What Are HLD and LLD?

In system design, HLD and LLD describe two different levels of thinking about a system:

  • High-Level Design (HLD) looks at the system from a bird’s-eye view.
    It focuses on overall architecture, main components, and how they talk to each other.

  • Low-Level Design (LLD) zooms in and looks at the system from a code perspective.
    It focuses on classes, interfaces, methods, data structures, and object interactions.

Both are about design, but they answer different questions and use different levels of detail.

What High-Level Design (HLD) Focuses On

HLD is about the “big picture” of the system. Typical concerns in HLD include:

  • Major components and services
    For example: User Service, Payment Service, Notification Service, Database, Cache.

  • Communication between components
    Whether they use REST APIs, message queues, events, or direct calls.

  • Technology choices
    Such as which database type (SQL/NoSQL), caching layer, load balancer, or external services to use.

  • Non-functional requirements
    Scalability, availability, reliability, performance, security, and monitoring.

At this level, you are not yet talking about classes and methods.
You are deciding how the system will be structured so it can handle requirements and scale properly.

What Low-Level Design (LLD) Focuses On

LLD is about the internal structure of each component decided in HLD. Typical concerns in LLD include:

  • Classes and interfaces
    Defining what classes exist, what responsibilities they have, and how they are grouped.

  • Methods and attributes
    Deciding what data each class holds and what functions it exposes.

  • Relationships between classes
    Association, aggregation, composition, and inheritance.

  • Detailed workflows
    How objects collaborate step-by-step to fulfil a use case.

At this level, you are close to code.
If HLD is the city map, LLD is the street-level drawing that shows each building and room.

Same Problem, Two Views: Simple Example

Consider an online food delivery system.

At the HLD level, you might talk about:

  • Services: User Service, Restaurant Service, Order Service, Delivery Service, Payment Service.

  • Data storage: relational database for orders, NoSQL for menus, cache for frequently accessed data.

  • External integrations: payment gateway, SMS/email provider, maps API.

  • Communication: services communicating over REST APIs or message queues.

At the LLD level, you might talk about:

  • Classes: User, Restaurant, MenuItem, Order, OrderItem, DeliveryPartner, Payment.

  • Methods: Order.create(), Order.cancel(), Payment.process(), DeliveryPartner.assignOrder().

  • Relationships: one Order has many OrderItems, one Restaurant has many MenuItems.

  • Detailed flows: how an Order object moves from “Created” to “Paid” to “Assigned to DeliveryPartner” to “Delivered”.

The problem is the same, but the level of detail and focus are very different.

Key Differences Between HLD and LLD

Here are the main differences in a compact form:

  • Scope

    • HLD: whole system, modules, and communication between them.

    • LLD: inside each module, down to classes and methods.

  • Abstraction level

    • HLD: abstract, technology and architecture focused.

    • LLD: concrete, implementation-oriented.

  • Audience

    • HLD: architects, senior engineers, stakeholders who care about system behaviour and scalability.

    • LLD: developers who implement features and write the actual code.

  • Outputs

    • HLD: architecture diagrams, component diagrams, technology choices.

    • LLD: class diagrams, sequence diagrams, detailed design documents.

Both are complementary: HLD gives structure, LLD gives implementation detail.

When to Do HLD vs LLD

In a real project, design usually moves in this order:

  1. Requirements & use cases
    Understand what the system should do and what constraints exist.

  2. High-Level Design
    Decide the architecture, components, data stores, and integrations.
    Check that the design can handle scale, performance, and reliability needs.

  3. Low-Level Design
    For each component, break it down into classes, interfaces, and detailed workflows.
    Focus on code organisation, maintainability, and clarity.

  4. Implementation
    Write code based on the LLD, using the structure defined.

Skipping HLD often leads to systems that do not scale or are hard to integrate.
Skipping LLD often leads to messy code and poor maintainability.

HLD vs LLD in Interviews

In interviews, questions are usually clearly oriented toward either HLD or LLD:

  • HLD-style questions
    “Design WhatsApp”, “Design YouTube”, “Design an e-commerce website.”
    These expect you to discuss components, APIs, databases, and scaling.

  • LLD-style questions
    “Design a parking lot system”, “Design an elevator system”, “Design a logging library.”
    These expect you to identify classes, methods, relationships, and object interactions.

Sometimes, a problem can have both phases: first the interviewer might ask for a brief high-level view, then ask you to dive into low-level design for a specific part.

Being able to recognise which level they are asking for helps you answer with the right amount of detail.

Summary

High-level design and low-level design are two essential but different layers of system design.
HLD focuses on the architecture: main components, how they connect, and how the system meets non-functional requirements.

LLD focuses on implementation details: classes, interfaces, methods, and object interactions inside those components.
Together, they form a clear path from problem statement to working, maintainable code.

Understanding LLD vs HLD helps you communicate better about systems, structure your thinking in interviews, and design software that is both scalable at the system level and clean at the code level.