Every second, Google handles billions of requests from all over the world. How do they distribute this load without any downtime or bottlenecks?

Answer: Through an advanced, multi-layered Load Balancing system, operating across edge locations, data centers, and internal services.

Real-World Analogy: Airport Traffic Control System

Imagine an airport like Indira Gandhi International:

  • Hundreds of flights (requests) arrive every hour.
  • Air Traffic Control (ATC) directs flights to the correct runway and gate, avoiding collisions or delays.
  • If a runway is under maintenance, ATC reroutes flights elsewhere.

Google’s Load Balancer acts like ATC — smartly directing incoming user traffic to healthy, optimal, and geographically nearest servers.

Two Key Layers of Google Load Balancing

Layer Responsibility
Global (Frontend) Routes traffic to the nearest, healthiest region
Regional (Backend) Balances between multiple servers/microservices within a data center

1. Global Load Balancer (Frontend)

Google’s Global HTTP(S) Load Balancer works across continents.

Features:

  • Routes based on geo-location, latency, and health
  • Uses Anycast IPs – same IP address served globally, routed to closest region
  • Can autoscale to millions of requests per second
  • Seamlessly shifts traffic during outages

Example:

  • A user in Patna hits google.com
  • DNS + Anycast sends them to Delhi GFE
  • Global Load Balancer checks:
    • Is the Delhi region healthy?
    • Is it overloaded?
  • If not, it may re-route to Mumbai or Singapore region

2. Regional Load Balancer (Backend)

Once inside a region (e.g., Mumbai DC):

  • A regional load balancer decides which backend pool to route to
  • Uses health checks, connection draining, and session affinity
  • Balances between:
    • Web servers
    • Microservices (e.g., indexer, video transcoder)
    • Databases or cache systems

Sample Load Balancing Flow (Real-Life Case)

User visits YouTube:

  1. Hits frontend Load Balancer in Delhi
  2. Sent to Mumbai backend
  3. Regional LB picks one from:
    • Cache server (if video cached)
    • Transcoder (if resolution is missing)
    • Recommendation service (for homepage)

This entire flow happens in <100ms.

Health Checking & Failover

Google’s LB constantly checks the health of services using:

  • HTTP checks for 200 OK
  • TCP handshakes
  • gRPC pings for microservices

If a service is:

  • Down → It's removed from rotation
  • Overloaded → New traffic is routed elsewhere
  • Slow → Weighted routing adjusts traffic volume

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