How would you design a web search engine like Google?

Components: Web Crawler (BFS crawl, politeness, URL frontier, deduplication), Indexer (inverted index - word to document IDs, index sharding by term or document), Query Processor (parse query, retrieve matching docs, rank), Ranking (PageRank for authority, TF-IDF for relevance, ML models). Scale: billions of documents, distributed crawling, index partitioning. Serving: query routing, result aggregation, caching popular queries. Freshness: continuous crawling, prioritize by importance. Features: autocomplete, spell correction, personalization. Challenges: spam detection, query understanding, real-time indexing.