Which comes first, HLD or LLD?
HLD first, then LLD. You decide architecture before detailing implementation. LLD depends on the choices made in HLD.
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More FAQs in HLD vs LLD: Interview Questions and Answers
HLD is the big picture (components, communication, tech stack, data flow). LLD is the detail (schemas, API contracts, logic, errors, edge cases). HLD settles architecture; LLD settles implementation.
Database schemas with types and indexes, API contracts (endpoints, methods, request/response shapes, status codes), internal logic, error cases, and edge cases. Enough for a developer to code without questions.
List them explicitly per endpoint: empty states, duplicates, race conditions, concurrent edits, large inputs. For each, document the behavior to implement. Do not leave edge cases implicit.
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