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LLD vs HLD: What Interviewers Actually Test

Where low-level design stops and high-level design starts — and why both matter in the same interview loop.

Fresher9 min read

Why this matters

  • Interviewers use LLD to test whether you can turn a vague product idea into maintainable code.
  • Confusing LLD with HLD leads to talking about Kafka when they wanted your class diagram.
  • CampusOS problems in this track all start in one JVM — scale-out belongs in System Design.
High-Level Design
Clients
API Gateway
ServicesParking, Payment, Notify
Databases & queues
Low-Level Design
ParkingLotfacade
ParkingFloorspots collection
ParkingSpotstate + vehicle

One service box from HLD expands into this class graph.

HLD names services and data flows. LLD names classes, interfaces, and in-process behaviour.

What LLD actually covers

Low-level design answers how code is organised inside a service:

  • Classes, interfaces, enums, and their relationships
  • Public APIs and invariants on domain objects
  • Design patterns applied to a concrete problem
  • Thread safety and data-structure choices in one process

You are not sizing clusters or picking databases — unless the interviewer explicitly bridges into HLD.

What HLD covers instead

High-level design names boxes and arrows between services:

  • Which microservices exist and what each owns
  • Data stores, caches, queues, and replication
  • Capacity, latency targets, and failure domains
Aspect
One service / moduleWhole platform
Class diagram + Java skeletonArchitecture diagram + data flow
ParkingLot allocates spotsParking service scales to 10M users
35–45 minutes45–60 minutes

  • One service / module
    Whole platform

  • Class diagram + Java skeleton
    Architecture diagram + data flow

  • ParkingLot allocates spots
    Parking service scales to 10M users

  • 35–45 minutes
    45–60 minutes

How they connect in CampusOS

Every CampusOS module in this track is one deployable slice. The parking module's ParkingLot class is one box inside the "Campus Mobility" service on an HLD diagram.

When an article ends with CampusOS extension, it points to where HLD picks up — sharding tickets, payment queues, display boards at the edge.

Quick recall

Everything you need if you only revisit this box.

  1. LLD = classes, APIs, patterns inside one process
  2. HLD = services, data stores, scale and reliability
  3. CampusOS modules map to one HLD service box each
  4. Say the boundary before drawing

Test yourself

Answer these before moving on — recall is what makes it stick.