Load Balancers in System Design

When building scalable and reliable systems, one of the most critical components is the load balancer. Its job is simple yet powerful: distribute incoming network traffic across multiple servers so that no single server becomes overwhelmed. This ensures smooth performance, high availability, and a better user experience.

🔑 Key Functions of a Load Balancer

• Traffic Distribution: Spreads requests evenly across servers.

• Redundancy & Reliability: Keeps services available even if one server fails.

• Scalability: Makes it easy to add or remove servers as demand changes.

• Health Monitoring: Continuously checks server status to avoid routing traffic to unhealthy nodes.

• Session Persistence: Ensures a user’s session stays on the same server when needed.

• SSL Termination: Offloads encryption/decryption tasks from backend servers.

🛠️ Types of Load Balancers

• Hardware Load Balancers 
  Physical devices dedicated to balancing traffic. Often used in enterprise setups.

•  Software Load Balancers  
  Applications running on standard hardware. Flexible and cost‑effective.

• Cloud‑Based Load Balancers
  Services offered by cloud providers (AWS ELB, Azure Load Balancer, GCP Load Balancing) that scale automatically and integrate seamlessly with cloud infrastructure.

⚙️ Common Load Balancing Algorithms

• Round Robin: Sequentially distributes requests across servers.

• Least Connections: Routes traffic to the server with the fewest active connections.

• IP Hash: Uses the client’s IP address to determine which server handles the request.

• Weighted Round Robin: Assigns more requests to servers with higher capacity.

🌐 How Requests Flow Through a Load Balancer

• Client Request: A user sends a request to the application’s public IP or domain.

• DNS Resolution: The domain name system translates the domain into the load balancer’s IP.

• Traffic Reception: The load balancer receives the request.

• Health Check: It verifies which backend servers are healthy and available.

• Algorithm Selection: Based on the chosen algorithm (e.g., round robin), it decides where to send the request.

• Request Forwarding: The request is passed to the selected backend server.

• Server Processing: The server handles the request and generates a response.

• Response Return: The server sends the response back to the load balancer.

• Client Response: Finally, the load balancer forwards the response to the client.

Diagram showing how a load balancer distributes client requests across backend servers for reliability and scalability.

NOTE: While sending response back, Load Balancer does not use DNS since IP of client is already known.

🚀 Why Load Balancers Matter

Without load balancers, systems risk downtime, bottlenecks, and poor user experience. By intelligently distributing traffic, they provide the foundation for scalable, resilient, and high‑performance applications.

✅ Pros of Using Load Balancers

• Improved Reliability: If one server fails, traffic is rerouted to healthy servers.

• Scalability: Easily add or remove servers to handle changing traffic loads.

• Optimized Performance: Distributes requests to avoid bottlenecks and reduce latency.

• Security Features: SSL termination and protection against DDoS attacks.

• Session Persistence: Maintains user sessions across requests when needed.

⚠️ Cons of Using Load Balancers

• Added Complexity: Requires configuration, monitoring, and maintenance.

• Cost: Hardware load balancers and cloud services can be expensive.

• Single Point of Failure: If not properly configured, the load balancer itself can become a bottleneck.

• Latency Overhead: Adds a small delay due to routing and health checks.

🧠 Conclusion

Load balancers are a cornerstone of modern system design. They ensure that applications remain resilient, scalable, and performant under varying loads. Whether you're deploying a small web app or architecting a global-scale platform, understanding how load balancers work — and choosing the right type and algorithm — is essential for building robust infrastructure.