Approaching An LLD Interview In 45 Minutes

For a long time, Low Level Design interviews felt fake to me.

Not because the concepts were fake.

Because most explanations online sounded nothing like how engineers actually think while designing systems under pressure.

Everything looked too clean.

People jumped straight into:

• UML diagrams
• design patterns
• SOLID principles
• fancy abstractions

Meanwhile during actual interviews your brain is usually doing this:

wait...
what exactly am I even building?

That’s the real starting point.

Especially in a 45-minute LLD interview.

Because the biggest mistake people make is treating LLD like:

“write perfect production architecture.”

That’s not the interview.

The interview is mostly evaluating:

• structured thinking
• decomposition
• object modeling
• communication
• tradeoff awareness
• extensibility

Not whether you invented Netflix.

Once I realized that, LLD became dramatically less intimidating.

Most Candidates Fail Before Writing A Single Class

The biggest issue is rushing into code too early.

Interviewer says:

Design StackOverflow

and candidates instantly start writing:

class User {}

Huge mistake.

Because you don't even know:

• what features exist
• what constraints matter
• what entities are required
• what relationships exist
• what edge cases matter

You're basically coding blind.

LLD interviews are not coding-first problems.

They're modeling problems.

The implementation comes later.

My Mental Flow For LLD Interviews Now

My process now is roughly:

1. Clarify requirements
2. Identify entities
3. Define relationships
4. Design behavior
5. Handle edge cases
6. Write implementation
7. Refactor while speaking

That’s honestly most of the interview.

Not memorizing design patterns.

Step 1 — Clarify Requirements Before Touching Code

This step is massively underrated.

Most candidates skip it because they think:

“asking questions wastes time.”

Wrong.

Clarification prevents terrible assumptions.

Example:

Design StackOverflow

What does that even mean?

Ask:

• Can users post questions?
• Are answers editable?
• Are comments nested?
• Do we support voting?
• Can questions close?
• Do we need persistence?
• Is concurrency important?
• Is this in-memory only?

You're narrowing scope.

Interviewers usually intentionally keep requirements vague.

They're testing whether you can shape ambiguity.

That’s an engineering skill.

Not a coding skill.

Step 2 — Identify Core Entities

Once requirements stabilize, stop thinking about classes immediately.

Think about nouns first.

For StackOverflow-style systems, obvious nouns become:

• User
• Question
• Answer
• Comment
• Tag
• Vote

These are your entities.

Most LLD problems become easier once you extract the core nouns from the system description.

Example:

Parking Lot

immediately suggests:

• Vehicle
• ParkingSpot
• Ticket
• Floor
• Gate

BookMyShow

suggests:

• Movie
• Theater
• Seat
• Booking
• Payment

This step is basically object decomposition.

Step 3 — Relationships Matter More Than Classes Initially

This was a huge realization for me.

The important thing is usually not:

“what classes exist?”

The important thing is:

“how do they interact?”

For example:

Question HAS answers
Answer BELONGS TO question
Post HAS comments
User CREATES posts

That relationship mapping shapes your entire design naturally.

Once relationships become clear:

• inheritance becomes obvious
• composition becomes obvious
• ownership becomes obvious

Example:

abstract class Post {
    private String id;
    private String content;
    private User author;
}

Then:

class Question extends Post
class Answer extends Post

Now inheritance actually has meaning.

Without relationship thinking, inheritance becomes random.

Good LLD Is Mostly About Responsibility Separation

This clicked late for me.

Bad LLD usually happens when one class does everything.

Example beginner mistake:

class StackOverflow {
    // users
    // questions
    // answers
    // voting
    // comments
    // searching
    // authentication
}

This becomes a god object immediately.

Good design separates responsibilities naturally.

Example:

class User
class Question
class Answer
class Comment
class VotingService
class SearchService

Each object owns a specific concern.

That separation matters much more than memorizing design patterns initially.

Inheritance Should Feel Natural, Not Forced

This is another common mistake.

People force patterns because they memorized them.

Example:

Question extends Post
Answer extends Post

This makes sense because:

• both have author
• both have content
• both support voting
• both support comments

So shared behavior belongs naturally inside Post.

abstract class Post {

    private String content;
    private User author;
    private int votes;

    public void vote(int value){
        this.votes += value;
    }

}

That's clean inheritance.

Not artificial inheritance.

Encapsulation Matters More Than Fancy Architecture

Interviewers care a lot about:

• state protection
• behavior ownership
• preventing invalid operations

Example:

public void addAnswer(Answer answer){

    if(isClosed){
        throw new IllegalStateException();
    }

    answers.add(answer);

}

This is important.

Because behavior rules belong inside the object owning the state.

Not somewhere random outside.

That’s actual object-oriented thinking.

Exception Handling Quietly Reveals Engineering Maturity

This is one thing strong candidates do that weaker candidates ignore.

They think about invalid states.

Example:

• answering closed questions
• booking already reserved seats
• withdrawing insufficient balance
• deleting non-existent objects

Most beginners only model happy paths.

Good candidates model failure paths too.

And interviewers notice this immediately.

LLD Interviews Are Usually About Tradeoffs, Not Perfection

This is another huge mental shift.

People think:

“I need perfect architecture.”

You don't.

You need reasonable tradeoffs under time pressure.

Example:

Should comments support nesting?

You can literally say:

“To keep scope manageable in 45 minutes, I'll support only one-level comments.”

That’s perfectly acceptable.

Strong communication beats overengineering.

Service Classes Usually Emerge Naturally

Initially I forced service classes everywhere because tutorials did that.

Eventually I realized: services appear naturally when orchestration becomes necessary.

Example:

class StackOverflowPlatform {

    private Map<String, User> users;
    private Map<String, Question> questions;

}

This object coordinates:

• registration
• posting
• searching
• retrieval

That’s orchestration responsibility.

Now service layers actually make sense.

The Biggest LLD Mistake Is Designing For Infinite Future Scale

This happens constantly.

Someone gets asked:

Design a parking lot

and suddenly they're discussing:

• Kafka
• sharding
• distributed consistency
• Kubernetes
• microservices

Relax.

That’s not LLD anymore.

That’s drifting into HLD/system design.

LLD interviews are mostly evaluating:

• object modeling
• clean APIs
• relationships
• extensibility
• encapsulation
• maintainability

Not distributed architecture.

My Actual Time Breakdown For 45 Minutes

This is roughly how I manage time now:

First 5–7 Minutes

Requirements clarification.

Next 5–8 Minutes

Entity identification + relationships.

Next 10–15 Minutes

Core class design.

Next 10–15 Minutes

Implementation of key methods.

Last 5 Minutes

Edge cases + improvements + tradeoffs.

Without structure, people spend:

• 35 minutes coding
• 10 minutes panicking

That usually goes badly.

The Biggest Realization

Eventually I realized LLD interviews are not really testing:

“Can you write Java classes?”

They're testing:

“Can you model real-world behavior cleanly under ambiguity?”

That’s the actual skill.

And honestly once you stop obsessing over:

• design pattern bingo
• perfect UML diagrams
• textbook architecture

LLD becomes much more approachable.

Because at its core, most LLD interviews are just:

• identifying entities
• defining relationships
• assigning responsibilities
• protecting state
• handling edge cases
• communicating tradeoffs

That's really it.