Reverse proxy concept in HLD - Scalability & System Analysis

Scalability Analysis - Reverse proxy concept

Growth Table: Reverse Proxy Scaling

Users	Requests per Second	Reverse Proxy Load	Network Traffic	Backend Impact
100	~10-50	Single reverse proxy server handles all requests easily	Low bandwidth, minimal latency	Backend servers lightly loaded
10,000	~1,000-5,000	Single reverse proxy may reach CPU/network limits	Moderate bandwidth, some latency possible	Backend servers under moderate load
1,000,000	~100,000-500,000	Single reverse proxy insufficient; need load balancing and multiple proxies	High bandwidth, potential network bottlenecks	Backend servers require scaling and caching
100,000,000	~10,000,000+	Distributed reverse proxy clusters with global load balancing	Very high bandwidth; CDN integration essential	Backend heavily sharded and cached

First Bottleneck

The reverse proxy server's CPU and network bandwidth become the first bottleneck as traffic grows beyond a few thousand requests per second. A single proxy can only handle limited concurrent connections and throughput before latency increases and requests queue up.

Scaling Solutions

• Horizontal scaling: Add multiple reverse proxy servers behind a load balancer to distribute incoming traffic.
• Load balancing: Use DNS round-robin or dedicated load balancers to spread requests evenly.
• Caching: Implement caching at the reverse proxy to reduce backend load and speed up responses.
• CDN integration: Offload static content delivery to Content Delivery Networks to reduce proxy and backend load.
• Connection pooling and keep-alive: Optimize connections between proxy and backend to reduce overhead.

Back-of-Envelope Cost Analysis

At 10,000 users generating ~5,000 requests/sec, a single reverse proxy server with 1 Gbps network and 8 CPU cores can handle the load.

At 1 million users (~500,000 requests/sec), you need ~100 reverse proxy servers (assuming 5,000 req/sec per server) and ~125 Gbps network bandwidth.

Storage is minimal at the proxy layer but caching requires memory (e.g., 64GB RAM per proxy for effective caching).

Interview Tip

Start by explaining the role of a reverse proxy in handling client requests and protecting backend servers. Then discuss how it scales with traffic growth, identifying bottlenecks like CPU and bandwidth. Finally, propose clear scaling solutions such as horizontal scaling, caching, and CDN integration, justifying each step with realistic capacity numbers.

Self Check

Your reverse proxy handles 1,000 requests per second. Traffic grows 10x to 10,000 requests per second. What do you do first?

Answer: Add more reverse proxy servers behind a load balancer to distribute the increased traffic and prevent CPU/network bottlenecks on a single proxy.

Key Result

A single reverse proxy server handles low to moderate traffic, but CPU and network bandwidth become bottlenecks as traffic grows. Horizontal scaling with multiple proxies and caching are key to scaling effectively.