A low-level design (LLD) round — often called the machine-coding or object-oriented design round — gives you a 45-minute problem like "design a parking lot" or "design a rate limiter" and asks you to produce a clean class-level model, frequently with compilable code. It sits between the algorithm round (data structures, single function) and the system-design round (services, databases, scale). The unit of work here is the class: responsibilities, relationships, interfaces, and the design patterns that connect them.
What this round actually measures
It is not about knowing exotic patterns. Interviewers are watching whether you can take an intentionally vague prompt, clarify it into concrete requirements, carve the domain into entities with single responsibilities, and let abstractions absorb future change. The recurring senior signal is restraint: the right amount of design, named patterns that earn their place, and a frank discussion of trade-offs.
The shape of a good 45 minutes
The whole module is built around one repeatable framework so you never freeze on a blank whiteboard:
- Clarify scope and requirements (≈5 min).
- Identify entities (the nouns).
- Map relationships (has-a, is-a, uses).
- Design the classes — fields, methods, interfaces.
- Apply patterns where a real axis of change demands one.
- Discuss trade-offs, concurrency, and extensions.
How to use this topic
The questions below decompose the framework into the skills you can drill independently: the six-step method, the clarifying questions to always ask, how to scope a vague prompt fast, the time budget for code versus diagrams, the anti-patterns that sink candidates, and the rubric interviewers grade against. Master these first; every concrete design problem in this module (parking lot, elevator, rate limiter…) is just this framework applied to a new domain.