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The transition from a single-file Streamlit script to a modular structure is the exact tipping point where a prototype becomes a real product. Decoupling core business logic from the Streamlit rendering layer not only makes writing unit tests far easier, but it also gives you the flexibility to expose that same logic via FastAPI or a CLI later on without a complete rewrite. In other words: moving business logic out of the presentation layer is a game-changer for long-term maintainability. Once you had everything decoupled into modules, did you add a test suite with pytest, or was the focus mainly on making the Streamlit UI cleaner to extend?

The point about workload contention hitting right during launch day is painfully accurate. Running heavy analytics queries or background jobs on the same cluster serving live user logins is a silent killer. Even within a monolith, introducing basic connection pool discipline, read replicas, and a caching layer before your first traffic spike is the difference between surviving a launch and taking down your entire app.