In the previous chapters, we have all concentrated on building a stateless cluster. That means we don't need to maintain a session for each user request, and a load balancer can freely choose a worker node to serve the user requests.
A stateless cluster is more flexible and can scale well, so it's always the first choice when we are building a cluster. In essence, HTTP is a stateless protocol, so it lacks the ability to maintain a session for user requests. To solve this problem, web servers usually pass a session ID to the users' web browsers to maintain a long conversation.
For example, if we are building an online shopping system, we have to maintain a shopping cart for each user. When a user is ...
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