Tech Byte - AWS ELB Notes from AWS Standard Docs (1/10)

A load balancer accepts incoming traffic from clients and routes requests to its registered & healthy EC2 instances in one or more Availability Zones (also monitors the health of registers instances)

• Stops routing traffic to unhealthy instances and restarts routing to them once they are found to be healthy again.
• With a Classic Load Balancer, you register instances with the load balancer. With an Application Load Balancer, you register the instances as targets in a target group, and route traffic to a target group.
• Load Balancer domain names are part of amazonaws.com domain.
• Each availability zone has a load balancer node.
• Client resolves Load Balancer's domain name and is given one or more IP address of load balancer nodes of the load balancer.
• As traffic to your application changes over time, Elastic Load Balancing scales your load balancer and updates the DNS entry. Note that the DNS entry also specifies the time-to-live (TTL) as 60 seconds, which ensures that the IP addresses can be remapped quickly in response to changing traffic.
• The client determines which IP address to use to send requests to the load balancer. 
• The load balancer node that receives the request selects a healthy registered instance and sends the request to the instance using its private IP address.
• Classic Load Balancer - the load balancer node that receives the request selects a registered instance using the round robin routing algorithm for TCP listeners and the least outstanding requests routing algorithm for HTTP and HTTPS listeners.
• Application Load Balancer - the load balancer node that receives the request evaluates the listener rules in priority order to determine which rule to apply, and then selects a target from the target group for the rule action using the round robin routing algorithm. Routing is performed independently for each target group, even when a target is registered with multiple target groups.
• For HTTP connections - Classic Load Balancers use pre-open connections but Application Load Balancers do not.
• The nodes of an Internet-facing load balancer have public IP addresses. The DNS name of an Internet-facing load balancer is publicly resolvable to the public IP addresses of the nodes. Therefore, Internet-facing load balancers can route requests from clients over the Internet.
• The nodes of an internal load balancer have only private IP addresses. The DNS name of an internal load balancer is publicly resolvable to the private IP addresses of the nodes. Therefore, internal load balancers can only route requests from clients with access to the VPC for the load balancer.
• The instances do not need public IP addresses to receive requests from an internal or an Internet-facing load balancer.
• If your application has multiple tiers, for example web servers that must be connected to the Internet and database servers that are only connected to the web servers, you can design an architecture that uses both internal and Internet-facing load balancers e.g. Internet-facing load balancer to web servers and an internal load balancer for the database servers.