Elastic Load Balancing (ELB)

ELB automatically distributes incoming application traffic across multiple targets, such as Amazon EC2 instances. This can be done across multiple AZs.

There are 4 types of Load Balancers:

1. Application Load Balancer (Intelligent Load Balancer). Best suited for load balancing of HTTP and HTTPS Traffic. They operate at Layer 7 and are application aware.

2. Network Load Balancer (Performance Load Balancer). Operating at the Transport Level (Layer 4) on the OSI Model, Network Load Balancers are capable of handling millions of requests per second, while maintaining ultra-low latencies.

3. Gateway Load Balancer (Or inline appliance load balancing). Operating at the Network Level (Layer 3).

4. Classic Load Balancer (Classic/Test/Dev Load Balancer). You can load balance HTTP/HTTPS applications and use Layer 7 specific features, such as X-Forwarded and sticky sessions.

Feature	Application Load Balancer (ALB)	Network Load Balancer (NLB)	Gateway Load Balancer (GWLB)	Classic Load Balancer (CLB)
OSI Layer	Layer 7 (Application)	Layer 4 (Transport)	Layer 3 (Network)	Layer 4 & 7
Protocols	HTTP, HTTPS, gRPC, WebSocket	TCP, UDP, TLS	IP	TCP, SSL/TLS, HTTP, HTTPS
Performance	Good	Best	High	Good
Latency	~400 ms	~100 ms	Ultra-low	~400 ms
Static IP	❌	✅	✅	❌
Elastic IP	❌	✅	❌	❌ (Only for CLB in EC2-Classic)
Connection Draining	✅ (Deregistration Delay)	✅ (Deregistration Delay)	N/A	✅
Health Checks	HTTP, HTTPS, TCP	TCP, HTTP, HTTPS	TCP, HTTP, HTTPS	TCP, HTTP, HTTPS, SSL
Sticky Sessions	✅	✅	N/A	✅
Path/Host-based Routing	✅	❌	❌	❌
URL Redirects	✅	❌	❌	❌
Fixed Response	✅	❌	❌	❌
Websocket Support	✅	✅	N/A	Limited
HTTP/2 Support	✅	❌	N/A	❌
gRPC Support	✅	❌	N/A	❌
SSL Offloading	✅	✅	N/A	✅
Server Name Indication (SNI)	✅	✅	N/A	❌
User Authentication	✅	❌	❌	❌
AWS WAF Integration	✅	❌	❌	❌
Cross-zone Load Balancing	Always on (Can't be disabled)	Disabled by default	Disabled by default	Disabled by default for EC2-Classic, Enabled by default for EC2-VPC
Target Types	IP addresses, EC2 instances, Lambda functions, Containers	IP addresses, EC2 instances	IP addresses, EC2 instances	EC2 instances
Use Case	HTTP/HTTPS applications, Microservices, Container-based applications	TCP/UDP applications, Extreme performance needs, Static IP requirements	Third-party virtual appliances, Firewalls, Intrusion detection and prevention systems	Legacy applications in EC2-Classic
Transparent Network Gateway	❌	❌	✅	❌
GENEVE Protocol Support	❌	❌	✅	❌