (if authentication is enabled)         |

### Blacklist Entry

| Field      | Type    | Description                    |
| ---------- | ------- | ------------------------------ |
| URL Pattern| VARCHAR | URL pattern to block           |

## 4. Endpoints

### Proxy Endpoints

*   **`/` (Default):** Accepts HTTP/HTTPS requests.
    *   **Request:**
        *   Method: GET, POST, PUT, DELETE, etc.
        *   Headers: Standard HTTP headers.
        *   Body: Request body (if applicable).
    *   **Response:**
        *   Headers: HTTP response headers.
        *   Body: Response body.

### Authentication Service Endpoints

*   **`/authenticate`:** Authenticates user credentials.
    *   **Request:**
        ```json
        {
          "username": "string",
          "password": "string"
        }
        ```
    *   **Response:**
        ```json
        {
          "success": true/false,
          "user_id": "string" (if successful)
        }
        ```

### Admin Endpoints (for managing the proxy itself)

*   **`/admin/cache/clear`:** Clears the cache (requires admin authentication).
*   **`/admin/blacklist/add`:** Adds a URL pattern to the blacklist (requires admin authentication).
    *   **Request:**
        ```json
        {
          "url_pattern": "string"
        }
        ```
*   **`/admin/blacklist/remove`:** Removes a URL pattern from the blacklist (requires admin authentication).
    *   **Request:**
        ```json
        {
          "url_pattern": "string"
        }
        ```

## 5. Tradeoffs

| Feature          | Approach                                     | Pros                                                                                              | Cons                                                                                                          |
| ---------------- | -------------------------------------------- | ------------------------------------------------------------------------------------------------- | ------------------------------------------------------------------------------------------------------------- |
| **Caching**      | In-memory cache (e.g., Redis, Memcached)   | Fast access, reduces load on origin servers                                                      | Requires more memory, potential cache invalidation issues                                                      |
| **HTTPS**        | Use SSL/TLS certificates                       | Secure communication, protects client data                                                         | Increased computational overhead for encryption/decryption                                                    |
| **Scalability**    | Load balancing, horizontal scaling            | Distributes load, handles more concurrent connections                                             | Increased complexity, requires careful coordination between servers                                          |
| **Authentication** | Basic authentication                         | Simple to implement                                                                               | Not very secure, transmits credentials in base64 encoding                                                      |
| **Logging**        | Centralized logging system (e.g., ELK stack) | Easy to analyze logs, identify issues                                                             | Requires setting up and maintaining a separate logging infrastructure                                        |
| **Blocking**       | URL pattern matching                          | Simple to implement, blocks access to unwanted websites                                            | Can be bypassed with URL obfuscation, requires regular updates to the blacklist                               |

## 6. Other Approaches

### Alternative Caching Strategies

*   **Disk-based Cache:** Store cached responses on disk. Slower than in-memory caching but can store more data. Can use a combination of in-memory (for frequently accessed items) and disk-based caching.
*   **Content Delivery Network (CDN):** Distribute cached content across multiple servers in different geographic locations. Reduces latency for users around the world.

### Alternative Authentication Methods

*   **OAuth 2.0:** More secure than basic authentication. Allows users to grant limited access to their accounts on other websites.
*   **JWT (JSON Web Tokens):** Stateless authentication. The server does not need to store session information.

### Alternative Blocking Techniques

*   **DNS-based Blocking:** Block access to websites by resolving their domain names to a non-routable IP address.
*   **Content Filtering:** Analyze the content of web pages and block access to pages that contain certain keywords or patterns.

## 7. Edge Cases

*   **Cache Stampede:** When a popular resource expires in the cache, multiple clients may request the resource at the same time, overloading the origin server. Solution: Use cache locks to prevent multiple clients from requesting the same resource at the same time.
*   **Denial-of-Service (DoS) Attacks:** Attackers may flood the proxy with requests, overwhelming the server and making it unavailable to legitimate users. Solution: Implement rate limiting to limit the number of requests that a client can make in a given period of time. Use a Web Application Firewall (WAF) to filter out malicious traffic.
*   **SSL/TLS Certificate Errors:** The proxy may encounter SSL/TLS certificate errors when connecting to origin servers. Solution: Properly configure the proxy to trust the root certificates of the certificate authorities. Allow users to bypass certificate errors (with a warning) in certain cases.
*   **Large Files:** Handling very large files can consume a lot of memory and disk space. Solution: Implement streaming to process the file in chunks. Use compression to reduce the size of the file.
*   **HTTP/2 and HTTP/3:** The proxy needs to support modern HTTP protocols like HTTP/2 and HTTP/3 to improve performance. Solution: Use a proxy server that supports these protocols.

## 8. Future Considerations

*   **Intrusion Detection System (IDS):** Implement an IDS to detect and prevent attacks on the proxy server.
*   **Web Application Firewall (WAF):** Use a WAF to protect the proxy server from web-based attacks.
*   **Machine Learning (ML) for Blacklist:** Use ML to automatically identify and block malicious websites.
*   **Support for WebSocket proxying:** Add support for proxying WebSocket connections.
*   **Integration with monitoring tools:** Integrate the proxy server with monitoring tools to track performance and identify issues.
*   **Dynamic Blacklist Updates:** Automatically update the blacklist based on threat intelligence feeds.
*   **Advanced Authentication:** Implement multi-factor authentication (MFA) to improve security.