Design a Highly Available DNS Service

To design a highly available DNS service at Google's scale, I would start by defining the requirements: handling billions of queries daily with sub-millisecond latency globally. The architecture should be hierarchical. First, we implement Anycast routing for our authoritative name servers. This allows us to advertise the same IP address from multiple geographic locations, ensuring users are automatically routed to the nearest operational node, which drastically reduces latency and provides inherent DDoS protection. For redundancy, every zone must be replicated across at least three distinct availability zones or regions to prevent data loss during regional outages. Next, we address caching. Recursive resolvers will cache responses based on Time-To-Live (TTL) values. A smart strategy involves dynamic TTL adjustment; shorter TTLs for frequently changing records to ensure freshness, and longer TTLs for stable records to reduce load on authoritative servers. We must also implement aggressive cache poisoning protection via DNSSEC. Finally, regarding failure handling, we need automated health checks. If a region fails, traffic instantly shifts to the next closest Anycast point. This combination of Anycast, multi-region replication, and intelligent caching ensures the system remains highly available even under extreme load or partial failures.