An Application Load Balancer functions at the application layer, the seventh layer of the Open Systems Interconnection (OSI) model.
After the load balancer receives a request, it 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.
You can configure listener rules to route requests to different target groups based on the content of the application traffic.
Routing is performed independently for each target group, even when a target is registered with multiple target groups.
You can add and remove targets from your load balancer as your needs change, without disrupting the overall flow of requests to your application.
Elastic Load Balancing scales your load balancer as traffic to your application changes over time.
Elastic Load Balancing can scale to the vast majority of workloads automatically.
You can configure health checks, which are used to monitor the health of the registered targets so that the load balancer can send requests only to the healthy targets.
APPLICATION LOAD BALANCER COMPONENTS
LISTENER - A PROCESS THAT CHECKS FOR CONNECTION REQUESTS, USING THE PROTOCOL AND PORT THAT YOU CONFIGURE
. ***** BEFORE YOU START USING YOUR APPLICATION LOAD BALANCER YOU MUST ADD ONE OR MORE LISTENERS *****
Target Group - a way of getting network traffic routed via specified protocols and ports to specified instances.
A target group contains EC2 instances to which a load balancer distributes workload.
They listen to HTTP/S request from a Load Balancer
IT'S BASICALLY LOAD BALANCING ON A PORT LEVEL
rules - The rules that you define for your listener determine how the load balancer routes requests to the targets in one or more target groups.
Each rule consists of a priority, one or more actions, and one or more conditions. For more information on listener rules:
https://docs.aws.amazon.com/elasticloadbalancing/latest/application/load-balancer-listeners.html#listener-rules
**** For an Application load balancer link the Auto Scaling Group with the target group (which itself is attached to the load balancer) ****
**** For the Classic load balancer link the auto scaling group with the load balancer directly. ****
