* you're building a web-ish application and need to store session data
* you don't want to go through the overhead of building a strongly typed relational table
* you know minimal operations stuff
* just use redis, its easy to deploy, easy to code for, and available on all major cloud platforms as a managed service
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The problem is there are tradeoffs and session storage becomes a fundamental architectural decision once your application matures. So something you added as a once-off so you can get back to feature development is now a foundational pillar.
However a simple redis instance in front of the database serving as a readable cache changes the rules of the game significantly - depending on the complexity of your calculation and your end result subsequent "page loads" or whatever you are doing can be tens of thousands (or more) times as efficient, and if you decide to use an expensive database or a cloud database this can help you a lot.
Eventually the hard part is you might have bugs in synchronizing the state of redis and your database, look to existing implementations for your stack instead of reinventing the wheel.