Sticky Session

What it is

A sticky session means the load balancer tries to send the same user back to the same instance repeatedly.

In practice, it “sticks” that session to one server for a while.

When it matters

This usually shows up when part of the state still lives in application memory:

• user session
• shopping cart
• multi-step flow

It is a quick way to make things work before externalizing that state.

Common mistake

The classic mistake is treating sticky session as an architectural solution when it is often just a temporary crutch.

If the instance dies, that local state is gone.

And if traffic gets uneven, some instances run much hotter than others.

Short example

A Node app stores session state in local memory.

With sticky sessions, the user keeps landing on the same instance and everything seems fine.

Without it, each request could hit a different server and “lose” the session.

Why it helps

Sticky session can simplify an early stage.

But it creates dependency on local state and often gets in the way of:

• failover
• autoscaling
• balanced traffic distribution

Sticky session fixes the symptom quickly. Externalizing state fixes the cause.