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Load Balancer in System Design – Part 1: Introduction & Fundamentals

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🌀 Load Balancer in System Design – Part 1: Introduction & Fundamentals

📑 Table of Contents

  1. What is a Load Balancer?
  2. Why Use a Load Balancer?
  3. What Does Load Balancing Actually Do?
  4. Layer 4 vs Layer 7 Load Balancing
  5. Real World Examples of Load Balancers
  6. Conclusion

🔹 What is a Load Balancer?

A load balancer is a system design component that distributes incoming traffic across multiple servers, ensuring no single server is overloaded.

Think of it as a traffic cop standing at the intersection of your application:

  • It takes every incoming request (cars).
  • Redirects them to the correct server (roads).
  • Ensures smooth flow without jams.

💡 Key Idea: The main purpose of a load balancer is to provide high availability, fault tolerance, and better performance.

📌 Example:
Imagine an e-commerce app during a sale. Instead of sending all users to one server (which will crash), the load balancer distributes them across multiple servers, ensuring the site stays up.

🔹 Why Use a Load Balancer?

✅ Increased Availability and Reliability

  • By spreading traffic across multiple servers, the system avoids single point of failure.
  • If Server A goes down, requests are automatically redirected to Server B or C.

💡 Real-world Example: Netflix uses global load balancers to reroute traffic if a regional server cluster goes down, so users rarely notice outages.

✅ Improved Performance

  • Prevents any single server from becoming overloaded.
  • Ensures fast response times by spreading requests evenly.

📌 Practical Use: Banking systems route requests like "Check Balance" and "Fund Transfer" across different app servers, reducing delays during peak hours (e.g., salary days).

✅ Scalability

  • As demand grows, you can add new servers to the pool.
  • Load balancers automatically start sending traffic to them without downtime.

📌 Example: During Black Friday, Amazon temporarily adds new servers to handle traffic spikes, balanced automatically by AWS Elastic Load Balancers.

✅ Maintenance

  • Servers can be updated, patched, or restarted without downtime.
  • Load balancers reroute traffic to healthy servers while maintenance happens.

💡 Use Case: Software companies roll out updates to one server at a time, keeping the system online throughout.

🔹 What Does Load Balancing Actually Do?

A load balancer performs 3 key jobs:

  1. Distributes Requests Evenly
  • Prevents overloading any single server.
  1. Detects Failures
  • Monitors servers using health checks (ping, HTTP status).
  • Redirects traffic away from failed servers.
  1. Optimizes User Experience
  • Directs requests to the least busy or fastest server.

📌 Simple Example Flow:

User → Load Balancer → {Server A, Server B, Server C}
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If Server A is overloaded:

Next request → Server B (because it's free)
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This makes the system resilient, fast, and highly available.

🔹 Layer 4 vs. Layer 7 Load Balancing

Load balancers operate at different OSI layers:

⚡ Layer 4 (Transport Layer)

  • Works with TCP/UDP connections.
  • Routes requests based on IP + Port.
  • Faster but less flexible.
  • Can’t inspect HTTP headers or payload.

📌 Example: A gaming server balancing raw TCP connections between players.

🌐 Layer 7 (Application Layer)

  • Works at application level (HTTP/HTTPS, gRPC, WebSockets).

  • Can make routing decisions based on:

    • HTTP headers
    • Cookies
    • Request paths (/images vs /api)

  • More flexible but slightly slower.

📌 Example:

  • /videos/* requests → Video server cluster.
  • /api/* requests → API server cluster.

💡 Real-World Use: YouTube uses Layer 7 load balancers to route video streaming requests vs. API calls differently.

🔹 Real World Examples of Load Balancers

Let’s see how major companies use them:

  1. Amazon (E-commerce)
  • Uses AWS Elastic Load Balancer to distribute millions of checkout and search requests.
  • Ensures scaling during festive sales.
  1. Netflix (Streaming)
  • Uses global load balancing + regional failover.
  • If EU servers fail, traffic reroutes to US clusters.
  1. Banking Apps
  • Internal load balancers distribute traffic across multiple app servers.
  • Transactions are routed to the least loaded server for speed.
  1. Gaming Servers
  • Online multiplayer games (e.g., PUBG, Fortnite) rely on load balancers to match players with the nearest, least-lag server.

📌 Diagram Reference:
(You could add a simple diagram like below when publishing)

[ Users ]  
   ↓  
[ Load Balancer ]  
   ↓ ↓ ↓  
[ Server A | Server B | Server C ]
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🔹 Conclusion

In this part, we covered the fundamentals of load balancing:

  • What a load balancer is and why it matters.
  • How it improves availability, reliability, scalability, and performance.
  • Layer 4 vs. Layer 7 differences.
  • Real-world examples from Amazon, Netflix, banks, and gaming.

👉 In Part 2, we’ll dive into the Types of Load Balancers (Hardware, Software, Cloud-based), with deeper analysis of their advantages, challenges, and best-fit use cases.

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