LLD vs HLD - Learning Module

Loading content...

0/246

How LLD Fits Within the Overall Design Process

Design as a Journey, Not a Destination

Software design is not a single activity—it's a process that unfolds in stages, each building on the previous. From the moment a stakeholder articulates a need to the moment code runs in production, design thinking permeates every step.

Low-Level Design occupies a specific position in this journey. It's the stage where abstract architectural concepts become concrete implementation plans—where boxes in diagrams transform into classes, interfaces, and methods. But LLD doesn't exist in isolation. It's shaped by what comes before (requirements, HLD) and shapes what comes after (implementation, testing).

Understanding where LLD fits in the overall design process helps you know when to do it, what inputs you need, what outputs you produce, and how to collaborate with teammates working at different levels of abstraction.

What You Will Learn

By the end of this page, you will understand the software design lifecycle, where LLD fits within it, how LLD connects to requirements and architecture, and how iteration refines design across all levels.

The Software Design Lifecycle

Software development involves multiple stages of design, each at a different level of abstraction. While the exact terminology varies by methodology (Waterfall, Agile, etc.), the fundamental stages remain consistent:

Requirements Gathering — Understanding what the system should do
High-Level Design (HLD) — Defining system architecture
Low-Level Design (LLD) — Designing component internals
Implementation — Writing actual code
Testing & Validation — Verifying the design works
Evolution & Maintenance — Adapting to change

Design Stages and Their Focus
Stage	Key Question	Outputs	Who's Involved
Requirements	What should the system do?	User stories, use cases, specs	Product, Users, Analysts
HLD	What components exist?	Architecture diagrams, data flow	Architects, Senior Engineers
LLD	How is each component built?	Class diagrams, API specs	Senior/Mid Engineers
Implementation	How do we code it?	Source code, tests	Developers
Validation	Does it work correctly?	Test results, bug reports	QA, Developers
Evolution	How do we adapt?	Refactored design, new features	Entire Team

Not Strictly Sequential

While presented as stages, modern development is iterative. You don't finish all LLD before starting implementation. Instead, you design, implement, learn, and refine in cycles. The stages overlap and inform each other continuously.

From Requirements to Design

Every design begins with requirements—the articulation of what the system should do. Requirements come in various forms:

Functional Requirements — What the system does (features, behaviors)
Non-Functional Requirements — How well it does it (performance, security, scalability)
User Stories — Narrative descriptions of user needs
Use Cases — Step-by-step interactions between users and system
Constraints — Limitations on the solution (budget, technology, timeline)

How Requirements Shape LLD

Requirements flow through HLD to LLD, becoming increasingly specific:

Requirement	HLD Decision	LLD Decision
"Users can place orders"	Order Service component	Order class, OrderRepository interface
"Must handle 1000 orders/second"	Horizontally scaled service, async processing	Thread-safe implementation, queue-based design
"Orders expire after 24 hours"	Scheduler component	OrderExpirationJob class, Timer abstraction
"Support multiple payment methods"	Payment Service + adapters	PaymentProcessor interface, Strategy pattern

Trace Requirements to Design

Every class in your LLD should trace back to a requirement. If you can't explain why a class exists in terms of user needs, question whether it's necessary. This traceability ensures you're building what's needed, not what's technically interesting.

From HLD to LLD: The Handoff

The transition from High-Level Design to Low-Level Design is one of the most critical handoffs in software development. HLD provides the context; LLD fills in the details.

What HLD Provides to LLD:

Component boundaries — What services exist and what each is responsible for
Communication patterns — How components talk (REST, gRPC, events)
Data ownership — Which component owns which data
Technology constraints — Languages, frameworks, databases to use
Non-functional requirements — Performance, scalability, availability targets

What LLD Adds:

Internal structure — Classes, interfaces, modules within each component
Detailed interfaces — Method signatures, parameter types, return types
Design patterns — Specific patterns applied to solve design challenges
Data structures — How data is organized in memory and storage
Algorithms — Specific procedures for key operations
Error handling — How failures are detected and managed

HLD Deliverables

•System context diagram
•Container/component diagram
•Data flow diagrams
•High-level sequence diagrams
•Technology stack decisions
•Deployment architecture

LLD Deliverables

•Class diagrams
•Detailed sequence diagrams
•State machine diagrams
•API specifications
•Database schemas
•Algorithm descriptions

Don't Start LLD Without HLD Context

A common mistake is diving into LLD (designing classes) before understanding HLD context (where those classes live). This leads to classes that work in isolation but don't integrate well into the system. Always understand the architectural context before designing internals.

From LLD to Implementation

LLD produces blueprints; implementation brings them to life. The transition from LLD to code should be straightforward if the design is detailed enough.

Characteristics of Implementation-Ready LLD:

Complete class specifications — All classes identified with their responsibilities
Detailed interfaces — Method signatures with types, not just method names
Clear relationships — How classes connect and collaborate
Error scenarios — How failures are handled
Edge cases addressed — Boundary conditions considered

From LLD to Code Example
TypeScript
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
// LLD Specification:
// Class: OrderService
// Responsibility: Orchestrate order creation and management
// Dependencies: OrderRepository, PaymentProcessor, InventoryService
// Methods:
//   - createOrder(userId, items[]): Order | throws InsufficientInventory
//   - cancelOrder(orderId): void | throws OrderNotFound
//   - getOrderStatus(orderId): OrderStatus
 
// Implementation follows directly from LLD:
class OrderService {
    constructor(
        private readonly orderRepository: OrderRepository,
        private readonly paymentProcessor: PaymentProcessor,
        private readonly inventoryService: InventoryService
    ) {}
 
    async createOrder(userId: string, items: OrderItem[]): Promise<Order> {
        // Check inventory (from LLD)
        const availability = await this.inventoryService.checkAvailability(items);
        if (!availability.allAvailable) {
            throw new InsufficientInventoryError(availability.unavailableItems);
        }
 
        // Reserve inventory (from LLD)
        await this.inventoryService.reserve(items);
 
        try {
            // Create order entity (from LLD)
            const order = Order.create(userId, items);
            
            // Persist order (from LLD)
            await this.orderRepository.save(order);
            
            return order;
        } catch (error) {
            // Rollback reservation on failure (from LLD error handling)
            await this.inventoryService.release(items);
            throw error;
        }
    }
    
    // Other methods follow same pattern...
}

Feedback from Implementation to LLD

Implementation often reveals issues in the design:

Missing classes — Discovered need for helper classes or utilities
Wrong abstractions — Interface doesn't fit actual usage patterns
Performance issues — Design works but is too slow
Complexity surprises — Something assumed simple is actually complex

When this happens, update the LLD. Design documents should be living artifacts, not historical records.

The Iterative Nature of Design

Real software design is rarely a clean, linear progression from requirements through HLD to LLD to code. Instead, it's an iterative process where insights at each level feed back to refine earlier decisions.

Why Design Is Iterative:

Incomplete information — You can't know everything upfront
Changing requirements — Business needs evolve
Technical discoveries — Implementation reveals unforeseen challenges
Learning — Better solutions emerge as understanding deepens

Common Iteration Patterns

•LLD reveals HLD issues — Designing a service's internals shows the service has too many responsibilities. Split it in HLD.
•Implementation reveals LLD issues — Coding shows an interface is awkward. Revise the interface design.
•Testing reveals design flaws — Tests are hard to write. The design isn't testable. Refactor for dependency injection.
•Performance testing reveals architecture issues — System is slow. Need caching layer. Update HLD and add cache-related LLD.
•User feedback reveals requirement gaps — Missing feature. Update requirements, flow through HLD and LLD.

Embrace Iteration, But Plan Checkpoints

While iteration is inevitable and healthy, uncontrolled iteration is chaos. Establish checkpoints: 'We'll lock HLD after Sprint 2' or 'LLD review before implementation begins.' This balances flexibility with predictability.

When to Do LLD

Not every feature needs a formal LLD document. The investment in detailed design should be proportional to the complexity and risk of the work.

When Detailed LLD Is Essential:

Complex features — Multiple interacting classes, non-obvious algorithms
Core domain logic — Business-critical functionality that must be correct
Shared components — Libraries or modules used by multiple teams
Public APIs — Interfaces that external systems will depend on
High-risk changes — Modifications to critical or fragile areas

Invest in LLD When

•Feature is complex or novel
•Multiple developers involved
•Code will be long-lived
•Mistakes are expensive to fix
•Component has external consumers
•Performance is critical

Light-Touch LLD When

•Feature is simple and isolated
•Single developer working alone
•Code is throwaway/prototype
•Easy to refactor later
•Component is internal only
•Performance isn't critical

LLD Can Be Documentation

Sometimes you do LLD after implementation—documenting what you built for future maintainers. This 'as-built' documentation is valuable, especially for complex systems where the code alone doesn't convey intent.

LLD in Different Development Methodologies

How LLD is practiced varies by development methodology. Understanding these variations helps you adapt LLD practices to your team's process.

LLD Across Methodologies
Methodology	LLD Approach	Timing	Documentation Level
Waterfall	Comprehensive upfront design	Before implementation	Heavy, formal documents
Agile/Scrum	Just-in-time, per-sprint	During sprint planning	Light, evolving artifacts
XP	Emergent design, refactor often	Continuous, with implementation	Minimal, code as documentation
Lean	Last responsible moment	As late as feasible	Just enough, no more
DevOps/CI-CD	Incremental, feature flags	Per-feature, continuous	Automated via tests and specs

Agile and LLD

In Agile environments, LLD happens in smaller increments:

Sprint Planning — Rough LLD to estimate complexity and identify risks
Refinement — Detailed LLD for upcoming work items
Implementation — Design refined as code is written
Retrospective — Design decisions reviewed and documented

The key is balancing 'enough design to start' with 'flexibility to adapt.' Over-designing upfront wastes effort on decisions that may change. Under-designing leads to costly rework.

Collaboration in the Design Process

Software design is a collaborative activity. Effective LLD requires communication with stakeholders at various levels.

Key Collaborators for LLD

•Architects — Ensure LLD aligns with HLD. Validate that component design fits architectural constraints and patterns.
•Product Owners — Clarify requirements. Ensure LLD captures business rules correctly. Validate acceptance criteria.
•Fellow Developers — Review design for feasibility, clarity, and best practices. Catch issues before implementation.
•QA Engineers — Ensure design is testable. Identify edge cases and failure scenarios early.
•DevOps — Validate deployment, monitoring, and operational considerations are addressed in design.
•Security — Review for security implications. Ensure sensitive data handling is designed correctly.

Design Reviews

Formal or informal, design reviews are essential for quality LLD:

Before implementation — Catch issues when they're cheap to fix
Focused scope — Review one component or feature at a time
Prepared materials — Share diagrams and descriptions ahead of time
Constructive feedback — Focus on improvement, not criticism
Documented decisions — Record what was decided and why

Two Sets of Eyes

At minimum, have one other engineer review your LLD before implementation. Fresh eyes catch assumptions you didn't know you made, complexity you've grown blind to, and alternatives you didn't consider. This small investment prevents large rework.

Summary: LLD in the Design Ecosystem

We've explored how Low-Level Design fits within the broader software design process. Let's consolidate the key takeaways:

Key Takeaways

•Design is a multi-stage process — From requirements through HLD and LLD to implementation, each stage adds detail and refinement.
•Requirements flow to design — Every class should trace back to a user need. Design artifacts should connect to requirements.
•HLD provides context for LLD — Understand architectural constraints before designing component internals.
•LLD guides implementation — A good LLD translates directly to code structure, making implementation straightforward.
•Design is iterative — Insights at each level feed back to refine earlier decisions. Living documents beat historical records.
•Investment scales with complexity — Not everything needs formal LLD. Match effort to risk and complexity.
•Methodology affects practice — Waterfall does LLD upfront; Agile does it incrementally. Adapt practices to your context.
•Collaboration improves design — Reviews, discussions, and diverse perspectives catch issues early and improve quality.

What's Next:

Now that we understand how LLD fits in the overall process, we'll address a practical question: When should you focus on HLD versus LLD? The final page of this module provides guidance for recognizing which level of design thinking your current situation requires.

Page Complete

You now understand where LLD sits in the software design lifecycle, how it connects to requirements and architecture, how iteration refines design, and how collaboration improves outcomes. Next, we'll learn to recognize when to think at the HLD level versus the LLD level.