🍪 What are Sticky Sessions?

🍪 What are Sticky Sessions?

Sticky Sessions, also known as Session Affinity, ensure that a user is always routed to the same backend server for the duration of their session.

✅ This is useful when your application stores session data locally on the instance, rather than in a centralized store like Redis or a database.

---

📦 How Sticky Sessions Work in AWS

AWS enables sticky sessions using a cookie:

🔹 Application Load Balancer (ALB):

• Uses the AWSALB cookie (default) or a custom app cookie.
• Sticky sessions can be configured per target group.

🔹 Classic Load Balancer (CLB):

• Uses the AWSELB cookie.
• Supports both duration-based and application-controlled stickiness.

❗ Sticky sessions are not supported in Network Load Balancer (NLB).

---

🔧 Use Case Example

Let’s say a user logs in to a shopping site. Their cart is stored in memory on EC2 instance A. With sticky sessions enabled:

• Their first request hits instance A
• All subsequent requests are also sent to instance A, until the session expires

Without sticky sessions:

• Load balancer may send requests to different EC2 instances, and the cart data might be lost if not stored centrally.

---

⚙️ Configuration (via AWS Console):

For ALB:

• Go to the Target Group settings
• Under Attributes, enable Stickiness
• Choose:
  • Duration-based (set time in seconds)
  • App-based cookie name (optional)

---

🧠 Pros and Cons

✅ Pros:

• Simple way to preserve session data
• Useful for legacy or stateful applications
• No need for external session storage

❌ Cons:

• Breaks load distribution (one server might get overloaded)
• Doesn’t work well with auto-scaling unless sessions are replicated
• Not recommended for stateless or microservice-based architectures

---

✅ Summary Table