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Vishnu
Vishnu

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Why Most Engineers Struggle With Low-Level Design Interviews (And How to Actually Practice)

#lld #systemdesign #interview #programming

Why Most Engineers Struggle With Low-Level Design Interviews (And How to Actually Practice)

Low-Level Design (LLD) interviews are weird.

Most engineers walk in knowing:

  • OOP concepts
  • SOLID principles
  • Design patterns

Yet many still struggle to explain a clean design under pressure.

I've been there.

After preparing for and discussing LLD interviews with multiple engineers, I realized the problem isn't lack of knowledge — it's lack of practice in the right way.

This article breaks down:

  • why LLD interviews feel harder than they should
  • what interviewers actually evaluate
  • and how to practice LLD so it sticks

The core problem: We prepare LLD the wrong way

Most LLD prep looks like this:

  • Read OOP theory
  • Memorize SOLID principles
  • Skim design pattern examples
  • Watch a few walkthrough videos

This builds awareness, but not skill.

In interviews, candidates usually fail at:

  • structuring their approach
  • identifying responsibilities cleanly
  • explaining trade-offs
  • evolving a design incrementally

LLD interviews are not about dumping knowledge — they're about how you think.

What interviewers actually look for in LLD

Across companies, interviewers tend to evaluate:

1. Problem understanding

  • Do you ask clarifying questions?
  • Do you understand scope and constraints?

2. Object modeling

  • Can you identify the right entities?
  • Are responsibilities well separated?

3. Design structure

  • Do relationships make sense?
  • Are you over-engineering or under-engineering?

4. Extensibility & trade-offs

  • Can your design evolve?
  • Can you explain why you chose this approach?

Notice something?

None of this is about "remembering the correct pattern".

A practical way to approach any LLD interview

A reliable mental model that works across problems:

1. Clarify requirements first

Before touching diagrams or code:

  • Ask about use cases
  • Identify constraints
  • Clarify what's in and out of scope

Jumping straight to design is a red flag.

2. Identify actors and core entities

Extract nouns from the problem statement:

  • Users
  • Systems
  • Objects

Then assign clear responsibilities to each.

This step alone prevents most "god object" designs.

3. Design class diagrams

Use diagrams to:

  • define relationships
  • decide ownership
  • choose composition vs inheritance

This is where good designs become visible.

4. Translate design into code

Your design should:

  • map cleanly to code
  • feel implementable
  • avoid hard-coding behaviors

If your design can't be expressed cleanly in code, it needs refinement.

5. Review and iterate

Strong candidates:

  • explain trade-offs
  • point out limitations
  • suggest improvements

LLD is about reasoning, not perfection.

Why "correct answers" don't exist in LLD

One important mindset shift:

There is rarely a single correct design in LLD.

Instead, there are:

  • reasonable trade-offs
  • better and worse abstractions
  • context-dependent decisions

Interviewers want to see:

  • clarity of thought
  • justification of choices
  • ability to adapt

Practicing only "final solutions" misses this entirely.

The missing piece: Interview-style practice

Here's what most prep resources lack:

  • Practicing the entire flow
  • Getting feedback on why a design works or doesn't
  • Iterating on designs, not just reading them

While preparing, I ended up building a visual-first LLD practice platform to mirror how interviews actually happen.

If you're curious, you can explore it here:
👉 https://www.lowleveldesignmastery.com

On top of in-depth concept explanations, there's an interactive LLD playground that guides you through the entire interview flow:

  • Pick a problem
  • Clarify requirements
  • Draw class diagrams
  • Write code
  • Get AI feedback + detailed solutions

The playground has 30+ problems across difficulty levels, with solutions available in Python, Java, JavaScript/TypeScript, C++, and C# — each with detailed explanations.

There's also an interactive playground to practice LLD problems end-to-end:
👉 https://www.lowleveldesignmastery.com/playground

Playground

Example problems to get started:

👉 Explore all concepts and problems

How to know if your LLD prep is working

Ask yourself:

  • Can I explain my design clearly without rushing?
  • Can I justify why I chose composition over inheritance?
  • Can I evolve the design when requirements change?
  • Can I defend trade-offs calmly?

If the answer is "yes", you're on the right track.

Final thoughts

Low-Level Design interviews aren't about brilliance.
They're about structured thinking, communication, and practice.

Stop optimizing for memorization.
Start optimizing for design reasoning.

That's what actually moves the needle.

👋 Feedback welcome

If you've interviewed recently:

  • What part of LLD did you struggle with?
  • What helped you improve?

Happy to learn from others' experiences.

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