Design a Real-Time AI Search Answer Engine
Last updated: September 4, 2025
Quick Overview
Build a system that answers questions about events that happened 30 minutes ago. Cover real-time web crawling, event detection, source prioritization, cache invalidation, and LLM integration.
Perplexity
System Design
Software Engineer
Perplexity
September 4, 2025Software Engineer
System Design Round
System Design
Hard
12
4
3,567 solved
Build a system that answers questions about events that happened 30 minutes ago. Cover real-time web crawling, event detection, source prioritization, cache invalidation, and LLM integration.
This is the canonical Perplexity system design question. It tests the full stack of AI-powered search from crawling to generation.
What the Interviewer Expects
- Design a real-time web crawling pipeline with freshness guarantees
- Implement hybrid retrieval combining BM25 and dense embeddings
- Address the freshness versus quality trade-off with concrete strategies
- Design the LLM integration layer with streaming token delivery
- Discuss cache invalidation for time-sensitive queries
Key Topics to Cover
Real-time web crawling
RAG pipeline architecture
Cache invalidation
LLM serving
Source freshness ranking
How to Approach This
- Start by clarifying functional and non-functional requirements with the interviewer.
- Estimate the scale: QPS, storage, bandwidth. This drives your design decisions.
- Draw a high-level architecture first, then deep dive into 1-2 critical components.
- Discuss trade-offs explicitly (e.g., consistency vs availability, SQL vs NoSQL).
- Address failure scenarios, monitoring, and how the system handles 10x traffic spikes.
Possible Follow-up Questions
- How do you detect when a cached answer is stale?
- What is your strategy for sources that are paywalled?
- How do you handle contradictory sources?
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Practice System Design ProblemsSample Answer
Requirements
Functional Requirements
- Real-Time Web Crawling: Continuously crawl the web to detect events that happened within the last 30 minutes.
- Event Detection: Identify and extract relevant e...
Capacity Estimation
Assuming Perplexity expects to handle up to 1 million queries per day:
- Peak Load: 12 queries per second.
- Crawling Frequency: Let's assume we crawl 10,000 pages every minute to identify new...
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