Monday, January 4, 2016

AWS Elastic Load Balancing session stickiness

In a previous post "Configuring and testing AWS Elastic Load Balancer" I described how to configure AWS ELB to distribute load on multiple web servers.
We observed that the same client might get routed to a different EC2 instance. Some applications require the user to always be directed to the same instance during his session. This is the case when an in-memory session state is used or any other application specific reason. This requirements is often referred to as session stickiness.
AWS ELB offers two ways to provide session stickiness: using a cookie provided by the application, or using a cookie generated by ELB.

Load Balancer Generated Cookie Stickiness

Using an Expiring cookie
Using the same configuration as the previous post, the load balancer will have the stickiness configuration set to "Disabled". 
To change this behavior:
  1. Click "Edit" link
  2. Select "Enable Load Balancer Generated Cookie Stickiness" option.
  3. As a testing value, enter 60 as an expiration period.

Editing stickiness properties of ELB

Now, let's start testing the effect of this new configuration. Open the test url (for example:, check the previous post for more details). Using fiddler or network tab in developer tools of your favorite browser, you can observe that the response includes this header:

Set-Cookie: AWSELB=A703B168326729FE0B7D2675641656C1889E580D7525169B4BE36A819D9F2A18BE64415E6B0C90F5F2AC8CB0CFAF8DABE929DB27D5077D6FF616065A5BAF81DDB430BE92;PATH=/;MAX-AGE=60

As we see, it's a cookie named "AWSELB" with max-age of 60 seconds. and it applies to the whole site.

AWSELB cookie in in the response as appears in Chrome dev tools

If you refresh the page, you'll find that the browser sends the cookie as expected:

Cookie: AWSELB=A703B168326729FE0B7D2675641656C1889E580D7525169B4BE36A819D9F2A18BE64415E6B0C90F5F2AC8CB0CFAF8DABE929DB27D5077D6FF616065A5BAF81DDB430BE92

The cookie is sent by the browser, and the response does not include a new cookie

But the response does not include the cookie. So it will expire after 60 seconds and the browser will not send it after expiration. Refreshing the browser several times will direct the traffic to the same EC2 instance, we can verify this by examining the response which looks like:

Host name:
Public IP:

As soon as the cookie is still active, the request is directed to the same instance. But what happens after the max-age passes?
A new cookie will be generated and you might be directed to one of the other web servers, and a new cookie with another value will be generated by ELB:


Notice that the value of the cookie has changed. And after this cookie expires, a new cookie might be generated using the old value pointing to the first instance.

Using an ELB cookie without expiration:
If the expiration values is left blank, then the behavior differs. The cookie will be generated without the max-age value. And the browser will send the same cookie until it's closed.


What happens when a server goes down?
To try this scenario, let's shut down the server which is getting the requests and refresh the browser. This time ELB generates a new cookie pointing to a healthy instance.

ELB has a built-in mechanism to support session stickiness with no code changes from the application side.
Using an expiring cookie might not be the best option to guarantee session affinity as the cookie is not renewed and it seems that there is no way to achieve a sliding expiration window for it. So you might prefer to go with a cookie without expiration.

In the next post, we'll use the other method available for session stickiness: using Application Generated Cookie Stickiness.