Load Balancer

1. Simple Meaning

A Load Balancer is like a traffic police 🚦 for your servers.
It stands in front of multiple backend servers and splits incoming requests among them so that no single server gets overloaded.

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2. Real-Life Example

Imagine you have a restaurant chain with 3 kitchens 🍽️:

• Kitchen 1

• Kitchen 2

• Kitchen 3

If all customers go to Kitchen 1, it will be crowded and slow.
Instead, a receptionist (load balancer) sends:

• First customer → Kitchen 1

• Second customer → Kitchen 2

• Third customer → Kitchen 3

• Fourth customer → Kitchen 1 again (and so on…)

This way:

• All kitchens work equally

• Customers get food faster

• No single kitchen is overloaded

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3. Why Load Balancing is Needed

Without load balancing:

• One server can crash from too much traffic.

• Other servers remain idle.

• Users get slow responses or timeouts.

With load balancing:

• Traffic is distributed evenly.

• If one server fails, others take over.

• Faster response time.

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4. Nginx as a Load Balancer

Nginx can sit in front of multiple servers and send requests to them based on different strategies.

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4.1 Basic Load Balancing Example

You have 3 Node.js servers:

• server1.example.com

• server2.example.com

• server3.example.com

Nginx config:

# Define backend servers
upstream backend_servers {
    server server1.example.com;
    server server2.example.com;
    server server3.example.com;
}

# Use them in a reverse proxy
server {
    listen 80;
    server_name example.com;

    location / {
        proxy_pass http://backend_servers;
    }
}

Here, Nginx sends requests in Round Robin style:
1st request → server1
2nd request → server2
3rd request → server3
4th request → server1 again…

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5. Load Balancing Methods in Nginx

Nginx supports multiple strategies:

1. Round Robin (Default) → Sends requests one by one to each server.

2. Least Connections → Sends request to the server with the fewest active connections.

   upstream backend_servers {
       least_conn;
       server server1.example.com;
       server server2.example.com;
   }
   

3. IP Hash → Same client always goes to the same server.

   upstream backend_servers {
       ip_hash;
       server server1.example.com;
       server server2.example.com;
   }
   

4. Weighted Round Robin → Give some servers more traffic if they are more powerful.

   upstream backend_servers {
       server server1.example.com weight=3;
       server server2.example.com weight=1;
   }
   

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6. Extra Benefits of Load Balancing

• High Availability → If one server is down, traffic is sent to others.

• Scalability → Add more servers as traffic grows.

• Failover → Automatic backup servers.

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7. Example: API with Load Balancer

You have:

• API Server 1 → localhost:3001

• API Server 2 → localhost:3002

Nginx Config:

upstream api_servers {
    server localhost:3001;
    server localhost:3002;
}

server {
    listen 80;
    server_name api.example.com;

    location / {
        proxy_pass http://api_servers;
    }
}

✅ Traffic gets distributed between the two API servers.

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8. Simple Flow

Client → Nginx Load Balancer → Multiple Backend Servers → Response