A Practical LLD Domain Modeling Framework for System Design Interviews

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A structured framework for low-level design (LLD) breaks down system modeling into a sequential checklist covering business flow, invariants, entities, state machines, aggregates, and bounded contexts. The approach emphasizes starting with user journeys and business rules rather than jumping directly to classes or design patterns. Engineers are advised to identify what changes over time as entities, what must never break as invariants, and what must remain consistent together as aggregates. The framework argues that interviewers assess a candidate's ability to reason about business correctness under constraints, not their knowledge of syntax or UML diagrams. Its core principle is that good domain modeling means designing systems where invalid business states are structurally difficult to create.

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