Reverse Proxy 🔄 — The Gateway Between Clients and Servers

A Reverse Proxy sits between clients and backend servers. Clients send requests to the reverse proxy, and the reverse proxy forwards those requests to the appropriate backend service.

flowchart LR
    C["Client"] --> RP["Reverse Proxy"]
    RP --> S1["Backend Server 1"]
    RP --> S2["Backend Server 2"]
    RP --> S3["Backend Server 3"]

1. Why Do We Need a Reverse Proxy?

• Hide backend servers from the internet
• Distribute traffic across multiple servers
• Terminate HTTPS
• Route requests
• Handle authentication/security policies
• Cache responses
• Compress responses
• Provide a single public endpoint

flowchart LR
    U["Internet"] --> RP["Reverse Proxy"]

    RP --> API1["API Server 1"]
    RP --> API2["API Server 2"]
    RP --> API3["API Server 3"]

    API1 --> DB["Database"]
    API2 --> DB
    API3 --> DB

2. Reverse Proxy vs Forward Proxy

3. Reverse Proxy with Multiple Services

A reverse proxy can route requests based on hostname or URL path.

flowchart LR
    C["Client"] --> RP["Reverse Proxy"]

    RP -->|"/users"| U["User Service"]
    RP -->|"/orders"| O["Order Service"]
    RP -->|"/payments"| P["Payment Service"]

For example:

https://example.com/users
https://example.com/orders
https://example.com/payments

4. HTTPS Termination

The reverse proxy can handle TLS/HTTPS so backend services do not need to perform the public TLS termination themselves.

flowchart LR
    C["Client"] -->|"HTTPS"| RP["Reverse Proxy"]
    RP -->|"HTTP or HTTPS"| API["Backend API"]

This is called TLS termination.

5. Reverse Proxy as a Load Balancer

A reverse proxy can also distribute requests between multiple backend instances.

flowchart LR
    C["Client"] --> RP["Reverse Proxy"]

    RP --> A["API Instance 1"]
    RP --> B["API Instance 2"]
    RP --> D["API Instance 3"]

Therefore, a reverse proxy and load balancer can overlap in functionality, although they are not exactly the same concept.

6. Reverse Proxy vs Load Balancer

7. Reverse Proxy in Microservices

In a microservices architecture, the reverse proxy provides a controlled entry point to backend services.

flowchart LR
    C["Client"] --> RP["Reverse Proxy"]

    RP --> A["Product Service"]
    RP --> B["Order Service"]
    RP --> C2["Payment Service"]
    RP --> D["Notification Service"]

    A --> DB["Databases"]
    B --> DB
    C2 --> DB

8. Popular Reverse Proxy Technologies

• Nginx
• HAProxy
• Apache HTTP Server
• Envoy
• Cloud load balancers such as AWS Application Load Balancer

9. Interview Scenario

Question: Your application has 10 backend servers. Should clients directly call those servers?

Usually, clients should call a single public endpoint. A reverse proxy can receive the request and forward it to an appropriate backend instance.

flowchart LR
    C["Clients"] --> RP["Reverse Proxy"]

    RP --> S1["Server 1"]
    RP --> S2["Server 2"]
    RP --> S3["Server 3"]
    RP --> SN["Server 10"]

Important Interview Questions

• What is a reverse proxy?
• Why do we need a reverse proxy?
• Reverse proxy vs forward proxy?
• Reverse proxy vs load balancer?
• How does a reverse proxy hide backend servers?
• Can a reverse proxy perform load balancing?
• What is TLS termination?
• How can a reverse proxy route microservice requests?
• What are Nginx, HAProxy and Envoy used for?

Key Takeaways

• Reverse proxy sits between clients and backend servers.
• It hides backend infrastructure from clients.
• It can perform routing and TLS termination.
• It can distribute traffic across backend instances.
• It is commonly used in microservices architectures.
• Load balancers can also provide reverse-proxy functionality.

Conclusion

In system design interviews, remember the simple flow: Client → Reverse Proxy → Backend Services. The reverse proxy provides a single controlled entry point while handling routing, security, TLS termination and, when applicable, load balancing.