🖥️ Server Administration
11 min read
Load Balancing Fundamentals
Understand load balancing algorithms, Layer 4 vs Layer 7 balancing, health checks, and how to set up HAProxy and Nginx as load balancers.
What Is Load Balancing?
A load balancer distributes incoming traffic across multiple backend servers. This provides:
- High availability — If one server fails, traffic is routed to healthy servers.
- Scalability — Add more backend servers to handle increased load.
- Performance — Spread requests to prevent any single server from being overwhelmed.
Layer 4 vs Layer 7
| Feature | Layer 4 (Transport) | Layer 7 (Application) |
|---|---|---|
| Operates on | TCP/UDP connections | HTTP requests |
| Speed | Faster (no content inspection) | Slightly slower |
| Routing decisions | IP + port only | URL, headers, cookies |
| SSL termination | Pass-through or terminate | Terminate and inspect |
| Use case | Generic TCP services | Web applications |
Load Balancing Algorithms
| Algorithm | How It Works | Best For |
|---|---|---|
| Round Robin | Sequential rotation | Equal-capacity servers |
| Weighted Round Robin | Rotation with capacity weights | Mixed-capacity servers |
| Least Connections | Sends to server with fewest active connections | Variable request duration |
| IP Hash | Same client IP always goes to same server | Session persistence |
| Random | Random selection | Large server pools |
Nginx as Load Balancer
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HAProxy Configuration
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Health Checks
Health checks prevent sending traffic to failed servers:
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Types of health checks:
- TCP — Can the load balancer open a connection? (Layer 4)
- HTTP — Does /health/ return 200 OK? (Layer 7)
- Custom — Application-specific checks (database connectivity, disk space).
Session Persistence
When a user's requests must go to the same backend (e.g., server-side sessions):
- Cookie-based — Load balancer inserts a cookie identifying the backend server.
- IP hash — Same source IP always routes to the same server.
- Application-managed — Use shared session storage (Redis, database) and any balancing algorithm.
Cookie-based or shared session storage are preferred over IP hash, which breaks when users are behind CGNAT (many users sharing one IP).