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How this question breaks down

1. 1TriesCore22 minStore twenty million past searches so that any prefix returns its ten most popular completions in microseconds, by compressing the trie and caching each busy node's top ten at build time.
2. 2ServingCore27 minAnswer 50,000 prefix requests a second inside a keystroke by caching popular prefixes at the edge, replicating a whole index on every server, and failing silently when anything is slow.
3. 3FreshnessCore26 minTurn a quarter of a billion searches a day into decayed scores, catch a breaking query within minutes, and ship a new index to every server without a pause or a bad suggestion.

Work through the parts in order and you’ve covered the full design. Each part is one topic.