What are proxies and reverse proxies?

Question

IT Interview Practice · Accepted Answer

A **proxy** acts as an intermediary for requests. A **forward proxy** sits in front of clients (representing them to servers), while a **reverse proxy** sits in front of servers (representing them to clients) — handling load balancing, caching, security, and more. Reverse proxies are key system-design components.

## Forward proxy vs reverse proxy

```text
FORWARD PROXY → sits in front of CLIENTS, forwarding their requests to servers:
  Client → FORWARD PROXY → Internet/Server
  → represents the CLIENT (hides client identity); for filtering, caching, anonymity,
    access control (e.g. corporate proxies)
REVERSE PROXY → sits in front of SERVERS, receiving requests on their behalf:
  Client → REVERSE PROXY → Backend servers
  → represents the SERVERS (clients don't see the backend); the common system-design component
```

## What a reverse proxy does

```text
✓ LOAD BALANCING → distribute requests across backend servers
✓ CACHING → cache responses (reduce backend load)
✓ SSL TERMINATION → handle HTTPS encryption/decryption (offload from backends)
✓ SECURITY → hide backend servers; a single point for filtering, WAF, DDoS protection
✓ COMPRESSION, request routing, rate limiting
→ tools: NGINX, HAProxy, Envoy; also CDNs and API gateways are forms of reverse proxies
```

## Why reverse proxies matter

```text
✓ Centralize cross-cutting concerns (load balancing, SSL, caching, security) at the edge
✓ Hide and protect the backend (clients only see the proxy)
✓ A single entry point → simpler client access, flexible backend changes
→ a fundamental building block of scalable, secure web architectures.
```

## Why it matters

Understanding proxies and reverse proxies is valuable because **reverse proxies are a fundamental building block of scalable, secure web architectures**, so it's important system-design knowledge.

Understanding the distinction — a **forward proxy** sitting in front of clients (representing them, for filtering and access control) versus a **reverse proxy** sitting in front of servers (representing them to clients) — clarifies these related but different concepts, with the reverse proxy being the common system-design component.

Understanding **what a reverse proxy does** — **load balancing** (distributing requests), **caching** (reducing backend load), **SSL termination** (handling HTTPS, offloading encryption from backends), **security** (hiding backend servers, providing a single point for filtering, WAF, and DDoS protection), and compression, routing, and rate limiting — clarifies the many cross-cutting concerns it handles at the edge.

Understanding **why reverse proxies matter** — centralizing cross-cutting concerns at the edge, hiding and protecting the backend (clients only seeing the proxy), and providing a single entry point (simpler access, flexible backend changes) — explains their fundamental role in web architectures.

Recognizing that load balancers, CDNs, and API gateways are essentially forms of reverse proxies connects these related concepts.

Reverse proxies (NGINX, HAProxy, Envoy) are ubiquitous in real systems, handling the load balancing, caching, SSL, and security that scalable web architectures need.

Since reverse proxies are a fundamental building block of scalable, secure web architectures (centralizing load balancing, caching, SSL, and security while hiding backends), and since understanding the forward/reverse distinction and what reverse proxies do is important for system design, understanding proxies and reverse proxies is valuable, practically-relevant system-design knowledge — a fundamental component of web architectures, connecting to load balancers, CDNs, and API gateways, and central to designing scalable, secure systems with proper edge handling of cross-cutting concerns.

Forward proxy vs reverse proxy

What a reverse proxy does

Why reverse proxies matter

Why it matters

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