What Is a Load Balancer?

Why Use a Load Balancer?

• High Availability: If one server fails, traffic can be redirected to others.
• Better Performance: Requests are shared among multiple servers.
• Scalability: Easy to add/remove servers based on traffic demand.

How It Works

Imagine 3 servers handling web requests. Without a load balancer, all users hit Server-1 → slow and unstable.

With a load balancer:

The load balancer decides which server handles each request.

Example

Imagine 3 web servers:

Least Connections algorithm → next request goes to Server-2.

Health Checks

Load balancers often check server health automatically.

• If a server fails → traffic is redirected to healthy servers.

Load Balancing Algorithms

A load balancer can use different algorithms to decide where each request goes. Here are the most common ones:

1. Round Robin

How it works:

• Requests are sent to servers in order, one after another.
• After the last server, it starts again from the first.

Example:

3 servers: S1, S2, S3 Incoming requests: R1, R2, R3, R4, R5

✅ Simple and fair for servers with similar capacity.

2. Least Connections

How it works:

• The server with the fewest active connections gets the next request.
• Good for servers where request processing time varies.

Example:

✅ Balances load dynamically based on current server usage.

3. IP Hash

How it works:

• Uses the client’s IP address to decide which server handles the request.
• Same client always goes to the same server (sticky session).

Example:

Client IP → Hash → Server

✅ Useful for session persistence, where users need to stay connected to the same server.

Summary

Key Benefits of Load Balancers:

• Distributes traffic to multiple servers.
• Prevents single-server overload.
• Improves performance, availability, and reliability.
• Performs health checks to avoid sending requests to failed servers.