073 Elastic Load Balancer

Load Balancer

A server that forwards traffic to multiple servers downstream

Why use a load balancer?

• Spread load across multiple downstream instances
• Expose a single point of access (DNS) for your app
• Seamlessly handle failures of downstream instances
• Health check
• SSL termination (HTTPS)
• Enforce stickiness with cookies
• High availability across zones
• Separate public traffic from private traffic

Why use Elastic Load Balancer

It is a managed load balancer

• AWS guarantess it will work
• AWS takes care of upgrade, maintenance, high availability
• AWS provides only a few config knobs

Your own load balancer

• cost less
• more effort

It is integrated with many AWS services

• EC2, EC2 Auto Scaling groups
• AWS Certificate Manager (ACM), CloudWatch
• Route 53, AWS WAF, AWS Global Accelerator

Health Check

• Crucial for load balancers
• It enables the load balancer to know if instances are available to replay to requests
• It is done on a port and a route (common endpoint: /health)
• If response is not 200, then load balancer will not forward request

Types of load balancer on AWS

Classic load balancer (2009)

• HTTP, HTTPS, TCP, SSL

Application load balancer (2016)

• HTTP, HTTPS, WebSocket

Network load balancer (2017)

• TCP, TLS, UDP

Gateway load balancer (2020)

• Operates at network layer 3 (IP protocol)

It is recommended to use the newer generation

Some load balancers can be setup as internal or external ELB

Load Balancer Security Groups

Load balancer security group:

• Looks like normal security group

Application security group:

• Allow traffic only from the load balancer
  • Set Source as specific security group (which is load balancer)