How Load Balancers Distribute Traffic Across Servers

Load balancing is the practice of distributing computational workloads between two or more computers. On the Internet, load balancing is often employed to divide network traffic among several servers. This reduces the strain on each server and makes the servers more efficient, speeding up performance and reducing latency. Load balancing is essential for most Internet applications to function properly.

To understand this better, imagine a checkout line at a grocery store with 8 checkout lines, but only one is open. All customers must stand in that single line, leading to long wait times. Now, if all 8 checkout lines are open, customers can spread out, and the wait time is about 8 times shorter (depending on how much food each customer is buying).

Load balancing works in a similar way. By dividing user requests among multiple servers, user wait times are significantly reduced. This results in a smoother and faster user experience. Without load balancing, just like customers avoiding an inefficient grocery store, users may turn away from slow or unreliable applications.

In this blog, we’ll explore how load balancers work, the strategies they use to distribute traffic, and why they’re critical for modern applications.

HOW DOES LOAD BALANCING WORK?

Load balancing is handled by a tool or application called a load balancer. A load balancer can be either hardware-based or software-based. Hardware load balancers require the installation of a dedicated device, while software-based load balancers can run on a server, inside a virtual machine, or in the cloud. Content delivery networks (CDNs) often include load balancing features.

When a request arrives from a user, the load balancer assigns the request to a given server, and this process repeats for each new request. This ensures that no single server is overloaded while others sit idle.

Load balancers determine which server should handle each request based on a number of algorithms. These algorithms generally fall into two main categories:

• Static algorithms: Rules are fixed in advance (e.g., round robin, IP hash).
• Dynamic algorithms: Rules adapt in real time based on server state (e.g., least connections, response time).

WHY DO WE NEED LOAD BALANCING?

Modern applications serve millions of users simultaneously. Without load balancing, servers can quickly become bottlenecks, leading to poor user experiences and downtime.

The key benefits include:

• High Availability – If one server fails, traffic is redirected to healthy servers.
• Scalability – More servers can be added seamlessly to handle increased demand.
• Performance – Requests are spread out, reducing latency and improving response time.
• Redundancy – Protects against system failures and improves reliability.

Example: During a flash sale on an e-commerce website, thousands of customers might check out at the same time. Without load balancing, the checkout service could crash. With a load balancer, requests are distributed across multiple servers, keeping the service stable.

WHAT AN EFFECTIVE LOAD BALANCER PROVIDES

An effective load balancer goes beyond simply splitting traffic. It should be able to:

• Distribute web traffic across servers using static or dynamic algorithms, with sticky sessions for seamless user experiences.
• Maintain high availability of web applications and APIs through health checks and automatic failover when a server becomes unhealthy.
• Scale efficiently to handle traffic fluctuations while maintaining high performance with low resource usage.
• Secure traffic with SSL termination, filtering, and rate limiting.
• Accelerate application performance with caching, compression, and SSL offloading.
• Operate at Layer 4 and/or Layer 7, depending on whether routing is based on IP/ports (L4) or application-level data like HTTP headers (L7).
• Be customizable and configurable to match the unique needs of the application.

WHAT KIND OF TRAFFIC DOES A LOAD BALANCER HANDLE?

Load balancers typically handle two main categories of traffic: transport layer traffic (Layer 4) and application layer traffic (Layer 7). Within these layers, a variety of protocols come into play. Some of the most common ones include:

• TCP/IP – Transport layer protocol that defines how data is broken down, exchanged, and routed between devices.
• HTTP (1.1, 2, 3) – Application layer protocol for communication between browsers and servers. HTTP/3 uses the QUIC protocol for better performance.
• UDP – Connectionless transport protocol, lightweight but without built-in reliability.
• QUIC – A new transport protocol built on top of UDP, combining speed and reliability.
• Other protocols – Such as DNS, SIP, RTSP, RADIUS, and Diameter.

Each of these protocols comes with its own challenges, and load balancers are designed to efficiently distribute traffic across servers while keeping performance and reliability in check.

LOAD BALANCING ALGORITHMS

Load balancing algorithms are logical methods of distributing web traffic across a fleet of servers. Algorithms can be separated into two main categories: dynamic and static.

STATIC LOAD BALANCING ALGORITHMS

In scenarios where the system’s state is stable and homogeneous, and where traffic load remains relatively constant, static load-balancing algorithms are highly efficient. These algorithms establish load-balancing rules at compile time and operate independently of the system’s state at runtime.

Examples of Static Algorithms:

1. Round Robin – Each request is distributed in order across servers. Simple, but doesn’t account for server load.
2. IP Hash – Requests from the same client IP always go to the same server. Useful for sticky sessions.

DYNAMIC LOAD BALANCING ALGORITHMS

In scenarios where the system’s state is heterogeneous and traffic patterns vary frequently, dynamic load-balancing algorithms are highly effective. These algorithms establish load-balancing rules based on the system’s state both before and during runtime. They adjust the load distribution in real time by continuously examining the current state of the system.

Examples of Dynamic Algorithms:

1. Least Connections – Routes new requests to the server with the fewest active connections. Great for apps with long-running sessions.
2. Weighted Distribution – Stronger servers handle more requests; weaker ones handle fewer.
3. Response Time – Sends traffic to the server responding fastest.

THE BENEFITS OF LOAD BALANCER

Load balancing is essential in guaranteeing the high availability of your website or networked application. High availability ensures that your application is accessible when your customers need it. Beyond availability, load balancers offer a range of benefits:

• Increased performance – With intelligent routing, load balancing ensures each server operates optimally, reducing processing delays and improving response times.
• Improved user experience – By maintaining availability and responsiveness, users consistently experience reliable services.
• Enhanced reliability – In case of failures, load balancers leverage redundant components to recover quickly and continue delivering applications seamlessly.
• High scalability – Infrastructure can scale out dynamically to meet rising demand, ensuring effective performance under heavy loads.
• Increased capacity – Scaling out allows organizations to serve more users simultaneously.
• Fortified security – Many load balancers include integrated security features that strengthen defenses against malicious traffic.
• Improved cost efficiency – Solutions like HAP Roxy support linear scalability, ensuring resources are utilized effectively without diminishing returns when adding servers.

REAL-WORLD USE CASES AND BUSINESS IMPACT

Load balancing powers many of the apps and platforms we use daily:

• E-commerce Websites – Handle traffic spikes during major sales.
• Streaming Platforms – Distribute users across multiple servers for smooth video playback.
• APIs – Handle millions of requests per second without downtime.
• SaaS Applications – Deliver reliable services to customers across different regions.

But beyond the technical side, load balancers also have a direct impact on business success.

• Business growth – Scale easily as demand rises, ensuring high performance without overspending.
• Business continuity – Keep services always available, minimizing downtime that could impact revenue.
• Business transformation – Simplify digital transformation by supporting migrations and hybrid environments with minimal disruption.
• Business optimization – Improve cost efficiency and maximize return on infrastructure investments.
• Reputation protection – Deliver a consistent user experience, strengthening trust and brand reputation.

Whether you’re running a small startup or managing a global platform, investing in a load balancer ensures smooth customer experiences and long-term resilience.

LOAD BALANCING AND REVERSE PROXY

Many load balancers also act as reverse proxies, meaning they sit in front of servers and process requests on their behalf.

HAProxy is an example of a reverse proxy load balancer. A reverse proxy receives a request, then relays it to an upstream server. HAProxy is one of the world’s fastest and most widely used software load balancers. Organizations deploy HAProxy products to deliver websites and applications with high performance, observability, and security at any scale and in any environment.

Key points about HAProxy:

• Open Source & Customizable – Backed by a strong community that continually enhances the platform.
• Enterprise Support – HAProxy Enterprise combines the open-source software with central control, enterprise-class security, integrations, and authoritative support.
• Versatile Deployment – Works anywhere, in any form factor, regardless of architecture.
• Broad Protocol Support – Handles multiple protocols, supports customizations and scripting for varied use cases.
• Flexible Workflows – Configurable using multiple methods, with REST APIs and integrations with common tools.
• High Performance – Industry-leading performance metrics with low resource usage, reducing costs in cloud or on-premises environments.
• Security & Observability – Provides visibility into traffic and enables teams to predict, prevent, and resolve issues efficiently.

HAProxy is a powerful choice for organizations looking to implement robust, efficient, and secure load balancing, whether for small-scale applications or enterprise-grade deployments.

For example, if a client makes a request to a website named example.com, the load balancer would receive the request first. Then, it would choose a web server from a list and pass the request along by opening a connection to that server (or re-using an already established connection). Meanwhile, the load balancer keeps its connection open to the client.

When the web server handling the request returns its response to the load balancer, the load balancer relays it back to the client over the original connection.

Reverse proxy load balancers can operate in various forms—hardware, software, or virtual—and can function at different layers of the application layer (Layer 7) or the transport layer (Layer 4).

Additional benefits:

• SSL Termination – The load balancer handles HTTPS encryption, reducing server load.
• Caching – Frequently requested content can be served faster.
• Compression – Reduces response size, improving performance.
• Security – Shields backend servers from direct access.

CONCLUSION

Load balancing is the backbone of modern web applications. By distributing traffic intelligently across servers, it improves availability, performance, and reliability. Without it, most Internet applications would struggle to function under heavy demand.

Load balancing delivers highly available and responsive web applications and APIs to clients by distributing web traffic across a pool of servers to process connections efficiently. The versatility of load balancing and the breadth of solutions it provides can be summarized with the following key takeaways:

• Benefits of Load Balancing – Increased performance, improved user experience, high scalability, fortified security, improved cost efficiency, fault-tolerant infrastructure, and reputation protection.
• Deployment Methods – Load balancers can function as a reverse proxy, sitting between clients and servers to manage traffic, or through DNS round-robin, distributing users across multiple servers by returning different IP addresses.
• Hardware vs. Software – Hardware load balancers provide high performance and reliability but can be expensive and less flexible. Software load balancers are scalable and versatile, though not all match the performance of dedicated hardware.
• Layer 4 vs. Layer 7 Load Balancing – L4 load balancing operates at the transport layer and makes decisions based on network-level information, while L7 load balancing works at the application layer and uses message content and server telemetry.
• Security & Reliability – Load balancers can encrypt traffic, block attacks, and perform health checks to remove vulnerable servers from rotation.
• HAProxy – The fastest and most widely used software load balancer, offering simple adoption, scalable performance, secure infrastructure, and observable operation. HAProxy Enterprise builds on this with enterprise-level support, community testing, and additional features.

Whether you use cloud-native load balancers or self-managed solutions like HAProxy and Nginx, understanding how load balancing works is essential for building scalable, resilient, and high-performing systems.

In short, implementing a load balancer is essential for scalable, resilient, and high-performing applications. A robust load balancing strategy ensures your users have a reliable, fast, and secure experience every time.