Elastic Load Balancing distributes incoming application traffic across multiple EC2 instances, in multiple Availability Zones.
You can configure health checks, which are used to monitor the health of the registered instances so that the load balancer can send requests only to the healthy instances. You can also offload the work of encryption and decryption to your load balancer so that your instances can focus on their main work.
You configure your load balancer to accept incoming traffic by specifying one or morelisteners. A listener is a process that checks for connection requests. It is configured with a protocol and port number for connections from clients to the load balancer and a protocol and port number for connections from the load balancer to the instances.
Elastic Load Balancing supports two types of load balancers: Application Load Balancers and Classic Load Balancers. There is a key difference between the way you configure these load balancers. 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.
You can configure health checks, which are used to monitor the health of the registered instances so that the load balancer can send requests only to the healthy instances. You can also offload the work of encryption and decryption to your load balancer so that your instances can focus on their main work.
You configure your load balancer to accept incoming traffic by specifying one or morelisteners. A listener is a process that checks for connection requests. It is configured with a protocol and port number for connections from clients to the load balancer and a protocol and port number for connections from the load balancer to the instances.
Elastic Load Balancing supports two types of load balancers: Application Load Balancers and Classic Load Balancers. There is a key difference between the way you configure these load balancers. 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.
When you enable an Availability Zone for your load balancer, Elastic Load Balancing creates a load balancer node in the Availability Zone. If you register instances in an Availability Zone but do not enable the Availability Zone, these registered instances do not receive traffic. Note that your load balancer is most effective if you ensure that each enabled Availability Zone has at least one registered instance.
With a Classic Load Balancer, we recommend that you enable multiple Availability Zones. With an Application Load Balancer, we require you to enable multiple Availability Zones. After you enable multiple Availability Zones, if one Availability Zone becomes unavailable or has no healthy instances, the load balancer can continue to route traffic to the healthy registered instances in another Availability Zone.
Cross-zone load balancing is always enabled for an Application Load Balancer and is disabled by default for a Classic Load Balancer. If cross-zone load balancing is enabled, the load balancer distributes traffic evenly across all registered instances in all enabled Availability Zones. If cross-zone load balancing is disabled, the load balancer distributes traffic evenly across all enabled Availability Zones. For example, suppose that you have 10 instances in Availability Zone us-west-2a and 2 instances in us-west-2b. If cross-zone load balancing is disabled, the requests are distributed evenly between us-west-2a and us-west-2b. As a result, the 2 instances in us-west-2b serve the same amount of traffic as the 10 instances in us-west-2a. However, if cross-zone load balancing is enabled, the load balancer distributes incoming requests evenly across all 12 instances.
When you create a load balancer, you must choose whether to make it an internal load balancer or an Internet-facing load balancer. Note that when you create a Classic Load Balancer in EC2-Classic, it must be an Internet-facing load balancer.
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.
Note that both Internet-facing and internal load balancers route requests to your instances using private IP addresses. Therefore, your instances do not need public IP addresses to receive requests from an internal or an Internet-facing load balancer.
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