How GitBook serves 30,000 sites with sub-second content updates

GitBook hosts 30,000 documentation sites on a single Vercel deployment, serving 120 million monthly page views. The engineering team solved multi-tenant cache invalidation by using Next.js tag-based caching with Vercel's `use cache` directive, enabling content updates to propagate globally within 300ms after a merge event. The system processes 40,000 cache invalidations daily without triggering broad purges across unrelated sites. A notable infrastructure challenge is that 41% of traffic now comes from AI crawlers, which create less predictable cache access patterns than human readers. Future work includes adaptive documentation that personalizes content per reader, adding further complexity to the caching model.