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Amazon Web Services

DNS & Traffic Routing — Route 53

Resolve names to endpoints and shift traffic between regions using routing policies and health checks.

Route 53 is DNS with routing logic and health checking attached.

A telephone switchboard that knows where every caller is ringing from, which offices are open, and how busy each one is — and connects the call accordingly.

Key Concepts

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The record types worth knowing.
    A / AAAA   name -> IP address
    CNAME      name -> another name. NOT allowed at the zone apex.
    ALIAS      an AWS-specific record that DOES work at the apex,
               points at an ALB, CloudFront or S3 website, and is free
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The apex problem is a favourite question. example.com cannot be a CNAME — the DNS specification forbids it alongside the SOA and NS records. ALIAS exists to solve exactly that, and it resolves without an extra lookup.
example.com
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Routing policies are where the real content is.
    simple       one record, one answer
    weighted     split by percentage -- canary and blue/green
    latency      send each user to the lowest-latency region
    failover     primary, and a secondary when health checks fail
    geolocation  route by the user's country -- data residency
    geoproximity shift traffic toward a region by a bias value
    multivalue   several healthy IPs, basic client-side spread
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Weighted is how you release gradually.
    v1  weight 95
    v2  weight 5     -> move to 50/50, then 0/100, then delete v1
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Health checks drive failover. Route 53 probes an endpoint from several locations and withdraws the record when it fails, so DNS stops handing out a dead address.
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TTL decides how fast a change takes effect, and this is the limitation to state plainly:
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    TTL 300  -> resolvers may serve the old answer for 5 more minutes
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So DNS failover is minutes, not seconds. For faster failover you want a load balancer or a global accelerator, not DNS.
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It is also a registrar, so the domain, the zone and the records can live in one place, with the zone delegated via NS records.
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What the interviewer is probing.1. "Why can you not CNAME the apex, and what do you use instead?" Probing: a standard DNS question. Stalls: "You can." Moves up: the specification forbids a CNAME alongside the SOA and NS records at the apex; an ALIAS record solves it and resolves without an extra lookup.
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2. "How fast is DNS failover really?" Probing: the TTL limit. Stalls: "Immediate, with health checks." Moves up: bounded by the TTL, because resolvers keep serving the old answer until it expires — minutes, not seconds.
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3. "Which routing policy for a canary release?" Probing: the policies. Stalls: "Failover." Moves up: weighted — 95/5, then shift the weights, with no deployment change needed.
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4. "Latency routing picked a region that is not the closest. Why?" Probing: what latency routing measures. Stalls: "It is broken." Moves up: it follows measured network latency, not geographic distance, and the two often differ.
Use cases:route53dnsfailoverlatency-routinghealth-check