Scaling
When an application needs to do more work, the historical approach has been to run the application on a bigger computer. Give it more memory, more CPU cores, and even more disk space. This does not increase the application's reliability, as it still relies on a single computer, and it comes with added complications once your application is large enough that there simply aren't any computers large enough to run it on.
Giving a program a larger computer to run on is called scaling vertically. By contrast, scaling horizontally is the approach of using many smaller computers. We came across this idea when discussing deploying on container-based services and increasing the number of instances that our Docker swarm used. An application must have a replicated, scalable idea of its current state to operate in this way, for client sessions, shopping basket contents, and anything else that a visitor would expect to be persistent between different pages of a website.
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