DNS turns a stable name into a usable destination#
Applications refer to api.example.com; network connections need an address or another resolvable endpoint. A recursive resolver finds or reuses the answer, consulting authoritative DNS when its cache cannot satisfy the query.
An A record maps a name to IPv4, AAAA maps to IPv6, and CNAME aliases one name to another. For a general System Design interview, the architectural role and caching behavior matter more than memorizing every record type.
TTL makes DNS fast and changes slow#
A record's TTL tells resolvers how long it may be cached. Longer caching reduces repeated resolution work; shorter caching can let planned changes become visible sooner. Existing cached answers do not vanish when authoritative data changes, so propagation is not instantaneous.
Three deployment regions
A DNS provider can answer users in North America, Europe, and Asia with region-appropriate endpoints using geography, measured latency, policy, and health signals available to that provider. The answer can steer a connection toward a region.
DNS is not a per-request L7 load balancer. Resolvers and clients cache answers, TTL controls how long they can remain, and health-aware behavior depends on the routing infrastructure and its checks.
DNS failover has a cache boundary#
If an endpoint becomes unhealthy, authoritative routing can stop returning it, but clients may continue using a cached answer until it expires. DNS also cannot inspect an HTTP path or choose a backend for every application request. Pair it with regional entry points and health-aware serving infrastructure.
- Claiming DNS routes every individual request.
- Assuming record changes propagate instantly.
- Confusing a resolver or authoritative service with an L7 load balancer.
- Spending interview time on packet fields when the decision is regional routing.
Related practice problems
Use the manifest-backed prompts below to apply this lesson in a complete design.
- RFC 1034: Domain Names - Concepts and FacilitiesIETF / RFC Editor