Clean explanation of a concept that trips up a lot of developers when they first move from single-server to load-balanced architectures. The cart example is perfect — nothing frustrates users more than losing their session state mid-checkout. One thing worth adding: with the rise of serverless and edge computing, session persistence strategies are evolving again. Token-based approaches with JWTs stored client-side avoid the sticky session problem entirely, though they come with their own tradeoffs around token size and revocation. What's your take on cookie-based affinity vs source IP-based for most production setups?