HLD vs LLD: Interview Questions and Answers

HLD vs LLD: Interview Questions and Answers

Interviewers often ask about High-Level and Low-Level Design. Here are the common questions with concise answers.

Q1: What is the difference between HLD and LLD?

HLD is the big picture: major components, how they talk, tech stack, data flow. LLD is the detail: schemas, API contracts, logic, errors, edge cases. HLD settles architecture; LLD settles implementation.

Q2: Which comes first, HLD or LLD?

HLD first, then LLD. You decide the architecture before detailing the implementation. LLD depends on the architecture choices made in HLD.

Q3: What does an HLD document contain?

Major components, how they communicate, external systems (database, cache, queue, email provider), tech stack, and data flow from input to output. Usually with a diagram.

Q4: What does an LLD document contain?

Database schemas (collections, fields, types, indexes, relationships), API contracts (endpoints, methods, request/response shapes, status codes), internal logic, error cases, and edge cases. Enough for a developer to code without questions.

Q5: How long should HLD take?

For small projects, a few hours. For bigger ones, a few days. The work is thinking and sketching, not writing long documents.

Q6: How long should LLD take?

Usually longer than HLD because it covers every endpoint. For an MVP, a few days. The test is: can a developer code from it without questions?

Q7: How do you decide if a design decision is HLD or LLD?

If it concerns the architecture (which components, how they talk, tech stack), it is HLD. If it concerns the implementation (schemas, endpoints, logic), it is LLD.

Q8: How do you handle edge cases in LLD?

List them explicitly per endpoint: empty states, duplicates, race conditions, concurrent edits, large inputs. For each, document the behavior the developer should implement. Do not leave edge cases implicit.

Q9: What is a bounded context and how does it relate to HLD?

A bounded context (from Domain-Driven Design) is a part of the domain that can be reasoned about independently. In HLD, each bounded context becomes a candidate component or service.

Q10: What is the biggest mistake teams make in design?

Skipping design entirely and jumping to code. Architecture and contracts emerge ad hoc, leading to rework. The second mistake is over-documenting instead of just enough.

The Takeaway

HLD is the big picture (components, comms, tech stack, data flow). LLD is the detail (schemas, contracts, logic, errors, edge cases). HLD first. Each should be just enough to enable the next step. The test for LLD: can a developer code from it without questions?