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System Design Interview for Beginners: HLD vs LLD and a Simple Way to Start

By Preeti Verma

System design interviews scare people because there is no single right answer. That is also the good news: interviewers are not checking whether you reproduce a textbook diagram. They are checking whether you can think in an organised way about a problem with trade-offs.

HLD versus LLD

High-level design (HLD) is the big picture: which services and databases exist, how data flows between them, how it scales and what happens when parts fail. Typical prompts: design a URL shortener, a ride-booking system, a video streaming platform.

Low-level design (LLD) zooms into the code structure of one component: classes, interfaces, relationships and design patterns. Typical prompts: design a parking lot, an LRU cache, a snake and ladders game.

A rough rule: HLD is boxes and arrows, LLD is classes and methods. Many companies ask both, in separate rounds.

A framework for HLD

Use the same steps every time so you never freeze.

Clarify requirements. What must the system do (functional) and how well (scale, latency, availability)? Ask questions before drawing anything.
Estimate roughly. Requests per second, data per day, read to write ratio. Round numbers are fine. You only need to know whether you are designing for a laptop or a data centre.
Define the API. Two or three main endpoints, with inputs and outputs.
Sketch the data model. What you store, and which database type fits.
Draw the main design. Client, load balancer, services, database, cache.
Go deep on the hard part. Every problem has one or two hard parts. Spend your time there.
Name the trade-offs. Say what you gave up and why.

Worked example: a URL shortener

Requirements: users submit a long URL and get a short one; visiting the short URL redirects. Say it must handle one million requests a day, with far more reads than writes.

Estimate: one million a day is only a few requests a second on average, so a single database can cope at first. Say what changes at one billion a day: you will need caching and a partitioned store.

API: a POST to create a short link and a GET on the short code that redirects.

Data model: a table mapping short code to long URL, with a creation time. A key-value store fits well because lookups are always by code.

Design: a load balancer in front of stateless application servers, a cache for hot links, and the database behind it.

Hard part: generating unique short codes. Options: a counter converted to base 62 (simple, but needs coordination across servers), a random string with a collision check, or pre-generated ranges handed out to each server. Compare them honestly.

Trade-offs: cache hit rate versus memory, and whether redirects should be permanent or temporary (permanent lets browsers cache but gives you less analytics).

A framework for LLD

Clarify what the system must do and list the main nouns (these become classes).
Decide each class's single responsibility.
Define relationships: which class owns which.
Use a pattern only where it solves a real problem.
Walk through one or two scenarios to test your design.

Example: a parking lot with different pricing on weekdays, weekends and business hours. The pricing rule changes independently of the parking logic, so a Strategy for pricing lets you add a new rule without touching existing code.

Common mistakes

Jumping to a diagram before clarifying requirements.
Naming technologies as a substitute for reasoning. Saying "Kafka" is not a design. Say why a queue helps here.
Ignoring failure. What happens when a service or database is down?
Over-engineering. Do not start with microservices for a problem a single service handles.
Going silent. Talk through your thinking. The interviewer can only credit what they hear.

How to practise

Pick one prompt, set a 35-minute timer and explain it out loud to nobody, or to a recording. Then compare with a reference and note what you missed. Repeat with a different prompt each day.

Our Resources section includes 50 high-level and 50 low-level design questions with descriptions of what each covers, and the AI Coach can explain any of them in simple words. If you are preparing for a specific company, read how Goldman Sachs and JP Morgan candidates describe their design rounds.