Problem Statement

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Google Search - Problem Statement

Overview

Design a web search engine that crawls, indexes, and searches billions of web pages with sub-second response times, relevance ranking, and personalized results. The system must handle billions of queries daily while maintaining freshness and accuracy.

Functional Requirements

Core Search Features

  • Web Search: Search across billions of web pages
  • Query Processing: Handle natural language queries, typos, synonyms
  • Result Ranking: Rank results by relevance using PageRank and ML
  • Instant Results: Return results in <500ms
  • Autocomplete: Suggest queries as user types
  • Spell Correction: Detect and correct misspelled queries
  • Rich Snippets: Show preview text, images, structured data

Crawling and Indexing

  • Web Crawling: Discover and fetch billions of web pages
  • Content Extraction: Parse HTML, extract text and metadata
  • Index Building: Create inverted index for fast search
  • Freshness: Re-crawl popular pages frequently
  • Duplicate Detection: Identify and handle duplicate content

Advanced Features

  • Image Search: Search images by content and metadata
  • Video Search: Search video content with transcripts
  • News Search: Real-time news with recency bias
  • Local Search: Location-based results with maps
  • Knowledge Graph: Entity recognition and structured answers
  • Personalization: Customize results based on user history

Search Quality

  • Relevance: Most relevant results first
  • Diversity: Show variety of sources and perspectives
  • Freshness: Recent content for time-sensitive queries
  • Authority: Prioritize authoritative sources
  • Safety: Filter spam, malware, explicit content

Non-Functional Requirements

Performance

  • Query Latency: <500ms for 95% of queries
  • Indexing Latency: Index new pages within 24 hours
  • Crawl Rate: 100 billion pages per month
  • Throughput: Handle 100K queries per second
  • Availability: 99.99% uptime

Scale

  • Web Pages: Index 100 billion+ web pages
  • Index Size: 100+ petabytes of index data
  • Daily Queries: 8 billion queries per day
  • Concurrent Users: 1 million concurrent users
  • Geographic Distribution: Serve users globally

Quality

  • Precision: >90% of top 10 results relevant
  • Recall: Find all relevant pages
  • Spam Detection: Block >99% of spam pages
  • Freshness: 80% of index updated within 24 hours

Key Challenges

1. Scale of the Web

  • 100 billion+ web pages to crawl and index
  • 1.5 billion websites
  • New pages created every second
  • Need distributed crawling and indexing

2. Query Understanding

  • Natural language queries
  • Ambiguous terms
  • Typos and misspellings
  • Intent detection (informational, navigational, transactional)

3. Ranking Quality

  • Billions of pages match most queries
  • Need to rank by relevance
  • Balance freshness, authority, diversity
  • Personalization without filter bubbles

4. Real-time Requirements

  • Sub-second query response
  • Fresh results for breaking news
  • Instant autocomplete
  • Low latency globally

Success Metrics

User Metrics

  • Query Success Rate: 85%+ users find what they need
  • Click-Through Rate: 60%+ queries result in click
  • Time to Success: <30 seconds average
  • Return Rate: 90%+ users return within 30 days

Technical Metrics

  • Query Latency: p95 <500ms, p99 <1s
  • Index Coverage: 90%+ of web indexed
  • Index Freshness: 80%+ updated within 24h
  • Crawl Efficiency: 95%+ successful crawls

Business Metrics

  • Market Share: #1 search engine globally
  • Daily Active Users: 1 billion+ DAU
  • Revenue per Search: $0.50 average (ads)
  • Ad Click-Through Rate: 3-5% of searches

This problem requires handling massive scale, understanding natural language, ranking billions of results, and delivering sub-second responses - making it one of the most complex system design challenges.