How we cut CDN metadata lookup latency by 91%

Vercel redesigned how its CDN retrieves routing metadata for deployments, replacing separate per-path cache objects with bounded, indexed metadata shards that warm the cache for many routes at once while allowing individual records to be found through binary search without parsing entire files. The change addresses frequent cache misses in large, regularly deployed projects, where new deployments previously generated fresh metadata keys for potentially hundreds of thousands of paths. Production testing identified roughly 200 KB as the best shard size, balancing transfer costs, process-level cache behavior, and regional cache hit rates; the approach reduced P99 metadata lookup latency from 215.8 ms to 19.1 ms and average latency by 79%. Vercel validated the migration through offline comparisons and feature-flagged shadow mode, which also exposed a rare legacy path-encoding bug involving split emoji. Removing now-redundant per-path metadata build work saved about 16.6 seconds per deployment, making deployment steps about 10% faster overall and an estimated 25% faster for metadata-heavy projects, while preserving the existing Build Output API contract.

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