How would you design a search autocomplete/typeahead system?

Data structure: Trie for prefix matching, with frequency counts at nodes. Architecture: prefix trees in memory, sharded by prefix ranges, replication for availability. Query flow: client sends prefix, service returns top-k suggestions. Ranking: frequency, recency, personalization, trending. Updates: offline batch processing for global frequencies, real-time updates for user history. Optimization: precompute top-k at each node, limit tree depth. Scale: horizontal sharding, CDN edge caching for common prefixes. Features: spelling correction (edit distance), phrase suggestions. Response time critical (<100ms).