What Is Load Balancing and How It Works

What is load balancing

Load balancing is a technique for distributing network traffic. This is particularly relevant for users of dedicated servers. It's the distribution of traffic between multiple servers to ensure high availability, reliability, and optimal application performance. Picture a load balancer as a traffic cop standing in front of a group of servers and directing each incoming request to the server that's currently least loaded. Without a load balancer, one server has to handle all the traffic, and when it hits its capacity, the site becomes slow or completely unavailable. With a load balancer, the load is distributed evenly and if one server fails, the others take over its work without interruption for users.

Load balancing is the foundation of modern web infrastructure. Every major web platform - Google, Facebook, Amazon, Netflix - uses sophisticated load balancing systems. But load balancing isn't reserved only for giants - any site with growing traffic or a high availability requirement can benefit from this technology. With the rise of cloud hosting, load balancing has become available to small and medium businesses at reasonable prices.

Types of load balancers

Layer 4 load balancing

A Layer 4 (L4) load balancer operates at the transport layer of the OSI model and makes routing decisions based on IP addresses and ports, without insight into traffic content. When a request arrives, the L4 balancer redirects the entire TCP or UDP connection to a selected backend server. Pros of L4 balancers are exceptional speed because they don't analyze packet content, lower resource requirements, and the ability to balance any type of traffic - not just HTTP. It's used in scenarios where pure traffic distribution is enough without the need for intelligent routing based on URL or headers.

Layer 7 load balancing

A Layer 7 (L7) load balancer operates at the application layer and understands the HTTP protocol. This lets it intelligently route based on URL, HTTP headers, cookies, content type, and other request attributes. For example, the L7 balancer can route image requests to servers optimized for static content, API requests to application servers, and websocket connections to servers configured for long-lived connections. L7 balancers can also perform SSL termination (decrypt HTTPS at the balancer), content compression, and caching. Most modern web applications use L7 load balancing.

Global server load balancing

GSLB (Global Server Load Balancing) distributes traffic between servers in different geographic locations. It uses DNS to route users to the nearest data center based on geographic location, latency, or server health. For example, users from Europe are routed to the European data center, while users from North America go to the American one. GSLB is key for global applications that require low latency worldwide and provides disaster recovery - if an entire data center becomes unavailable, traffic is automatically redirected to another.

Traffic distribution algorithms

Round Robin

Round Robin is the simplest algorithm, distributing requests sequentially in order across available servers. The first request goes to server 1, the second to server 2, the third to server 3, then back to server 1, and so on. The advantage is absolute simplicity of implementation and predictability of distribution. The downside is that it doesn't take into account current server load - a server processing a heavy operation gets the same number of new requests as a server that's idle. The Weighted Round Robin variant assigns different weights to servers proportional to their capacities, so more powerful servers get more requests.

Least Connections

The Least Connections algorithm routes every new request to the server that currently has the fewest active connections. This is more intelligent than Round Robin because it takes actual load into account. If one server processes long-running requests while another quickly finishes short requests, Least Connections will route more requests to the faster server. Weighted Least Connections combines this approach with weight coefficients for servers of different capacity. This algorithm is ideal for applications with requests that vary in duration.

IP Hash

The IP Hash algorithm calculates a hash value from the client's IP address and uses it to determine the server. This guarantees that requests from the same client always go to the same server, which matters for applications that store the session on the server (session persistence). Without IP Hash or a similar mechanism, a user can be moved to another server that doesn't have their session, which results in losing the shopping cart or being asked to log in again. The modern alternative is to use centralized session storage with Redis, eliminating the need for session persistence at the load balancer level.

Least Response Time

This algorithm combines fewest connections with shortest response time. The request is routed to the server with the fewest active connections and fastest average response time. This is the most intelligent approach because it considers both load and performance of each server. If one server has faster disks or more RAM, it will naturally have shorter response times and receive proportionally more requests. It requires monitoring of responses from each server which adds a small overhead but yields optimal results.

Hardware vs software load balancers

Hardware load balancers

Hardware load balancers are specialized appliances designed exclusively for traffic distribution. Vendors like F5 Networks (BIG-IP), Citrix (NetScaler), and A10 Networks offer appliances that can handle millions of connections per second with extremely low latency. The advantages are predictable performance, dedicated hardware, and advanced features like SSL offloading, DDoS protection, and an application firewall. The downside is high cost - hardware load balancers cost from a few thousand to hundreds of thousands of dollars, plus maintenance and licensing costs.

Software load balancers

Software load balancers install on standard servers and offer flexibility, lower costs, and ease of scaling. The most popular open-source solutions are Nginx, which is both a web server and a load balancer with excellent performance, HAProxy, which is specialized for load balancing with exceptional reliability and low latency, and Traefik, a modern load balancer designed for containerized environments like Docker and Kubernetes. Cloud providers offer managed load balancers as a service, eliminating the need to manage infrastructure.

Cloud load balancers

AWS Elastic Load Balancer, Google Cloud Load Balancing, and Azure Load Balancer are managed services that offer load balancing without managing hardware or software. They automatically scale with your traffic, integrate with other cloud services, and you pay for what you use. For most modern applications, cloud load balancers are an optimal choice because they eliminate operational complexity, offer high availability with built-in redundancy, and require minimal configuration to get started.

When you need a load balancer

Use scenarios

You need a load balancer when one server can't handle your traffic and the site becomes slow during peaks, when you need high availability and can't afford downtime, when you have a critical application like an e-commerce store where every minute of downtime means lost revenue, when you want the ability to update servers without service interruption using rolling deployment, or when your traffic varies significantly and you need dynamic scaling.

When a load balancer isn't needed

For a small site with moderate traffic, a load balancer brings unnecessary complexity and costs. If your site receives fewer than 10,000 visitors per day, one well-optimized server with caching is completely enough. Before adding a load balancer, optimize the existing server - implement page caching, optimize the database, use a CDN for static files, and compress images. These optimizations often eliminate the need for a load balancer and are much cheaper.

Health checks and failover

• Active health checks: The load balancer periodically sends requests to backend servers to check whether they're alive and functional - usually an HTTP GET to a specific endpoint.
• Passive health checks: The load balancer tracks responses from servers during normal traffic and detects errors without special requests.
• Automatic failover: When a server fails the health check, it's automatically removed from rotation and traffic is redirected to remaining servers.
• Graceful recovery: When a server becomes healthy again, it's gradually returned to rotation with fewer requests to avoid a sudden load.
• Custom health checks: Beyond the basic HTTP check, configure checks specific to your application - database status, availability of external APIs, and free disk space.

Conclusion

Load balancing is a fundamental technique for building reliable and scalable web applications. From the simple Round Robin algorithm to sophisticated L7 balancers with intelligent routing, there's a solution for every budget and need. Software load balancers like Nginx and HAProxy have democratized this technology, making it accessible to everyone. At BeoHosting we offer cloud hosting with load balancing capability for sites that require high availability, and our team can help configure optimal infrastructure for your specific needs.