🚀 30 Days of System Design – Day 3: Load Balancers
By Alvion Technologies
In early-stage systems, a single server may handle all traffic efficiently. But as users increase, the same architecture becomes fragile. A sudden spike can overwhelm the server, causing downtime and lost user trust.
This is why load balancers are foundational in modern distributed systems. They distribute traffic intelligently, improve availability, and enable safe horizontal scaling.
Without load balancing, scaling is risky. With it, scaling becomes predictable.
🔹 What is a Load Balancer?
A Load Balancer (LB) sits between users and backend servers, distributing incoming traffic across multiple servers to ensure no single server is overloaded.
It acts as a traffic controller, monitoring server health and routing requests only to healthy instances.
Load balancers can operate at Layer 4 (Transport Layer) or Layer 7 (Application Layer), depending on routing complexity.
🔄 How Traffic Flows
Understanding request flow clarifies the importance of load balancing:
- User sends request to a domain
- DNS resolves domain to Load Balancer IP
- Load Balancer selects a healthy backend server
- Response travels back through LB to user
This design keeps backend servers private and protected from direct internet exposure.
🛣️ Load Balancing Strategies
Different systems require different routing strategies based on workload behavior.
Round Robin distributes traffic evenly. Least Connections routes to the least busy server. IP Hashing ensures session consistency. Weighted routing prioritizes stronger servers.
The right strategy depends on state management, traffic spikes, and SLA requirements.
❤️ Health Checks & Failover
Load balancers continuously monitor backend servers to ensure reliability.
- Unhealthy servers are removed from rotation
- Traffic is redirected automatically
- Recovered servers are added back seamlessly
This enables zero-downtime deployments and high availability in production systems.
🧠 Final System Design Insight
Load balancers do more than distribute traffic — they enable controlled growth, improve uptime, and protect infrastructure.
In modern cloud-native systems, scaling servers is easy. Scaling without a load balancer is not.