How Load Balancing Works

When a server receives hundreds of requests per second, it's critical that the system continues to run smoothly. That’s where a load balancer comes in — a tool that distributes incoming requests across multiple servers to keep everything fast and stable. In this article, we’ll look at what a load balancer does, how it works, the methods and algorithms used in real-world infrastructure, and why cloud-based solutions are increasingly replacing hardware-based ones.

What is Load Balancing?

The easiest way to imagine load balancing is like a traffic controller at a busy intersection. Their job is to keep traffic flowing and prevent collisions. In IT systems, the same principle applies: incoming traffic (user requests) is distributed across different servers. One may handle the website, another the database, a third the application logic.

This approach helps to:
— Reduce the load on each individual server;
— Speed up response times;
— Prevent failures and downtime;
— Scale the system — just add a new server, and it joins the work immediately.

Why Use a Load Balancer?

A load balancer isn’t just a distributor — it’s the “brain” that decides exactly where each request should go. It can:

— Automatically redirect traffic if a server goes down;
— Evenly spread out requests to avoid overloading a single node;
— Filter suspicious traffic and assist in DDoS protection;
— Scale the system as the user base grows;
— Recover from failures if a data center goes offline.

Important: A load balancer can itself become a bottleneck, so it’s often duplicated and configured with failover mechanisms.

How It Works on Different Network Layers

Load balancing operates on different layers of the OSI model. The most relevant ones are:
— L4 (Transport Layer): analyzes IP addresses, ports, and protocol types;
— L7 (Application Layer): takes into account HTTP content, headers, cookies, etc.

For example, an L7 balancer can route mobile traffic to one server and desktop traffic to another to optimize page loading.

Common Load Balancing Methods

Load balancers use various methods depending on the goals:
— By active connections: send the request to the least busy server;
— By performance: prioritize the fastest-responding server;
— Geographically: choose the nearest data center;
— By response time: select the quickest responder;
— Static routing: useful in stable infrastructure setups;
— Hash-based routing: “sticks” users to specific servers;
— Anycast (via BGP): one IP address, traffic is routed to the closest server.

These methods can be combined — the main goal is to keep the system responsive under load.

Load Balancing Algorithms

An algorithm determines how the balancer makes decisions. The most common ones are:
— Round Robin: Requests are distributed sequentially among servers. Simple and efficient when all servers have equal capacity, but doesn’t consider current load.
— Least Connections: Sends traffic to the server with the fewest active sessions. Ideal for web apps with sessions of varying length.
— Sticky Sessions: Keeps a user connected to the same server to avoid reauthentication. Useful for applications with login sessions.
— BGP Anycast: One IP address, multiple servers across the globe. Users get a response from the nearest one.

Load Balancer vs Proxy — What's the Difference?

A load balancer can also act as a proxy — receiving, forwarding, and returning requests. This is especially true for L7 load balancers, which often provide extra features like content caching, traffic filtering, and attack protection.

Even with a single backend server, a proxy can boost performance and enhance security.

Why Cloud Load Balancers Are Winning

In the past, load balancing was done using dedicated hardware. Now, cloud-based balancers are more common because they:

— Scale more easily;
— Deploy in minutes;
— Aren’t tied to physical infrastructure, making them more resilient;
— Are often more cost-effective with pay-as-you-go models.

They’re also easy to manage using tools like Terraform — the whole infrastructure can be configured as code.

Conclusion

Load balancing is not a luxury — it’s a necessity for any system that wants to grow and stay available 24/7. It helps get the most out of your servers, ensures fast response times, and prevents breakdowns.
The right approach depends on your system’s architecture, goals, and fault-tolerance requirements. Just remember: even the most reliable load balancer needs good failover and proper configuration.