How to use load balancing to distribute traffic

· Category: System Design

Short answer

Load balancers distribute incoming requests across multiple backend servers to optimize resource utilization and prevent overload.

Steps

  1. Place a load balancer between clients and application servers.
  2. Choose an algorithm such as round robin, least connections, or weighted distribution.
  3. Configure health checks to remove failed servers from the pool automatically.
  4. Enable session persistence only when required by application state.
  5. Scale load balancers themselves to avoid becoming a bottleneck.

Tips

  • Use application-layer load balancers for path-based routing.
  • Employ geo-DNS or anycast for global traffic distribution.
  • Terminate TLS at the load balancer to offload cryptography from backends.
  • Monitor backend latency to adjust weights dynamically.

Common issues

  • Uneven load due to long-running requests concentrating on certain servers.
  • Health checks too aggressive causing flapping.
  • Session affinity breaking elasticity by pinning users to overloaded instances.
  • Load balancer failures taking down the entire entry point.

Example

# Consistent hashing for service discovery
import hashlib

def get_node(key, nodes):
    hash_val = int(hashlib.md5(key.encode()).hexdigest(), 16)
    return nodes[hash_val % len(nodes)]

node = get_node('user-123', ['node-a', 'node-b', 'node-c'])

This snippet implements consistent hashing to distribute keys across nodes, a foundational technique in scalable distributed systems.