What high level design encompasses in HLD - Deep Dive

Overview - What high level design encompasses

What is it?

High level design (HLD) is a broad plan that shows how a system will work overall. It breaks down the system into big parts and explains how these parts connect and interact. It does not go into tiny details but focuses on the main structure and flow. This helps everyone understand the system's purpose and layout before building it.

Why it matters

Without high level design, teams would struggle to agree on how a system should work. It prevents confusion and costly mistakes by setting clear goals and boundaries early. Imagine building a house without a blueprint; the result would be chaotic and unsafe. HLD ensures the system is scalable, maintainable, and meets user needs.

Where it fits

Before learning HLD, you should understand basic system concepts like components and data flow. After HLD, you can dive into low level design (LLD), which details how each part works internally. HLD sits between understanding requirements and writing detailed code or configurations.

Mental Model

Core Idea

High level design is the system's blueprint that outlines major parts and their connections without detailing every small step.

Think of it like...

It's like drawing a map of a city showing neighborhoods and main roads, but not every street or house.

┌─────────────────────────────┐
│       High Level Design      │
├─────────────┬───────────────┤
│  Components │  Interactions │
│  (Big parts)│  (Connections) │
├─────────────┴───────────────┤
│       Overall System Flow    │
└─────────────────────────────┘

Build-Up - 7 Steps

1

Foundation

Understanding System Components

Concept: Learn what system components are and why they matter.

A system is made of parts called components. Each component does a specific job. For example, in a car, the engine, wheels, and brakes are components. In software, components could be modules like user interface, database, or payment service.

Result

You can identify and name the main parts of any system.

Understanding components helps you see how complex systems are built from simpler pieces.

2

Foundation

Grasping System Interactions

Concept: Learn how components communicate and work together.

Components don't work alone; they send messages or data to each other. For example, the car's brake pedal tells the brakes to slow down. In software, the user interface sends requests to the server. These interactions define how the system behaves as a whole.

Result

You understand that connections between parts are as important as the parts themselves.

Knowing interactions prevents building isolated parts that don't fit together.

3

Intermediate

Defining System Boundaries

🤔Before reading on: do you think system boundaries include internal details or just external limits? Commit to your answer.

Concept: Learn to set clear limits on what the system includes and excludes.

System boundaries define what is inside the system and what is outside. For example, a bank app's boundary includes user login and transactions but excludes the user's phone hardware. Defining boundaries helps focus design and avoid scope creep.

Result

You can clearly say what your system will and will not do.

Understanding boundaries keeps the design focused and manageable.

4

Intermediate

Creating Component Diagrams

🤔Before reading on: do you think diagrams should show every detail or just main parts? Commit to your answer.

Concept: Learn to visually represent components and their connections.

Component diagrams use boxes for parts and arrows for interactions. They help teams see the big picture quickly. For example, a diagram might show a user interface box connected to a server box, which connects to a database box.

Result

You can draw simple diagrams that explain system structure.

Visuals make complex ideas easier to share and understand.

5

Intermediate

Specifying Data Flow and Interfaces

Concept: Learn how data moves between components and how they connect.

Data flow shows what information travels where. Interfaces define how components talk, like rules or formats. For example, a payment service might accept data in a specific format from the user interface. Clear data flow and interfaces prevent miscommunication.

Result

You can describe how data moves and how parts connect.

Knowing data flow and interfaces ensures components work smoothly together.

6

Advanced

Balancing Scalability and Simplicity

🤔Before reading on: do you think adding more components always makes a system better? Commit to your answer.

Concept: Learn to design systems that can grow without becoming too complex.

Scalability means the system can handle more users or data by adding resources. But too many parts can make the system hard to understand and maintain. High level design balances these by grouping related functions and planning for growth.

Result

You can plan systems that grow well without becoming messy.

Balancing growth and simplicity prevents future headaches and costly redesigns.

7

Expert

Anticipating Integration Challenges Early

🤔Before reading on: do you think integration problems only appear during coding? Commit to your answer.

Concept: Learn to foresee and plan for how different parts will work together in real environments.

Integration means combining components so they function as one system. Problems like mismatched data formats or timing issues often arise. High level design includes strategies like standard interfaces and clear contracts to reduce these risks before coding starts.

Result

You can reduce costly integration bugs by planning ahead.

Anticipating integration issues early saves time and improves system reliability.

Under the Hood

High level design works by abstracting the system into major components and defining their relationships. It uses models and diagrams to represent these parts and their interactions. This abstraction hides internal details, focusing on how components fit together and communicate. It guides developers and stakeholders by providing a shared understanding and a roadmap for building the system.

Why designed this way?

HLD was created to manage complexity in large systems. Early software projects failed due to unclear structure and poor communication. By separating concerns and focusing on big-picture design, teams can coordinate better and avoid costly rework. Alternatives like jumping straight into coding proved inefficient and error-prone, so HLD became a standard practice.

┌───────────────┐      ┌───────────────┐
│   Component   │─────▶│   Component   │
│      A        │      │      B        │
└───────────────┘      └───────────────┘
       │                      │
       ▼                      ▼
┌───────────────┐      ┌───────────────┐
│   Component   │◀─────│   Component   │
│      C        │      │      D        │
└───────────────┘      └───────────────┘

Myth Busters - 4 Common Misconceptions

Quick: Does high level design include detailed code logic? Commit yes or no.

Common Belief: High level design means writing detailed code or algorithms.

Reality: HLD focuses on big parts and their connections, not detailed code or logic.

Why it matters: Confusing HLD with detailed design wastes time and causes confusion among team members.

Quick: Is high level design only for big systems? Commit yes or no.

Common Belief: Only large systems need high level design; small projects can skip it.

Reality: Even small systems benefit from HLD to clarify structure and avoid mistakes.

Why it matters: Skipping HLD can lead to unclear goals and wasted effort, even in small projects.

Quick: Does high level design guarantee a perfect system? Commit yes or no.

Common Belief: Once HLD is done, the system will have no problems.

Reality: HLD reduces risks but does not guarantee perfection; issues can still arise during detailed design and coding.

Why it matters: Overconfidence in HLD can cause teams to overlook testing and refinement.

Quick: Is high level design a one-time task? Commit yes or no.

Common Belief: HLD is created once and never changed.

Reality: HLD evolves as requirements change and new insights emerge.

Why it matters: Treating HLD as fixed can make the system inflexible and outdated.

Expert Zone

1

HLD often balances competing goals like performance, security, and cost, requiring tradeoffs that are not obvious at first glance.

2

Experienced designers know that clear interface definitions in HLD prevent most integration bugs later in development.

3

HLD documents serve as communication tools across diverse teams, so clarity and simplicity often matter more than technical perfection.

When NOT to use

HLD is less useful for very small or throwaway projects where speed matters more than structure. In such cases, lightweight or agile approaches focusing on quick prototyping may be better.

Production Patterns

In real systems, HLD is used to create component diagrams, define APIs, and set system boundaries. It guides team roles and responsibilities and helps plan for scaling and fault tolerance.

Connections

Software Requirements Specification

HLD builds directly on requirements by translating them into system structure.

Understanding requirements deeply helps create a meaningful and accurate high level design.

Project Management

HLD informs project planning by defining scope and major deliverables.

Knowing HLD helps managers allocate resources and schedule tasks realistically.

Urban Planning

Both involve designing large systems by dividing space or functions into zones and connections.

Seeing HLD like city planning reveals the importance of clear zones and smooth connections for system health.

Common Pitfalls

#1 Including too much detail in high level design.

Wrong approach:

Designing every function and line of code in the HLD document.

Correct approach:

Focus on major components and their interactions, leaving details for low level design.

Root cause:

Misunderstanding the purpose of HLD as a broad overview rather than detailed instructions.

#2 Ignoring system boundaries and scope.

Wrong approach:

Trying to design features outside the system's intended limits.

Correct approach:

Clearly define what the system includes and excludes before designing components.

Root cause:

Lack of clarity on project goals and scope leads to wasted effort and confusion.

#3 Not updating HLD when requirements change.

Wrong approach:

Using the original HLD without revisions despite new needs.

Correct approach:

Regularly revise HLD to reflect changes and keep the design relevant.

Root cause:

Treating HLD as a fixed document rather than a living guide.

Key Takeaways

High level design is a broad blueprint showing major system parts and their connections without detailed code.

It helps teams understand the system structure, set clear boundaries, and plan for growth and integration.

Good HLD balances simplicity and scalability to avoid complexity and future problems.

HLD is a living document that evolves with requirements and guides detailed design and development.

Confusing HLD with detailed design or skipping it altogether leads to costly mistakes and miscommunication.

Practice

1. What does a high level design (HLD) primarily show in a system?

easy

A. The user interface colors and fonts

B. The main components and how they connect

C. The detailed database schema

D. The exact code for each module

Solution

1. Step 1: Understand the purpose of HLD

   HLD focuses on the big picture, showing main parts and their relationships.

2. Step 2: Differentiate from detailed design

   Detailed code or UI specifics are not part of HLD but come later in design phases.

3. Final Answer:

   The main components and how they connect -> Option B

4. Quick Check:

   HLD = Big parts and connections [OK]

Hint: HLD = big parts + connections, not code or UI [OK]

Common Mistakes:

• Confusing HLD with detailed design
• Thinking HLD includes UI styling
• Assuming HLD shows exact code

2. Which of the following is a correct element to include in a high level design document?

easy

A. Component diagrams showing system parts

B. Detailed API code snippets

C. Exact SQL queries for database

D. Pixel-perfect UI mockups

Solution

1. Step 1: Identify typical HLD contents

   HLD includes diagrams of components and their interactions, not code or UI details.

2. Step 2: Eliminate incorrect options

   Code snippets, SQL queries, and UI mockups belong to detailed design or implementation.

3. Final Answer:

   Component diagrams showing system parts -> Option A

4. Quick Check:

   HLD = component diagrams [OK]

Hint: HLD shows diagrams, not code or UI details [OK]

Common Mistakes:

• Including code snippets in HLD
• Confusing HLD with low-level design
• Adding UI mockups in HLD

3. Given a high level design showing components A, B, and C connected in sequence, what is the most likely data flow?

medium

A. Data flows from A to B to C

B. Data flows from C to B to A

C. Data flows randomly between components

D. No data flows between components

Solution

1. Step 1: Analyze component connections

   Sequence connection means data moves in order from first to last component.

2. Step 2: Determine flow direction

   Data flows from A to B to C as per the sequence shown.

3. Final Answer:

   Data flows from A to B to C -> Option A

4. Quick Check:

   Sequence = A -> B -> C data flow [OK]

Hint: Follow arrows in sequence for data flow direction [OK]

Common Mistakes:

• Reversing data flow direction
• Assuming random data flow
• Ignoring component connections

4. A high level design diagram shows a database connected directly to the user interface without any backend service. What is the main issue here?

medium

A. Database should connect only to frontend

B. Too many components included

C. Missing backend layer to handle business logic

D. User interface should connect directly to database

Solution

1. Step 1: Identify missing architectural layer

   Backend services handle logic and security between UI and database.

2. Step 2: Understand why direct UI to database is bad

   Direct connection risks security and breaks separation of concerns.

3. Final Answer:

   Missing backend layer to handle business logic -> Option C

4. Quick Check:

   Backend needed between UI and database [OK]

Hint: UI never connects directly to database [OK]

Common Mistakes:

• Thinking UI can safely connect to database
• Ignoring backend role
• Confusing component count with design quality

5. When creating a high level design for a scalable web application, which of the following should be included?

hard

A. Detailed SQL queries for data retrieval

B. Exact CSS styles for responsive design

C. User passwords and encryption keys

D. Components like load balancers, web servers, and databases with their interactions

Solution

1. Step 1: Identify scalable system components

   Load balancers, web servers, and databases are key parts for scalability.

2. Step 2: Exclude irrelevant details

   CSS styles, SQL queries, and sensitive data are not part of HLD.

3. Final Answer:

   Components like load balancers, web servers, and databases with their interactions -> Option D

4. Quick Check:

   HLD = scalable components + interactions [OK]

Hint: Include scalable components, exclude code and secrets [OK]

Common Mistakes:

• Adding UI styles in HLD
• Including sensitive data in design
• Confusing HLD with implementation details