Sticky Sessions in AWS: What They Are and Why Your Website Might Need Them 🍪

Imagine you’re shopping online. You add a few things to your cart, click to check out, and suddenly your cart is empty. What happened?

Behind the scenes, the website might have sent your request to a different server, and that new server didn’t know what you were doing. This kind of issue is exactly what sticky sessions are meant to solve.

Let's walk through what they are and how they work in AWS.

What Are Sticky Sessions?

Sticky sessions, also called session affinity, are a way to make sure that a user keeps getting sent to the same backend server during their visit to a website. Normally, a load balancer sends each request to any available server. But with sticky sessions turned on, it remembers who you are and keeps sending you to the same server.

This is useful for things like login sessions, shopping carts, or any personalized experience.

How Sticky Sessions Work in AWS

If you're using an AWS Classic Load Balancer, you can enable sticky sessions by using duration-based cookies.

There are two cookie options:

• ELB cookie (AWSELB): This is a built-in cookie that AWS uses to track your session.

• Application cookie: If your application already creates its own session cookie, AWS can use that instead to keep the session sticky.

Think of cookies as name tags that help AWS recognize you and keep you connected to the same server you started with.

When to Use Sticky Sessions

Sticky sessions are especially helpful when:

• Users need to stay logged in

• Your application uses a shopping cart

• The site delivers personalized content

• You want a consistent user experience throughout a session

Things to Watch Out For

Sticky sessions can sometimes cause problems if too many users get attached to the same server. This can lead to an unbalanced workload where one server is doing most of the work while others sit idle.

To avoid this, developers often:

• Replicate session data across all servers

• Design the application to be stateless, meaning it does not rely on remembering individual users between requests

Final Thoughts

Sticky sessions help keep users on the same server, so their experience stays smooth and consistent. They are especially useful in web applications that rely on memory, such as keeping a user logged in or tracking items in a cart.

If you're working with AWS and want your site to feel more reliable and personal for each visitor, sticky sessions are a simple and effective option to explore.

References: 

Configure sticky sessions for your Classic Load Balancer - Elastic Load Balancing. (2017). Amazon.com. https://docs.aws.amazon.com/elasticloadbalancing/latest/classic/elb-sticky-sessions.html

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