|Using Elastic Load Balancing / Eucalyptus Load Balancing Usage Examples|
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 load balancer uses a special load-balancer-generated cookie to track the application instance for each request. When the load balancer receives a request, it first checks to see if this cookie is present in the request. If so, the request is sent to the application instance specified in the cookie. If there is no cookie, the load balancer chooses an application instance based on the existing load balancing algorithm. A cookie is inserted into the response for binding subsequent requests from the same user to that application instance. The stickiness policy configuration defines a cookie expiration, which establishes the duration of validity for each cookie. The cookie is automatically updated after its duration expires.
In this example, you create a stickiness policy and then use it to enable sticky sessions for a load balancer that has load balancer-generated HTTP cookies. Before you get started, be sure you've created a load balancer with Elastic Load Balancing.
To enable duration-based sticky sessions for a load balancer:
The load balancer uses a special cookie to associate the session with the original server that handled the request, but follows the lifetime of the application-generated cookie corresponding to the cookie name specified in the policy configuration. The load balancer only inserts a new stickiness cookie if the application response includes a new application cookie. The load balancer stickiness cookie does not update with each request. If the application cookie is explicitly removed or expires, the session stops being sticky until a new application cookie is issued.
If an application server fails or is removed, the load balancer will try to route the sticky session to another healthy application server. The load balancer will try to stick to new healthy application server and continue routing to currently stick application server even after the failed application server comes back. However, it is up to the new application server on how it'll respond to a request which it has not seen previously.
In this example, you configure a load balancer for session stickiness when the life of the session follows that of an application-generated cookie. Before you get started, be sure you've created a load balancer with Elastic Load Balancing.