Elastic Load Balancing automatically distributes incoming application traffic across multiple Amazon EC2 instances. It enables you to achieve greater levels of fault tolerance in your applications, seamlessly providing the required amount of load balancing capacity needed to distribute application traffic.

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Achieve higher levels of fault tolerance for your applications by using Elastic Load Balancing to automatically route traffic across multiple instances and multiple Availability Zones. Elastic Load Balancing ensures that only healthy Amazon EC2 instances receive traffic by detecting unhealthy instances and rerouting traffic across the remaining healthy instances. If all of your EC2 instances in one Availability Zone are unhealthy, and you have set up EC2 instances in multiple Availability Zones, Elastic Load Balancing will route traffic to your healthy EC2 instances in those other zones.

Elastic Load Balancing automatically scales its request handling capacity to meet the demands of application traffic. Additionally, Elastic Load Balancing offers integration with Auto Scaling to ensure that you have back-end capacity to meet varying levels of traffic levels without requiring manual intervention.

Elastic Load Balancing works with Amazon Virtual Private Cloud to provide robust networking and security features. You can create an internal (non-internet facing) load balancer to route traffic using private IP addresses within your virtual network. You can implement a multi-tiered architecture using internal and internet-facing load balancers to route traffic between application tiers. With this multi-tier architecture, your application infrastructure can use private IP addresses and security groups, allowing you to expose only the internet-facing tier with public IP addresses.

Elastic Load Balancing provides integrated certificate management and SSL decryption, allowing you to centrally manage the SSL settings of the load balancer and offload CPU intensive work from your instances.

You can build fault tolerant applications by placing your Amazon EC2 instances in multiple Availability Zones. To achieve even more fault tolerance with less manual intervention, you can use Elastic Load Balancing. You get improved fault tolerance by placing your compute instances behind an Elastic Load Balancer, as it can automatically balance traffic across multiple instances and multiple Availability Zones and ensure that only healthy Amazon EC2 instances receive traffic. You can setup an Elastic Load Balancer to load balance incoming application traffic across Amazon EC2 instances in a single Availability Zone or multiple Availability Zones. Elastic Load Balancing can detect the health of Amazon EC2 instances. When it detects unhealthy Amazon EC2 instances, it no longer routes traffic to those unhealthy Amazon EC2 instances. Instead, it spreads the load across the remaining healthy Amazon EC2 instances. If all of your Amazon EC2 instances in a particular Availability Zone are unhealthy, but you have set up Amazon EC2 instances in multiple Availability Zones, Elastic Load Balancing will route traffic to your healthy Amazon EC2 instances in those other zones. It will resume load balancing to the original Amazon EC2 instances when they have been restored to a healthy state.

You can use Amazon Route 53 health checking and DNS failover features to enhance the availability of the applications running behind Elastic Load Balancers. Route 53 will fail away from a load balancer if there are no healthy EC2 instances registered with the load balancer or if the load balancer itself is unhealthy.

Using Route 53 DNS failover, you can run applications in multiple AWS regions and designate alternate load balancers for failover across regions. In the event that your application is unresponsive, Route 53 will remove the unavailable load balancer endpoint from service and direct traffic to an alternate load balancer in another region. To get started with Route 53 failover for Elastic Load Balancing, visit the Elastic Load Balancing Developer Guide and the Amazon Route 53 Developer Guide.

Let’s say that you want to make sure that the number of healthy Amazon EC2 instances behind an Elastic Load Balancer is never fewer than two. You can use Auto Scaling to set these conditions, and when Auto Scaling detects that a condition has been met, it automatically adds the requisite amount of Amazon EC2 instances to your Auto Scaling Group. Or, if you want to make sure that you add Amazon EC2 instances when latency of any one of your Amazon EC2 instances exceeds 4 seconds over any 15 minute period, you can set that condition, and Auto Scaling will take the appropriate action on your Amazon EC2 instances — even when running behind an Elastic Load Balancer. Auto Scaling works equally well for scaling Amazon EC2 instances whether you’re using Elastic Load Balancing or not.

Elastic Load Balancing makes it easy to create an internet-facing entry point into your VPC or to balance load between tiers of your application within your VPC. You can assign security groups to your ELB to control which ports are open to a list of allowed sources. Because Elastic Load Balancing is attached to your VPC, all of your existing Network Access Control Lists (ACL’s) and Routing Tables continue to provide additional network controls.

When you create a load balancer in your VPC, you can specify whether the load balancer is internet-facing (the default) or internal. If you select internal, you do not need to have an internet gateway to reach the load balancer, and the private IP addresses of the load balancer will be used in the load balancer’s DNS record.