Perplexity system design interview

Round 4 of 7 · 3 questions reported

See the whole loop
Length
60 min
Format
Design discussion
They grade
Distributed systems for search and AI serving: RAG, LLM serving limits, semantic caching, real-time crawling
Heads-up
Justify storage and architecture choices from access patterns.

Questions Perplexity asks in this round 3

  • Design durable execution for long-running agent queriesAI systemAn answer service runs each query in memory, but agentic queries can now take hours. Redesign it so a query keeps running and finishes after the browser disconnects, resumes after…
    Last asked Sep 2026
  • Design a top-K popular items systemSystem designReport the K most viewed or played items over sliding windows (last minute, hour, day) from a firehose of billions of events per day.
    Possibly retired
  • Design typeahead suggestionsSystem designSuggest completions as the user types into a search box, within tens of milliseconds, ranked by popularity and fresh with trending queries.