Intro

• Regional Service
• Supports Multi AZ
• Spread load across multiple EC2 instances
• Separate public traffic from private traffic
• Health checks allow ELB to know which instances are working properly (done on a port and a route, /health is common)
• Does not support weighted routing
• If no targets are associated with the target groups ⇒ 503 Service Unavailable
• Targets are unreachable (possibly due to NACL or SG rules) ⇒ 504 Timeout Error
• Using ALB & NLB, instances in peered VPCs can be used as targets using IP addresses.

Types

Classic Load Balancer (CLB) - deprecated

• Load Balancing to a single application
• Supports HTTP, HTTPS (layer 7) & TCP (layer 4)
• Health checks are HTTP or TCP based
• Provides a fixed hostname (xxx.region.elb.amazonaws.com)

Application Load Balancer (ALB)

• Load balancing to multiple applications (target groups) based on the request parameters