HLD, System Design & Architecture Rounds: seemingly synonymous, but each demands a distinct set of expectations
A definitive guide to the three zoom‑levels of on‑site design interviews—what each round really tests, how to structure your answers, and the trade‑off stories that turn a “maybe” into a “strong hire.
1. Why the Distinction Matters
Recruiters often toss around “HLD,” “System Design,” and “Architecture” as if they were interchangeable. Yet in the panel debrief room each label signifies a different level of abstraction, depth, and business impact.
Mixing them up hurts candidates in two ways:
Misdirected prep time. Spending nights memorising TCP retransmission windows when the round expects a 20‑minute whiteboard sketch is wasted effort.
Misaligned conversation. Entering an Architecture round with a green‑field “let’s start fresh” mindset signals you don’t grasp real‑world legacy constraints that staff engineers juggle daily.
Understanding the intent behind each label lets you choose the correct framework, pacing, and vocabulary—and that is often the tie‑breaker between a “hire” and “no‑hire” at onsite.
Below Design questions are previously covered topics in this blog-
System Design of Airbnb Part1 - User, Listings, Search & Availability
System Design of Airbnb Part2 - Booking & Reservations, Payment, & Reviews & Ratings
System Design of Airbnb Part3 - Messaging, Notifications, Trust & Safety, and Customer Support
System Design of Airbnb User QnA post articles - Prevent Double Booking
System Design of Twitter - Most in depth article you could find
Check out my profile as a coach/mentor and feedback from people I mentor who got placed Meta, Google, Twitter, Tesla, Microsoft, Stripe, Walmart, Uber and many more:
🔗 I Got An Offer – Coach Profile
🔗 Meetapro – L4 to L8 System Design Interviews
🚀 Promotion: Prep Smarter With Mock Interviews And Career Guidance – Get $75 Off and Free Resume Review!
If you're aiming for top tech roles at companies like Google, Microsoft, Meta, or leading fintechs — TechInterview.Coach is your go-to platform for 1:1 mock interviews and coaching from engineers who've been on the other side of the table.
🎯 Get personalized help with:
System design
Coding interviews
Behavioral rounds
Career strategy
🧠 Coaches include professionals from FAANG, Paytm, Stripe, and more. I am also available on that Platform, visit my profile. Ramesndra Kumar
🎁 Special Offer: Use coupon 75OFFFIRSTMONTH to get $75 off your first month
👉 Sign up here
HLD Example:
System Design Example:
Architecture Example
2. 30‑Second Cheat‑Sheet — HLD vs System Design vs Architecture
Keep this grid on a sticky note; it tells you which hat to wear the moment you walk into the room.
3. High‑Level Design (HLD) Round
3.1 Canonical Interview Flow
2 min — Clarify Use‑Case & Constraints
Feature set? SLA? Single region or global?3 min — List Functional (FR) & Non‑Functional Requirements (NFR)
2 min — Call out Obvious Out‑of‑Scope Items (video upload, payments, etc.)
10 min — Draw the Big Boxes
Clients → API GW → Service Layer → Persistence / Cache5 min — Explain Data Flow & Protocols
3 min — Highlight Trade‑offs & Alternatives
Remaining Time — Q&A + Optional Mini Deep‑Dive
Clock‑Management Tip: If you’ve spent > 10 min on requirements in a pure HLD round, you’ve ceded precious drawing time.
3.2 Skills & Evaluation Matrix
3.3 Example Walk‑Through – “Design a URL Shortener”
Prompt
Build a service like bit.ly that converts long URLs into short ones and redirects users.
Sketch
Clients → Load Balancer → API Gateway
Write Path
API GW → Shortening Service
Generates 6‑char code (Base62), stores mapping in Relational DB (primary‑replica)
Publishes event to Kafka for async analytics
Read Path (Redirect)
API GW → Redirect Service
Redis cache lookup → fallback to DB → 301 redirect
Admin Path — dashboard + metrics
In an HLD round you can hand‑wave hash‑collision mitigation: “In practice we add a checksum; happy to dive deeper if required.” Interviewer usually moves on.
3.4 Preparation Playbook & Pitfalls
4. System Design Round
4.1 Canonical Interview Flow
5 min — Requirement Deep‑Dive (FR, NFR, anti‑requirements)
5 min — Back‑of‑Envelope (BOE) Capacity Estimation
TPS, storage, peak bandwidth, cache RAM5 min — API & Data Model Sketch
10 min — High‑Level Architecture
10 min — Deep Dive Subsystem (e.g., feed ranking, push / pull fan‑out)
5 min — Bottlenecks & Mitigations
5 min — Evolution / Future Work
Rhythm Hint: Unlike HLD, BOE math is mandatory; skipping it triggers a red flag.
4.2 Skills & Evaluation Matrix
4.3 Deep‑Dive Example – “Design Instagram Stories (24 h ephemeral)”
Requirements Excerpt
FR – upload story (photo/video <= 15 s), view stories ordered by follow graph, reactions.
NFR – 50 M DAU, P99 view latency < 300 ms, global availability, GDPR delete ≤ 30 s.
Out of Scope – ML ranking, ad‑insertion, billing.
BOE Summary
Peak stories uploaded / day ≈ 15 M × 5 stories = 75 M; @ 1 MB avg ⇒ 75 TB/day raw → 27 PB/yr (replicated ×3).
Hot cache: 20 % active users ⇒ 10 M story sets; store in Redis cluster ~40 GB.
HLD
Clients → CDN → Story Upload Svc → Chunk Storage (S3)
→ async event to Kafka → Fan‑out Worker → Redis Feed Cache
→ Edge GraphQL fetch story list.
Deep‑Dive: Feed Fan‑Out vs Fan‑In
Fan‑out‑on‑write for < 500 followers creators (90 % users).
Fan‑in‑on‑read for celebrities; story IDs resolved at read time.
Consistency: eventual (<1 s). Discuss Redis Sorted Set per user.
Bottlenecks
4.4 Preparation Playbook & Pitfalls
Master BOE primitives — bitrate ≈ (#req × payload) / seconds; storage ≈ QPS × size × retention.
Practice one deep‑dive per domain (cache, queue, DB, CDNs, consensus).
Teach‑back method: explain designs to a rubber duck; if the duck looks confused, simplify.
Pitfall: forgetting operational story: run‑books, dashboards, SLOs, canaries.
5. Architecture / Deep‑Dive Round
5.1 Canonical Interview Flow
5 min — Current Context Recap (often provided in prompt).
10 min — Pain‑Point Identification (latency spikes, cost blow‑up, on‑call pager fatigue).
10 min — Goal & Constraint Alignment (budget, compliance, freeze window).
15 min — Target State Architecture (diagram + component rationale).
10 min — Migration / Roll‑out Plan (phased, canary, rollback).
10 min — Failure Modes & Observability
10 min — Trade‑off Debate (interviewer “what if…?” scenarios).
5.2 Skills & Evaluation Matrix
5.3 Real‑World Example – “Re‑architect Payments for 100× Scale”
Current State – monolithic Java service behind F5 LB, MySQL master‑replica, TPS 2 k, P95 latency 300 ms, batch settlement nightly.
Pain‑Points – vertical DB scaling hitting limit, 40 % downtime during schema alters, on‑call burnt out.
Target State Overview
Split monolith into Auth, Ledger, Settlement, Webhook services.
Introduce Kafka for event sourcing; Postgres‑Citus shards for ledger; CockroachDB optional for global spanner‑like writes.
Idempotency via UUID‑v7 tokens, Sagas for multi‑service atomicity.
gRPC with deadline propagation + circuit breakers (Hystrix style).
Migration Plan
Operational Excellence
SLO: 3 nines availability, ≤ 200 ms P99.
Prometheus + Grafana dashboards: auth_latency, ledger_drift, settlement_lag.
Chaos test: kill Kafka broker, ensure retry w/backoff works.
5.4 Preparation Playbook & Pitfalls
Own war stories. Prepare 2–3 narratives (outage, migration, cost‑savings).
Risk ledger — have a table of risks → mitigation.
Pitfall: drawing “ideal” target but offering no migration detail—the surest no‑hire.
6. Head‑to‑Head Comparison
Role Mapping
SDE‑II / Mid‑Level – often just HLD.
Senior Engineer – combo HLD + System Design, sometimes mini Architecture probing.
Staff / Principal – at least one Architecture round; HLD is assumed baseline.
7. End‑to‑End Preparation Roadmap (8‑Week Plan)
8. Appendix
A. Universal HLD Checklist
Clients, Auth, LB, API GW, Service Layer, DB, Cache, Queue, Analytics
Failure modes: LB down, DB master down, network partition
Observability: metrics, logs, traces
B. Clarifying Questions Bank (excerpt)
“Is traffic global or region‑specific?”
“What P99 latency is acceptable on the critical path?”
“Should deletes propagate within seconds or minutes?”
C. Story Bank Template
Final Takeaway
Think of the three rounds as nested zoom layers: HLD sketches the skyline, System Design wires the metro beneath, and Architecture reroutes trains while the city stays awake. Mastering the shifting depth—and signalling why you picked each trade‑off—will make you stand out, no matter what label the recruiter puts on your calendar invite.
Good luck, and happy white‑boarding!