A load balancer accepts incoming traffic from clients and routes requests to its registered targets (such as EC2 instances) in one or more Availability Zones. The load balancer also monitors the health of its registered targets and ensures that it routes traffic only to healthy targets. When the load balancer detects an unhealthy target, it stops routing traffic to that target. It then resumes routing traffic to that target when it detects that the target is healthy again.
You configure your load balancer to accept incoming traffic by specifying one or more listeners. 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. Likewise, it is configured with a protocol and port number for connections from the load balancer to the targets.
Elastic Load Balancing supports three types of load balancers:
- Application Load Balancers
- Network Load Balancers
- Classic Load Balancers
Availability Zones and Load Balancer Nodes
You need enable the Availability Zone also ensure that each enabled Availability Zone has at least one registered target, as when you enable Availability Zone, the Elastic Load Balancing creates a load balancer node in the Availability Zone. If target registered but the Availability Zone not enabled, the registered target will not receive any traffic.
Multiple Availability Zones are recommend. (With an Application Load Balancer, we require you to enable multiple Availability Zones.) If one Availability Zone becomes unavailable or has no healthy targets, the load balancer can route traffic to the healthy targets in another Availability Zone.
After you disable an Availability Zone, the targets in that Availability Zone remain registered with the load balancer. However, even though they remain registered, the load balancer does not route traffic to them.
Cross-Zone Load Balancing
The nodes for your load balancer distribute requests from clients to registered targets. When cross-zone load balancing is enabled, each load balancer node distributes traffic across the registered targets in all enabled Availability Zones. When cross-zone load balancing is disabled, each load balancer node distributes traffic only across the registered targets in its Availability Zone.
Cross-zone load balancing is disabled
Cross-zone load balancing is enabled
Request Routing
Before a client sends a request to your load balancer, it resolves the load balancer's domain name using a Domain Name System (DNS) server. The DNS entry is controlled by Amazon, because your load balancers are in the amazonaws.com domain. The Amazon DNS servers return one or more IP addresses to the client. These are the IP addresses of the load balancer nodes for your load balancer. With Network Load Balancers, Elastic Load Balancing creates a network interface for each Availability Zone that you enable. Each load balancer node in the Availability Zone uses this network interface to get a static IP address. You can optionally associate one Elastic IP address with each network interface when you create the load balancer.As traffic to your application changes over time, Elastic Load Balancing scales your load balancer and updates the DNS entry. The DNS entry also specifies the time-to-live (TTL) of 60 seconds. This helps ensure 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 target and sends the request to the target using its private IP address.
Scheme
- 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.
- Both internet-facing and internal load balancers route requests to your targets using private IP addresses. Therefore, your targets do not need public IP addresses to receive requests from an internal or an internet-facing load balancer.
Network Load Balancer
- Operates at the connection level (Layer 4), routing connections to targets – EC2 instances, microservices ,containers and IP addresses based on IP protocol data.
- Suited for TCP/UDP traffic.
- Capable of handling millions of requests per second while maintaining ultra-low latencies.
- Network Load Balancer is optimized to handle sudden and volatile traffic patterns while using a single static IP address per Availability Zone.
- Integrated with other AWS services such as Auto Scaling, EC2 Container Service (ECS), and CloudFormation.
Application Load Balancer
- Operates at the request level (layer 7), routing traffic to targets – EC2 instances, containers, IP addresses and Lambda functions based on the content of the request.
- Ideal for advanced load balancing of HTTP and HTTPS traffic, Application Load Balancer provides advanced request routing targeted at delivery of modern application architectures, including microservices and container-based applications.
- Simplifies and improves the security of your application, by ensuring that the latest SSL/TLS ciphers and protocols are used at all times.
Classic Load Balancer
- Basic load balancing across multiple Amazon EC2 instances and operates at both the request level and connection level.
- Intended for applications that were built within the EC2-Classic network.
