Elastic Load Balancing (ELB) Overview¶

What is load balancing?¶

Load Balances are servers that forward traffic to multiple servers (e.g. EC2 instances) downstream.

Why use a load balancer?¶

Spread load accross multiple downstream instances
Expose a single point of access (DNS) to your application
Seamlessly handle failures of downstream instances
Do regular health checks to your instances
Provide SSL termination (HTTPS) for your websites
Enforce stickiness with cookies
High Availability across zones
Separate public traffic from private traffic

Why use an Elastic Load Balancer?¶

An Elastic Load Balancer is a managed load balancer
- AWS guarantees that it will be working
- AWS takes care of upgrades, maintenance, high availability
- AWS provides only a few configuration knobs
It costs less to setup your own load balancer but it will be a lot more effort on your end
It is integrated with many AWS offerings / services
- EC2, EC2 Auto Scaling Groups, Amazon ECS
- AWS Certificate Manager (ACM), CloudWatch
- Route 53, AWS WAF, AWS Global Accelerator

Health Checks¶

Health Checks are crucial for Load Balancers
They enable the load balancer to know if instances it forwards traffic to are available to reply to requests
The health check is done on a port and a route (/health is common)
If the response is not 200 (OK), then the instance is unhealthy

Types of load balancers on AWS¶

AWS has 4 kinds of managed Load Balancers: - Classic Load Balancer (v1 old generation) - 2009 - CLB - HTTP, HTTPS, TCP, SSL (secure TCP) - Application Load Balancer (v2 - new generation) - 2016 - ALB - HTTP, HTTPS, WebSocket - Network Load Balancer (v2 - new generation) - 2017 - NLB - TCP, TLS (secure TCP), UDP - Gateway Load Balancer - 2020 - GWLB - Operates at layer 3 (Network layer) - IP Protocol

Overall, it is recommended to use the newergeneration load balancers as they provide more features.

Some load balancers can be setup as internal (private) or external (public) ELBs.