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08_Proxies_and_LB.md

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Proxy Servers, Load Balancers, and Algorithms - A Deep Dive

1. Proxy Servers Overview

A proxy server is an intermediary between a client (user) and a destination server. It can be used for security, performance enhancement, and anonymity. There are two main types of proxy servers:

  1. Forward Proxy (Client-Side Proxy)
  2. Reverse Proxy (Server-Side Proxy)

2. Forward Proxy (Client-Side Proxy)

How It Works

  • A forward proxy sits between the client and the internet.
  • When a client makes a request, the forward proxy forwards the request to the target server on behalf of the client.
  • The server sends the response back to the proxy, which then delivers it to the client.

Functionality and Uses of Forward Proxy

✅ 1. Anonymity and VPNs (Virtual Private Networks)

  • Hides user IP addresses from websites.
  • Users can bypass geo-restrictions (e.g., access blocked content).
  • Used by VPN services to provide anonymous browsing.

✅ 2. Content Filtering and Blocking

  • Organizations use forward proxies to block access to restricted sites.
  • Used in schools, offices, and government networks for censorship.

✅ 3. Caching and Performance Optimization

  • Forward proxies cache frequently requested content to reduce server load.
  • Example: A school proxy caches YouTube videos so students don’t repeatedly download them.

3. Reverse Proxy (Server-Side Proxy)

How It Works

  • A reverse proxy sits in front of backend servers and receives requests from clients.
  • Instead of clients connecting to origin servers, they connect to the reverse proxy, which forwards requests.

Functionality and Uses of Reverse Proxy

✅ 1. Load Balancing

  • Distributes traffic across multiple backend servers.
  • Improves performance and prevents server overload.

✅ 2. Caching (CDN Functionality)

  • Caches static content (images, CSS, JavaScript) to reduce backend load.
  • Used in CDNs (Content Delivery Networks) to serve content efficiently.

✅ 3. Security and DDoS Protection

  • Hides backend server IPs, preventing direct attacks.
  • Blocks malicious traffic before it reaches the origin.

✅ 4. SSL Termination

  • Offloads SSL/TLS encryption and decryption from backend servers.
  • Used in HTTPS-based services for performance.

Example of a Reverse Proxy Using Nginx

Install Nginx:

sudo apt install nginx -y

Configure as a Reverse Proxy:

sudo nano /etc/nginx/sites-available/reverse_proxy

Add the following:

server {
    listen 80;
    server_name mysite.com;
    
    location / {
        proxy_pass http://backend_server_ip;
        proxy_set_header Host $host;
        proxy_set_header X-Real-IP $remote_addr;
    }
}

Enable the Configuration and Restart Nginx:

sudo ln -s /etc/nginx/sites-available/reverse_proxy /etc/nginx/sites-enabled/
sudo systemctl restart nginx

Now, requests to mysite.com are forwarded to the backend.

4. Load Balancers

What Is a Load Balancer?

A load balancer distributes incoming traffic across multiple servers to:

  • Ensure high availability (prevent a single server from being overloaded).
  • Improve performance (use multiple servers efficiently).
  • Provide fault tolerance (if one server fails, others take over).

Types of Load Balancers

  • Software-based: Nginx, HAProxy
  • Hardware-based: F5 Networks, Citrix ADC
  • Cloud-based: AWS ELB, GCP Load Balancing

5. Load Balancing Algorithms

A. Round Robin (RR)

How It Works:

  • Distributes requests evenly across all servers in a circular manner.

Example:

Request 1 → Server A  
Request 2 → Server B  
Request 3 → Server C  
Request 4 → Server A

Pros: Simple and effective for equally powerful servers.
Cons: Does not consider server load or performance.

B. Weighted Round Robin (WRR)

How It Works:

  • Assigns weights to servers based on capacity.
  • More powerful servers get more requests.

Example:

Server A (weight = 3)  
Server B (weight = 1)  
Server C (weight = 1)  

Requests: A → A → A → B → C → A → A → A → B → C → …

Pros: Accounts for server capacity differences.
Cons: Still does not account for real-time load.

C. Least Connections (LC)

How It Works:

  • Sends new requests to the server with the fewest active connections.

Example:

Server A (5 active connections)  
Server B (2 active connections)  
Request goes to Server B.

Pros: Ideal for handling varying workloads.
Cons: Requires tracking real-time server loads.

D. IP Hashing

How It Works:

  • Assigns a client to a server based on its IP address.
  • Ensures that repeat requests from the same client go to the same server.

Example:

Client 192.168.1.10 → Server A  
Client 192.168.1.11 → Server B

Pros: Useful for session persistence (e.g., authentication).
Cons: If a server fails, those users lose their session.

6. Load Balancing in AWS, GCP, and Nginx

A. AWS Elastic Load Balancer (ELB)

  • Application Load Balancer (ALB): Best for HTTP/HTTPS traffic.
  • Network Load Balancer (NLB): Best for low-latency TCP/UDP traffic.
  • Classic Load Balancer (CLB): Legacy option.

B. GCP Load Balancer

  • Supports global load balancing (single IP worldwide).
  • Autoscaling to handle traffic spikes.

C. Nginx Load Balancer Example

Edit nginx.conf:

upstream backend {
    server server1.com;
    server server2.com;
    server server3.com;
}

server {
    listen 80;
    location / {
        proxy_pass http://backend;
    }
}

Restart Nginx:

sudo systemctl restart nginx