Handle concurrent users in web application

I'm planning on creating a kind of recommender, where the user asks questions and gets a subset of recommendations. My idea would be to have 3 layers, front end, logic in Java or Python and SQL such as Postgres.

Some questions related to this:

• How do I handle private data for the user? I mean, the user answers question A and gets 100 options with a select from a DB. The user answers question B but where should I store those first 100 options? in a temporary table in the DB? in a matrix in the logic layer, and if this is the case, how do I know to which user this data belongs?

• How do I handle multiple runs a the same time? I want several users to be able to keep their executions in parallel, when someone starts the questionnaire with interim answers, or even user dependent questions, how do I maintain this state?