What is Amazon Elastic Load Balancer

Amazon Elastic Load Balancing (Amazon ELB):-

• ELB distributes the incoming application or network traffics across multiple targets.
•  multiple targets could be EC2 instances, containers, and IP address in multiple AZ.

The advantage of ELB:

• ELB distribute workload across multiple compute resources.
• ELB increases the availability and fault tolerance of your applications.
• Easily add and remove compute resource without disturbing your applications.
• ELB use health check to monitor the status of compute resource so that ELB forward the request to an only healthy instance.

How ELB works:

• ELB accept incoming traffic and route the request to its registers targets (such as EC2) in one or more AZ.
• ELB also monitor the health of its compute resource to ensure traffic goes to only healthy targets.
• ELB accept incoming traffic via listeners and you can set up one more listener.
• A Listener is a process that checks the connections request. 
• Listener use protocol and port number for connections from clients to ELB and a protocol and port number for connection from targets to ELB.

Features of Amazon ELB: -

Amazon ELB supports three type of Load balancing: You can choose load balancer as per your application's need.

• Classic Load Balancer 
• Applications Load Balancer
• Network Load Balancer

1- Classic Load Balancer: 

• Classic ELB distributes traffic evenly across the AZ that you enable for your ELB.
• To distribute traffic evenly for each AZ it's recommended to enable cross-zone load balancing. 
• When you have enabled cross-zone load balancing ELB distributes traffic evenly, let say you have two AZ one AZ have register 3 instances and Second AZ have 5 register instances. in this scenario, cross-zone load balancing will help to distribute traffic evenly.
• Support for an EC2 classic.
• Support TCP and SSL listeners.
• Support for sticky sessions using application-generated cookies

2- Application Load Balancer: 

• Application LB works on the Application layer of the OSI model.
• It uses target groups to distribute the coming request.
• Application LB forwards the request to target groups.
• After receiving the request, it evaluates the listener rules in priority order to determine which rule to apply.
• Can configure listener rules to route requests to different target group based on the content of application traffic.
• Support path based routing.
• Support host-based routing.

3- Network Load Balancer: 

• Operate on the fourth layer of the OSI model.
• It can handle millions of request per second.
• Use the target group to distribute the coming request.
• Support for Static IP address for the load balancer.
• Support for registering targets by IP address including outside target of LB VPC.
• Support AWS ECS(Elastic container Service)

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