Load Balancer (L4 vs L7)

Distributes incoming traffic across multiple servers so no single server gets overwhelmed.

Clients
   │
   ▼
Load Balancer
   ├── Server 1
   ├── Server 2
   └── Server 3

L4 — Transport Layer Load Balancer

Operates at TCP/UDP level. Doesn't look inside the packet, just routes based on:

• Source/Destination IP
• Source/Destination Port
• Protocol (TCP/UDP)

Client --- LB sees: IP + Port only --- forwards to server
           (never opens the packet)

• Very fast — no deep inspection.
• Can't make smart routing decisions (doesn't know if it's HTTP, video, API).
• One TCP connection: Client → LB → Server (NAT-based or TCP passthrough).
• Use case: Raw speed, non-HTTP traffic, database connections, game servers.

L7 — Application Layer Load Balancer

Operates at HTTP/HTTPS level. Actually reads the request content:

• URL path
• Headers
• Cookies
• HTTP method

Client --- LB reads full HTTP request --- smart routing decision
           "oh this is /api → send to API servers"
           "oh this is /video → send to media servers"

• Smarter routing decisions.
• Can terminate TLS (decrypt, inspect, re-encrypt).
• Can modify requests/responses.
• Slightly more overhead than L4.
• Use case: Web apps, microservices, API gateways.

L4 vs L7 Comparison

	L4	L7
Layer	Transport (TCP/UDP)	Application (HTTP)
Sees	IP + Port	Full request content
Speed	Faster	Slightly slower
Routing based on	IP/Port	URL, headers, cookies
TLS termination	No	Yes
Use case	DB, games, raw TCP	Web, APIs, microservices

Load Balancing Algorithms

Algorithm	How
Round Robin	Requests go 1→2→3→1→2→3 in order
Weighted Round Robin	Powerful servers get more requests
Least Connections	Send to server with fewest active connections
IP Hash	Same client IP always goes to same server (sticky)
Random	Pick a server randomly

Key Concepts

Health Checks

• LB periodically pings servers.
• If server fails → removed from pool automatically.

Session Persistence (Sticky Sessions)

• Same user always hits same server.
• Needed when session state is stored locally on server.
• L7 can do this via cookie, L4 via IP hash.

TLS Termination

• L7 LB decrypts HTTPS traffic.
• Backend servers receive plain HTTP.
• Offloads crypto work from app servers.

Client --HTTPS--> L7 LB (decrypts) --HTTP--> Backend servers

Examples

• L4 — AWS NLB, HAProxy (TCP mode)
• L7 — AWS ALB, Nginx, Cloudflare, HAProxy (HTTP mode)