Practice: Design Top-K Trending Topics

Trending topics inspired by Twitter/X Trends and YouTube Trending: hashtag events on a partitioned stream, windowed counting with a count-min sketch and heavy-hitter heaps, trends ranked by velocity against a baseline, a precomputed top 50 per region in a cache, a replayable raw-event archive and bot filtering. A teaching design.

Build this system yourself on the canvas below. Add components, connect them, and run traffic simulations to see where your design holds up and where it breaks.

Functional requirements

  • Every post and search with a hashtag is recorded as an event.
  • Users can see the top trending topics for their region.
  • Trends reflect what is rising fast now, not what is always popular.
  • Trends refresh at least every minute.
  • Activity from bot accounts does not push a topic into trends.
  • Counts can be rebuilt from raw events after a bug.

Non-functional requirements

  • Trending tab latency: p99 < 200ms.
  • Freshness: a topic that starts rising appears in trends within 1 minute.
  • Posting is never slowed down by counting.
  • Scale: 50M daily users, 5K requests/s at peak, mostly events.
  • Availability: 99.9%; stale trends are acceptable, errors are not.
  • Approximate counts are acceptable for ranking.
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Phase 01: Thinking in Systems
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Components

User
CDN
Load Balancer
Server
Cache
Database
Blob Storage
Search Index
Queue
Worker
Rate Limiter
Service
API Gateway
Reverse Proxy
WebSocket Server
Third-party API

Inspector

Notes
Use clear names so your design intent is easy to understand.
Good
Name by business meaning
"Order API", "Restaurant Service"
Avoid
Generic names = zero signal
"Server 1", "API", "Queue"
A short description for each component makes feedback much better.
100%
Start by identifying:
  • Users & entry points
  • APIs & services
  • Databases & storage
  • Traffic flow & scale

Round 1 of 4 Architecture Design

Run a simulation to see results.

Time Remaining
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System Design Interview

What are the core functional requirements?
What are the key non-functional constraints?

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EVALUATE MODE

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A structured 4-module evaluation that mirrors how top companies assess system design candidates.

Architecture Design
Draw your system on the canvas. Define components, connections, and data flow.
FR & NFR Requirements
Answer functional and non-functional requirement questions about your design.
MCQ Round
Multiple choice questions testing your depth on the chosen system.
Tradeoff Analysis
Justify your design decisions and defend your architectural tradeoffs.
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Round 2 of 4
MCQ Round
Answer multiple-choice questions based on your design.

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My Evaluations

Your past evaluation sessions

Here’s a simple request flow that follows the expected layer order.

External User
→
Edge CDN → API Gateway → Load Balancer
→
Compute App Servers / Services
→
DataAccess Cache
→
Storage Database / Search Index
→
Async Queue → Worker

Tip: keep arrows moving forward through layers (Edge → Compute → Storage). Avoid sending storage back to compute.

Evaluation Instructions

Read the rules carefully before starting. The test auto-submits on refresh.

Before you start

  • Build your architecture on the canvas. The timer starts when you click Start Evaluation.
  • Don't forget to answer Functional Requirement and Non Functional Requirements.
  • When satisfied with your design, click Next to lock it and view the questions.
  • Please answer final step questions to complete the evaluation.

Dos

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  • Do include required components to maximize component coverage.
  • Do save a copy of your design if you want to keep it before submission.

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  • Don't switch app modes or open another tab while the evaluation is running.
  • Don't attempt to edit the design after clicking Next; the workspace will be locked.

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