🌍 Introduction
When you run an application on AWS, having just one server (EC2 instance) is risky:
- If that server fails → your app goes down ❌
- If too many users visit → server may crash ❌
👉 That’s where Elastic Load Balancer (ELB) comes in.
AWS ELB automatically distributes traffic across multiple servers (EC2, containers, Lambda). This ensures:
✅ High availability
✅ Scalability
✅ Fault tolerance
By the end, you’ll understand:
👉 Browser → ELB (smart routing + health checks) → Healthy Servers (EC2, ECS, Lambda)
🔒 Step 1: Understand ELB Types
AWS provides different types of load balancers for different use cases:
1️⃣ Application Load Balancer (ALB)
- Works at Layer 7 (Application Layer)
- Best for HTTP/HTTPS traffic
- Supports content-based routing (host, path, headers)
- Example:
/api/*→ API servers,/images/*→ image servers
2️⃣ Network Load Balancer (NLB)
- Works at Layer 4 (Transport Layer)
- Handles TCP/UDP/TLS traffic
- Extremely fast + scalable (millions of requests/sec)
- Provides static IPs
- Example: gaming, IoT, high-performance apps
3️⃣ Gateway Load Balancer (GWLB)
- Works at Layer 3 (Network Layer)
- For 3rd-party appliances (firewalls, intrusion detection)
4️⃣ Classic Load Balancer (CLB)
- Legacy version
- Works at Layer 4 & 7 (limited features)
- ⚠️ AWS recommends using ALB or NLB instead
⚡ Step 2: Learn Key ELB Terminologies
Here are the basic terms you’ll see when setting up ELB:
| Term | Meaning |
|---|---|
| Target | Resource receiving traffic (EC2, Lambda, IP) |
| Target Group | A pool of targets (with health checks) |
| Listener | Process that checks for connections (protocol + port) |
| Listener Rule | Routing conditions (e.g., /api/* → API group) |
| Health Check | ELB test to ensure only healthy servers get traffic |
| AZ (Availability Zone) | AWS data centers where servers run |
| Cross-Zone LB | Distribute traffic across all AZs |
| Sticky Sessions | Keep same user on the same server |
| Security Groups | Firewall for ELB |
| Idle Timeout | Time before idle connections close |
| SSL/TLS Cert | Enables HTTPS (via AWS ACM) |
📜 Step 3: How ELB Works
-
User visits your app (e.g.,
myapp.com) - Request goes to ELB Listener (port 80 or 443)
- ELB checks Listener Rules
- ELB forwards request to the correct Target Group
- Health Check ensures only healthy servers receive traffic
- Response goes back → ELB → user
👉 Flow: Browser → ELB → Healthy Server → ELB → Browser
🌐 Step 4: Benefits of ELB
✅ Scalability – Auto-scales with traffic
✅ High Availability – Multi-AZ support
✅ Security – SSL/TLS, WAF integration
✅ Monitoring – CloudWatch metrics, access logs
✅ Cost Efficient – Pay-as-you-go pricing
🧪 Step 5: Real-World Example
Imagine an E-commerce website:
- You run 3 EC2 instances in different AZs
- Deploy an Application Load Balancer in front
- A user goes to
www.shop.com→ ELB receives request - ELB checks which server is healthy + least busy
- Routes request → Server responds → User gets page
- If one server crashes → traffic reroutes to healthy ones 🎉
🎯 Conclusion
With ELB, you get:
✅ Better performance
✅ Zero downtime (if a server fails)
✅ Intelligent routing
✅ Secure connections
👉 For websites → ALB
👉 For high-performance TCP apps → NLB
👉 For firewalls/security → GWLB
This is the recommended AWS setup for building highly available, production-grade applications 🚀
✅ Next Steps
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