LLD vs HLD: Low-Level Design vs High-Level Design Compared
Understanding the distinction between Low-Level Design (LLD) and High-Level Design (HLD) is critical for cracking technical interviews. Learn what interviewers expect, how problems differ, and how to structure your preparation.
LLD vs HLD: Quick Comparison Table
Key differences at a glance for system design interview candidates.
| Dimension | Low-Level Design (LLD) | High-Level Design (HLD) |
|---|---|---|
| Core Question Answered | "How is this component built in code?" (Micro) | "What components exist and how do they interact?" (Macro) |
| Primary Audience | Software Engineers, Code Reviewers, Module Owners | System Architects, Engineering Managers, DevOps, SREs |
| Key Artifacts | UML Class Diagrams, Sequence Diagrams, Interface Signatures, DB Schemas | Architecture Block Diagrams, Dataflow Graphs, Network Topology, API Contracts |
| Core Technologies / Concepts | OOP, SOLID Principles, GoF Design Patterns, Mutexes, Clean Code | Load Balancers, Caching (Redis), Message Queues (Kafka), Sharding, CAP Theorem |
| Typical Interview Length | 45–60 min (or 90–120 min Machine Coding) | 45–60 min Architecture Whiteboard |
| Candidate Level | SDE-1, SDE-2, and Backend Roles | SDE-2, Senior (SDE-3), Staff, and Principal |
| Common Failure Reason | Rigid monolithic classes, violating Single Responsibility, unhandled concurrency | Single Point of Failure (SPOF), failing to address bottleneck scale, no math |
What is Low-Level Design (LLD)?
Low-Level Design (LLD), also called Object-Oriented Design (OOD) or Detailed Design, specifies how a single software service or module is implemented internally. It translates abstract business requirements into clean, modular, extensible, and maintainable source code.
Core Pillars of LLD:
Single Responsibility, Open/Closed, Liskov Substitution, Interface Segregation, Dependency Inversion.
Creational (Factory, Singleton, Builder), Structural (Adapter, Decorator), Behavioral (Strategy, Observer, State).
Thread safety, thread pools, locks (Mutex, ReadWriteLock), enums, and exact class relationships (IS-A vs HAS-A).
What is High-Level Design (HLD)?
High-Level Design (HLD), commonly referred to simply as System Design in interview contexts, defines the overall topology of a distributed software system. It focuses on the communication between services, scaling from 1,000 to 100,000,000 users, handling infrastructure failures, and fulfilling Service Level Objectives (SLOs).
Core Pillars of HLD:
Horizontal vs Vertical scaling, stateless application tiers, multi-datacenter replication, active-active failover.
SQL vs NoSQL trade-offs, consistent hashing, database sharding keys, write buffers (LSM trees), read replicas.
Message queues (Kafka, RabbitMQ), cache eviction strategies (LRU/LFU), CDN edge delivery, rate limiters.
Concrete Example: Designing a Rate Limiter in LLD vs HLD
Notice how the exact same prompt ("Design a Rate Limiter") requires completely different discussions depending on the round:
- Classes:
RateLimiterinterface with concrete implementations:TokenBucketRateLimiter,SlidingWindowRateLimiter. - Patterns: Strategy Pattern to swap algorithms at runtime; Factory Pattern to instantiate limiters based on config.
- Thread Safety: Using
AtomicIntegerorReentrantLockto prevent race conditions when multiple concurrent threads consume tokens. - Code: Writing real, compilable methods:
boolean allowRequest(String clientId).
- Placement: API Gateway tier (Envoy / Kong) sitting in front of microservices cluster.
- Storage: Distributed Redis cluster using Lua scripts for atomic increments (evaluating 100,000 QPS).
- Scale & Latency: Local in-memory caching with asynchronous batch sync to Redis to keep latency under 2ms.
- Fail-Open vs Fail-Closed: What happens if Redis cluster drops? (Fail-open to protect customer experience).
How to Structure Your 45-Minute Interview
LLD 45-Minute Interview Framework LLD
- Clarify Scope (0–5 min): Identify core entities, actors, and functional use cases.
- Class Diagram (5–15 min): Draw classes, attributes, enums, and inheritance/composition links.
- Apply Patterns (15–25 min): Justify Strategy, Factory, or Observer pattern applications.
- Code Implementation (25–40 min): Write clean code for critical interfaces and core algorithms.
- Edge Cases & Concurrency (40–45 min): Handle race conditions, null checks, and extensibility.
HLD 45-Minute Interview Framework HLD
- Requirements & Scope (0–5 min): Agree on 3 functional and 3 non-functional SLOs.
- Capacity Math (5–10 min): DAU → QPS → Storage per day → Network bandwidth.
- High-Level Architecture (10–25 min): Client → CDN → Load Balancer → Services → DB → Cache.
- Deep Dive (25–40 min): Sharding key selection, replication lag, queue dead-lettering.
- Bottlenecks & SPOF (40–45 min): What breaks at 10x traffic? Monitoring, caching, and rate limiting.