HLD Versus LLD Rounds

What each round is looking for

A high-level design (HLD) round asks you to design a large system such as a ride-hailing service: components, data stores, APIs, data flow, scaling and failure handling. Interviewers look for structured requirement gathering, sensible capacity reasoning, sound choices of storage and communication, and honest trade-offs. A low-level design (LLD) round, sometimes called object-oriented or machine-coding design, asks you to model something smaller, such as a parking lot, as classes and interfaces, and often to write runnable code. The signals there are clean decomposition, clear responsibilities, extensibility, correct handling of edge cases and concurrency, and readable code. Many companies run one or both; ask the recruiter which format to expect.

Two kinds of design interview

High-level design rounds test system architecture; low-level design rounds test object modelling and clean code.

The two rounds side by side

What you produce in each.

HLD (system design)                      LLD (object-oriented design)
-----------------------------------      -----------------------------------
scope: whole product / service           scope: one module or program
artefacts: boxes, arrows, APIs,          artefacts: classes, interfaces,
  data model, estimates                    methods, sometimes working code
key topics: scale, storage, caching,     key topics: responsibilities, SOLID,
  partitioning, consistency, failures      patterns, state, thread safety
typical length: 45-60 minutes            typical length: 45-90 minutes

City planner versus architect

HLD is planning a city: roads, utilities and districts. LLD is designing one building: rooms, doors and wiring. Both need structure, but the level of detail differs.

Quick check: Which signal matters most in an LLD round?

• Estimating global bandwidth
• Choosing between two cloud vendors
• Clear class responsibilities and extensible, correct code
• Designing a CDN topology