Load Balancing

A load balancer sits in front of servers and distributes incoming requests. It’s the key to horizontal scaling.

Visual: LB Spreading Requests

flowchart LR
    Users["👥 Users"] --> LB["Load Balancer"]
    LB --> S1["Server 1<br/>🟢 Healthy"]
    LB --> S2["Server 2<br/>🟢 Healthy"]
    LB --> S3["Server 3<br/>🟢 Healthy"]
    LB --> Sn["Server N<br/>🔴 Unhealthy"]

    style LB fill:#7c3aed,color:#fff
    style S1 fill:#059669,color:#fff
    style S2 fill:#059669,color:#fff
    style S3 fill:#059669,color:#fff
    style Sn fill:#dc2626,color:#fff

Load Balancing Algorithms

Algorithm	How It Works	Best For
Round Robin	Requests go to each server in turn	Simple, equal-capacity servers
Least Connections	Send to server with fewest active connections	Uneven request processing times
IP Hash	Hash client IP → same server always	Sticky sessions (stateful apps)
Weighted Round Robin	Heavier servers get more requests	Mixed-capacity servers
Random	Pick a server at random	Stateless, any server works

L4 vs L7 Load Balancing

Layer	How It Routes	Speed	Features
L4 (Transport)	By IP + port	⚡ Fast	No content awareness
L7 (Application)	By HTTP path, headers, cookies	Slower	Can route `/api` → one pool, `/web` → another

Analogy: L4 is like a mail sorter that reads only the ZIP code. L7 reads the full letter and decides based on content.

What Else a Load Balancer Does

Health checks — regularly pings servers, removes dead ones
SSL termination — handles HTTPS decryption, servers get plain HTTP
Session persistence — sends the same user to the same server (if needed)
Rate limiting — rejects excess requests

Trade-offs

The load balancer itself is a Single Point of Failure (SPOF) — always run at least two in active-passive mode.
L7 gives more control but adds latency.
Most cloud providers offer managed LBs (AWS ALB, GCP HTTP LB).

In Simple Words

Load balancer = traffic cop. Spreads requests so no server gets overwhelmed.
Round robin is simplest. Least connections is smarter for uneven work.
L4 is fast but dumb. L7 is slower but can route by URL.