What Are Sticky Sessions and When to Avoid Them?

Sticky sessions (session affinity) are a load-balancing method that ensures all requests from the same user are routed to the same server to maintain session continuity.

When to use/Use Cases

Use sticky sessions in small-scale apps, legacy systems, or when quickly enabling user login continuity without building a centralized session store. They’re suitable for prototypes or low-traffic apps where simplicity matters more than scalability.

Example

Imagine a shopping cart: sticky sessions ensure all your requests hit the same server so your cart doesn’t reset mid-checkout.

Want to go deeper?

Check out Grokking System Design Fundamentals, Grokking the System Design Interview, or practice with Mock Interviews with ex-FAANG engineers.

Why Is It Important

Sticky sessions highlight the trade-off between simplicity and scalability. They help beginners understand why modern systems favor stateless architectures.

Interview Tips

In interviews, define sticky sessions clearly. Then contrast them with alternatives like stateless tokens or distributed caches. Always mention that sticky sessions don’t scale well.

Trade-offs

You gain simplicity (no external store needed) but lose load balance and fault tolerance. If the chosen server fails, the user’s session is lost.

Pitfalls

Beginners often assume sticky sessions are fine at scale. In reality, they can cause uneven server load, session loss, and scaling headaches.