Reverse proxy concept in HLD - System Design Exercise

Design: Reverse Proxy System

Design focuses on the reverse proxy component between clients and backend servers. Does not cover backend server internals or client applications.

Functional Requirements

FR1: Accept client requests and forward them to appropriate backend servers

FR2: Hide backend server details from clients

FR3: Distribute incoming traffic to multiple backend servers for load balancing

FR4: Cache responses to improve performance

FR5: Provide security features like filtering and SSL termination

FR6: Handle failures of backend servers gracefully

Non-Functional Requirements

NFR1: Must handle 10,000 concurrent client connections

NFR2: API response latency p99 should be under 150ms

NFR3: Availability target of 99.9% uptime

NFR4: Support HTTPS connections from clients

NFR5: Support backend servers running HTTP

Think Before You Design

Questions to Ask

❓ Question 1

❓ Question 2

❓ Question 3

❓ Question 4

❓ Question 5

❓ Question 6

Key Components

Listener to accept client connections

Load balancer to distribute requests

Cache layer for storing responses

Health checker for backend servers

SSL termination module

Logging and monitoring system

Design Patterns

Load balancing patterns

Caching strategies (write-through, write-back)

Circuit breaker for backend failures

SSL termination and offloading

Reverse proxy as a gateway pattern

Reference Architecture

Client
  |
  v
Reverse Proxy
  |-- Listener (HTTPS)
  |-- SSL Termination
  |-- Load Balancer
  |-- Cache
  |-- Health Checker
  |-- Security Filters
  |
  v
Backend Servers (HTTP)

Components

Listener

Nginx or HAProxy

Accepts incoming client connections over HTTPS

SSL Termination

OpenSSL library integrated in proxy

Decrypts HTTPS requests to HTTP for backend communication

Load Balancer

Round robin or least connections algorithm

Distributes requests evenly across backend servers

Cache

In-memory cache like Redis or built-in proxy cache

Stores frequently requested responses to reduce backend load

Health Checker

Periodic HTTP checks

Monitors backend server availability and removes unhealthy servers

Security Filters

IP filtering, rate limiting modules

Protects backend servers from malicious or excessive requests

Request Flow

1. Client sends HTTPS request to reverse proxy listener

2. SSL Termination decrypts request to HTTP

3. Load Balancer selects a healthy backend server

4. If cached response exists, return it directly to client

5. Otherwise, forward request to backend server

6. Backend server processes and sends response to proxy

7. Cache stores response if eligible

8. Reverse proxy encrypts response and sends back to client

Database Schema

Not applicable as reverse proxy typically does not store persistent data. Cache is in-memory or external store like Redis.

Scaling Discussion

Bottlenecks

Single reverse proxy instance limits concurrent connections

Cache size and eviction policy may limit hit rate

Load balancer algorithm may cause uneven load

SSL termination CPU overhead under high traffic

Health checker frequency impacts detection speed

Solutions

Deploy multiple reverse proxy instances behind a DNS load balancer

Use distributed cache with sharding and eviction policies

Implement adaptive load balancing based on server load

Use hardware SSL accelerators or offload SSL to dedicated devices

Tune health check intervals and use asynchronous checks

Interview Tips

Time: Spend 10 minutes understanding requirements and clarifying scope, 20 minutes designing components and data flow, 10 minutes discussing scaling and trade-offs, 5 minutes summarizing.

Explain how reverse proxy improves security and scalability

Discuss SSL termination benefits and challenges

Describe load balancing strategies and cache usage

Highlight failure handling with health checks

Mention scaling techniques and bottleneck mitigation