Elastic Load Balancing Concepts

This section describes the terminology and concepts you need for understanding and using the Elastic Load Balancing service.


When a new load balancer is created, Eucalyptus will assign a unique DNS A record to the load balancer. After the load balancer has been launched and configured, the IP address of its interface will be added to the DNS name. If more than one cluster is specified, there can be more than one IP addresses mapped to the DNS name.

The load balancers in one cluster will distribute traffic to the instances only in the same cluster. Application users can query the application service using the DNS name and the Eucalyptus DNS service will respond to the query with the list of IP addresses on a round-robin basis.

Eucalyptus generates a DNS name automatically (e.g., lb001.euca-cloud.example.com). Typically there will also be a CNAME record that maps from the meaningful domain name (e.g., service.example.com) to the automatically-generated DNS name. The CNAME records can be serviced anywhere on Internet.

Health Check

In order to route traffic to healthy instances, and prevent traffic from being routed to unhealthy instances, the Elastic Load Balancing service routinely checks the health of your service instances. The health check uses metrics such as latency, RequestCount and HTTP response code counts to ensure that service instances are responding appropriately to user traffic.

Load Balancer

Load balancers are the core of the Elastic Load Balancing service. Load balancers are special instances created from a Eucalyptus-provided VM image, and are managed by Eucalyptus. Instances off the image forward packets to your service instances using load-balancing software to ensure no instances are overloaded.

There may be a brief delay between the time that the first service instance is registered to a load balancer and the time the load balancer becomes operational. This is due to the virtual machine instantiation, and will not adversely affect the operation of your load balancer.

Each balancer VM will service only one load balancer. Launching unnecessary load balancers may be detrimental to your cloud’s performance.

Service Instance

A service instance is an instance created from an image in your cloud. These are the instances that serve your application and users. The Elastic Load Balancing service monitors the health of these instances and routes traffic only to healthy instances.

Sticky Sessions

By default, a load balancer routes each request independently to the application instance with the smallest load. However, you can use the sticky session feature (also known as session affinity), which enables the load balancer to bind a user’s session to a specific application instance. This ensures that all requests coming from the user during the session will be sent to the same application instance.

The key to managing the sticky session is determining how long your load balancer should consistently route the user’s request to the same application instance. If your application has its own session cookie, then you can set Elastic Load Balancing to create the session cookie to follow the duration specified by the application’s session cookie. If your application does not have its own session cookie, then you can set Elastic Load Balancing to create a session cookie by specifying your own stickiness duration. You can associate stickiness duration for only HTTP/HTTPS load balancer listeners.

An application instance must always receive and send two cookies: A cookie that defines the stickiness duration and a special Elastic Load Balancing cookie named AWSELB, that has the mapping to the application instance.


HTTPS Support is a feature that allows you to use the SSL/TLS protocol for encrypted connections (also known as SSL offload). This feature enables traffic encryption between the clients that initiate HTTPS sessions with your load balancer and also for connections between the load balancer and your back-end instances.

There are several advantages to using HTTPS/SSL connections with your load balancer:

  • The SSL server certificate used to terminate client connections can be managed centrally on the load balancer, rather than on every individual application instance.

  • The work of encrypting and decrypting SSL traffic is moved from the application instance to the load balancer.

  • The load balancer can ensure session affinity or “sticky sessions” by terminating the incoming HTTPS request and then re-encrypting the content to send to the back-end application instance.

  • All of the features available for HTTP can be used with HTTPS connections.

Using HTTPS/SSL protocols for both front-end and back-end connections ensures end-to-end traffic encryption. If you are using SSL and do not want Elastic Load Balancing to terminate, you can use a TCP listener and install certificates on all the back-end instances handling requests.

To enable HTTPS support for your load balancer, you’ll have to install an SSL server certificate on your load balancer by using Identity and Access Management (IAM) to upload your SSL certificate and key. After you upload your certificate, specify its Amazon Resource Name (ARN) when you create a new load balancer or update an existing load balancer. The load balancer uses the certificate to terminate and then decrypt requests before sending them to the back-end instances.