🔹 How Application Load Balancer (ALB) works
Metaphor: Think of ALB as a restaurant receptionist.
- You (the customer) arrive at the restaurant (the ALB).
- The receptionist greets you, asks: “What do you want?” (parses your HTTP request).
- Depending on what you ask for (e.g., menu section: sushi vs. pizza), the receptionist decides which waiter/kitchen section to send you to (routing rules: path-based, host-based).
- You don’t walk directly into the kitchen — the receptionist terminates your request, figures it out, and then forwards it on your behalf.
- Because the receptionist handled your order, the waiter/kitchen may see the receptionist’s name, not yours — unless the receptionist leaves a note (the
X-Forwarded-Forheader with your original IP).
Key idea:
- ALB = layer 7 (application-aware), reads your request.
- It acts as a middleman and may hide your original IP (unless passed in headers).
🔹 How Network Load Balancer (NLB) works
Metaphor: Think of NLB as a highway traffic cop at a busy junction.
- Cars (client requests) are flying in millions per second.
- The cop doesn’t care about who’s inside the car or what you’re carrying (no parsing of HTTP).
- The cop just looks at the car’s license plate (IP/port info at layer 4), then waves it directly into the correct lane (the EC2 instance private IP).
- The cop does not open the trunk, check your documents, or slow things down.
- Because the cop just redirects the car, when it arrives at the destination (EC2 instance), the driver’s original identity (the client’s real IP) is still intact.
Key idea:
- NLB = layer 4 (transport-level only), blind to the contents of the request.
- It’s designed for speed, scale, and preserving the client IP.
- It routes directly to the target’s private IP, staying entirely in AWS’s backbone network.
🔹 Core Difference (Metaphor Summary)
| Feature | ALB (Receptionist) | NLB (Traffic Cop) |
|---|---|---|
| Layer | Layer 7 (Application) | Layer 4 (Transport) |
| Understands request? | Yes — parses HTTP headers, URLs, cookies | No — only cares about IP address + port |
| Adds overhead? | Yes — terminates and re-creates connections | Minimal — just forwards at lightning speed |
| Client IP preserved? | No (unless X-Forwarded-For header used) |
Yes — client IP is preserved automatically |
| Best for | Smart routing (websites, APIs, microservices) | Massive scale (gaming, IoT, VoIP, TCP apps) |
Top comments (0)