Proxy and Reverse Proxy

Introduction

Modern distributed systems rarely allow clients to communicate directly with backend servers.

Instead, an intermediary layer is introduced between clients and servers.

This intermediary is called a Proxy.

Proxies serve many purposes:

security
caching
request filtering
anonymity
load balancing
traffic control

There are two primary types of proxies:

Forward Proxy
Reverse Proxy

Understanding the difference between them is fundamental for high level system design.

Real World Analogy

Imagine a corporate office building.

Visitors do not walk directly into internal offices.

Instead, they first interact with a reception desk.

The receptionist:

verifies identity
decides where the visitor should go
blocks unauthorized access
sometimes handles requests directly

A proxy acts similarly:

it sits between two parties
controls communication
forwards requests when appropriate

What is a Proxy?

A proxy server is an intermediary that forwards requests from clients to servers.


Client → Proxy → Server

The proxy hides the client's identity from the server.

Forward Proxy

Definition

A Forward Proxy sits between the client and the internet.

It represents the client side.

Architecture

Diagram

flowchart LR Client --> ForwardProxy ForwardProxy --> InternetServer InternetServer --> ForwardProxy ForwardProxy --> Client

Request Flow

Client sends request to proxy
Proxy forwards request to destination server
Server responds to proxy
Proxy sends response back to client

Key Property

The server sees the proxy as the client, not the real user.

Example Scenario

A company wants to control employee internet access.

Employees must route internet traffic through a corporate proxy.

Diagram

flowchart LR Employee --> CorporateProxy CorporateProxy --> Internet

The proxy can:

block websites
log activity
enforce security policies

Forward Proxy Features

Feature	Explanation
anonymity	hides client identity
access control	blocks certain sites
caching	store frequently accessed content
monitoring	track user activity

Example: Content Filtering

A school network blocks social media websites.

Diagram

flowchart TD Student --> Proxy Proxy -->|Allowed| Website Proxy -->|Blocked| Denied

If the student tries accessing restricted sites:

Access Denied

What is a Reverse Proxy?

A Reverse Proxy sits in front of servers.

It represents the server side.

Architecture

Diagram

flowchart LR Client --> ReverseProxy ReverseProxy --> BackendServer1 ReverseProxy --> BackendServer2 ReverseProxy --> BackendServer3

Clients believe they are communicating directly with the server.

In reality, they are communicating with the reverse proxy.

Key Property

The client does not know which backend server handles the request.

Reverse Proxy Request Flow

Diagram

sequenceDiagram participant Client participant ReverseProxy participant Server Client->>ReverseProxy: HTTP Request ReverseProxy->>Server: Forward Request Server-->>ReverseProxy: Response ReverseProxy-->>Client: Response

Why Reverse Proxies Are Used

Reverse proxies are extremely important in large-scale systems.

Use Case	Description
load balancing	distribute traffic across servers
caching	store responses to reduce backend load
SSL termination	handle HTTPS encryption
security	hide internal servers
compression	reduce bandwidth

Reverse Proxy vs Forward Proxy

Feature	Forward Proxy	Reverse Proxy
Represents	client	server
Client awareness	client knows proxy	client unaware
Server awareness	server unaware of client	server unaware of proxy
Use cases	anonymity, filtering	load balancing, caching

Example System Architecture

Large scale applications place reverse proxies in front of microservices.

Diagram

flowchart TD Users --> ReverseProxy ReverseProxy --> AppServer1 ReverseProxy --> AppServer2 ReverseProxy --> AppServer3

The proxy distributes traffic across servers.

Load Balancing with Reverse Proxy

Reverse proxies often implement load balancing algorithms.

Example algorithms:

Algorithm	Explanation
Round Robin	distribute requests sequentially
Least Connections	send traffic to least busy server
IP Hash	same user routed to same server

Example Load Balancing Flow

Diagram

flowchart LR Client --> ReverseProxy ReverseProxy --> ServerA ReverseProxy --> ServerB ReverseProxy --> ServerC

Each incoming request is routed to an available server.

Caching with Reverse Proxy

Reverse proxies can store frequently requested responses.

Example:

GET /images/logo.png

If cached:

Response served directly by proxy

No request reaches the backend server.

Caching Architecture

Diagram

flowchart TD Client --> ReverseProxy ReverseProxy -->|Cache Hit| Response ReverseProxy -->|Cache Miss| BackendServer BackendServer --> ReverseProxy

SSL Termination

Handling HTTPS encryption is computationally expensive.

Instead of every backend server handling SSL:

A reverse proxy performs SSL termination.

Architecture

Diagram

flowchart LR Client --> HTTPS HTTPS --> ReverseProxy ReverseProxy --> HTTP ReverseProxy --> BackendServers

Steps:

Client sends HTTPS request
Proxy decrypts traffic
Proxy forwards plain HTTP to backend

This reduces CPU load on application servers.

Security Benefits

Reverse proxies improve security in several ways.

Security Feature	Benefit
hide backend IPs	attackers cannot access servers directly
request filtering	block malicious requests
rate limiting	prevent abuse
DDoS mitigation	absorb traffic spikes

Web Application Firewall (WAF)

Many reverse proxies integrate Web Application Firewalls.

WAF detects:

SQL injection
cross-site scripting
malicious bots

Example architecture:

Diagram

flowchart TD Client --> WAF WAF --> ReverseProxy ReverseProxy --> ApplicationServers

Reverse Proxy in Microservices

Microservice architectures rely heavily on reverse proxies.

Example:

Diagram

flowchart TD Users --> ReverseProxy ReverseProxy --> AuthService ReverseProxy --> PaymentService ReverseProxy --> OrderService

The proxy routes traffic to the appropriate service.

Edge Proxy vs Internal Proxy

Large systems often deploy multiple proxy layers.

Type	Location
Edge Proxy	entry point from internet
Internal Proxy	between microservices

Multi-layer Architecture

Diagram

flowchart TD Internet --> EdgeProxy EdgeProxy --> API_Gateway API_Gateway --> Services

Each layer provides additional control.

Popular Proxy Technologies

Many production systems rely on high-performance proxy servers.

Examples include:

NGINX — high-performance reverse proxy and web server
HAProxy — advanced load balancing proxy
Envoy — modern proxy used in service meshes

These tools are widely used in cloud infrastructure.

Reverse Proxy + CDN

Large platforms combine reverse proxies with Content Delivery Networks.

Example architecture:

Diagram

flowchart LR Users --> CDN CDN --> ReverseProxy ReverseProxy --> ApplicationServers

CDN caches content near users, reducing latency.

Real World Examples

Many large internet platforms rely heavily on reverse proxies.

Examples include infrastructure used by organizations like:

Netflix streaming platform architecture
Google global web services infrastructure
Amazon large-scale cloud platforms

Reverse proxies allow these systems to handle millions of requests per second.

Key Advantages

Advantage	Explanation
scalability	distribute traffic
reliability	isolate failures
security	hide backend systems
performance	caching and compression

Summary

Proxy servers act as intermediaries that manage network traffic.

Two major types exist:

Forward Proxy

Represents clients and hides their identity.

Reverse Proxy

Represents servers and manages incoming traffic.

In high-level architecture design, reverse proxies are essential because they provide:

load balancing
security
caching
SSL termination
traffic control

They serve as the front door of modern distributed systems, ensuring that backend services remain scalable, secure, and resilient.