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What if we have 2 servers, and a load balancer which alternatively sends requests to either server?
A user adds an item to their cart that is stored in-memory (RAM) of the first server, but upon refreshing, the request is sent to the second server and user gets an empty cart.
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How to solve this issue?
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The mistake the developer made, is storing the "state" on a single node.
So instead of your server gatekeeping the cart items in it's RAM, everyone shares a central lightening-fast cache database.
And congratulations! You just unlocked 2 new items:
Because no “state” is being stored on a single server. By offloading state to an external, shared databases, we can effortlessly horizontally scale our applications, as much as we’d like to.