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Clarify requirements before you design — kept separate from your stage notes.

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Search Engine System Design

Visual Problem Diagram

Scenario

Crawlers discover 10B pages but users expect Google-quality results in < 200 ms—the index cannot fit on one machine and PageRank cannot run on every query. The interview splits crawl/index, ranking, and serving with honest freshness vs cost tradeoffs.

Design a web search engine that crawls pages, builds an index, ranks results, and serves queries at low latency. Scale forces distributed indexing and two-phase query processing—retrieval then ranking.

You should support crawling, indexing, ranking, query serving, and snippets. Be ready to explain inverted indexes, sharding, and offline vs online ranking.

Constraints

Functional

Crawl web, build inverted index, rank, serve queries, snippets, autocomplete optional

Non-functional

< 200 ms p95 query, index billions of pages, high crawl throughput, degrade rank under load

Scale

10B+ pages, 100K QPS search peak, petabyte index, continuous crawl

Stages ahead

1Requirement Analysis

2API Design

3High-Level Design

4HLD Extensions

5Trade-offs

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