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DNS finds the right destination, a CDN serves cacheable files from nearby edge locations, and cloud load balancers distribute remaining requests across healthy origin servers. Together, these layers prevent one server, data center, or network path from becoming a single point of failure. If an edge location is unavailable, anycast routing can send users to another site; if an origin is overloaded, a load balancer can shift traffic among backends; and if a response is already cached, the CDN can answer without contacting the origin at all.
That resilience is not automatic. Availability depends on sensible DNS TTLs, correct cache headers, health checks, redundant backends, certificate management, and a tested plan for purging or bypassing bad content. The sections below show what each layer does, how a request moves through them, and which failure modes each one can—and cannot—handle.
What happens when a browser opens a website?
A page load is a chain of lookups, routing decisions, and HTTP requests rather than a single connection to a server. The usual path is:
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- Edge selection: If the hostname is behind a reverse-proxy CDN, routing sends the connection to an edge point of presence.
- Cache check: The edge checks whether it has a fresh response for that URL and request.
- Origin request: On a miss, the CDN forwards the request through a load balancer or directly to an origin backend.
- Response delivery: The origin response travels back through the edge, which may store it for later requests, and then to the browser.
DNS does not carry the web page. It supplies a destination. The CDN and the cloud origin tier perform the delivery work after that destination has been selected.
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How does DNS direct my browser?
DNS translates a name such as www.example.com into one or more IP addresses. Your device normally asks a recursive resolver operated by an internet provider, enterprise, or public DNS service. If the resolver has no usable cached answer, it follows the DNS hierarchy from the root and top-level-domain servers to the domain’s authoritative name servers, then returns the result to your device.
Recursive and authoritative servers have different jobs
A recursive resolver performs lookups for clients and caches the answers. An authoritative name server is the source of truth for a domain’s zone records. Delegation connects the two: the parent zone identifies which name servers are authoritative, and those servers return records such as A, AAAA, CNAME, MX, and TXT.
What anycast changes
Managed DNS services commonly advertise the same service IP address from many facilities using anycast. Google Cloud DNS documents anycast delivery for managed zones, and Google Public DNS routes queries to a nearby anycast location. A query can therefore reach a geographically close, healthy site instead of one fixed machine. Anycast improves reachability and latency, but it does not make an incorrect record correct or instantly remove an old answer cached elsewhere.
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Every record has a time to live (TTL). Resolvers may continue returning a cached answer until that TTL expires, even after you edit the authoritative zone. A low TTL can make a planned migration visible sooner, but it creates more query traffic and does not revoke answers already cached under other policies. During an incident, assume some users will continue reaching the previous address until their resolver caches age out.
What does a CDN do?
A content delivery network is a distributed reverse proxy. It accepts requests for your hostname, chooses an edge location, and serves a cached response when one is available. Cloudflare describes its CDN as a globally distributed network that caches content in many data centers; static images, CSS, and JavaScript are generally cacheable, while dynamic HTML is not cached by default.
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Cache hit versus cache miss
On a cache hit, the edge returns the stored response without contacting your infrastructure. This lowers latency for the visitor and removes work from the origin. On a cache miss, the edge fetches the object from the origin, returns it to the client, and stores it only if the response and cache policy allow storage. The first request after an eviction or expiration is therefore slower than later requests.
Freshness, revalidation, and eviction
HTTP response headers such as Cache-Control, Expires, ETag, and Last-Modified control freshness. When an object becomes stale, an edge can revalidate it with an ETag or modification date instead of downloading the full body. Objects can also be evicted before their TTL because of capacity pressure or an explicit purge. A purge removes stored copies, but it does not repair an origin that is returning an error.
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Some CDN policies allow an expired object to be served while the edge revalidates it or while the origin is temporarily unreachable. Google Cloud CDN’s documented defaults, accessed in 2026, include a 3,600-second client TTL, a 3,600-second default cache TTL, an 86,400-second maximum TTL, and up to 86,400 seconds of serve-while-stale. These are documentation defaults, not a universal rule: your response headers, cache-mode settings, and product configuration can override them.
How does a cloud load balancer protect the origin?
A load balancer is the traffic director for dynamic or uncached requests. It maintains a pool of backend services or servers, checks their health, and selects a backend according to a policy such as capacity, locality, or connection load. If one backend fails its health check, the balancer stops sending it new traffic while healthy backends continue serving.
The request path in a managed cloud CDN
Google Cloud CDN documents a path in which a Google Front End receives the request, checks the Cloud CDN cache, and, on a miss, forwards through an external Application Load Balancer to origin backends. A cached response is sent directly from the front end. This arrangement lets the same public endpoint handle static cache hits, dynamic application traffic, and failover decisions.
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DNS-based versus HTTP load balancing
DNS-based load balancing returns an address selected by policy, such as geography or endpoint health. It is useful for directing new connections among regions, but resolver caching means a decision can persist until the DNS TTL expires. An HTTP or transport-level load balancer sees each connection or request and can react faster to backend health, but it must itself be deployed redundantly. Many architectures use both: DNS gets clients to a regional entry point, and a load balancer distributes traffic inside that region.
How do anycast and multiple regions handle an outage?
Anycast lets multiple locations announce the same IP prefix. Normal internet routing tends to send a client to a nearby announcement, while withdrawal or route changes can move traffic when a site is unavailable. Cloudflare’s 2026 reference architecture describes hundreds of cities worldwide, more than 405 Tbps of network capacity, and coverage placing 95% of the world’s Internet-connected population within 50 milliseconds. Those figures describe Cloudflare’s network claims, not a guarantee for every customer’s path or every cached object.
Redundancy works only when failure detection and routing boundaries are designed correctly. A CDN edge outage can be masked by another edge. A regional load-balancer outage requires another regional entry point or DNS policy. An origin database failure may still take the application offline even if the CDN and web servers remain reachable.
What a CDN can shield
- Loss of an individual edge machine or point of presence.
- Short traffic surges for cacheable files, because repeated requests terminate at edges.
- Some network-layer floods, when the provider’s traffic filtering and capacity absorb them.
- Origin overload caused by popular static assets that would otherwise be downloaded repeatedly.
What it cannot shield by itself
- A broken deployment that produces invalid HTML or incorrect API data.
- An unavailable database, payment service, or other uncached dependency.
- A DNS zone with incorrect delegation or expired registration.
- Every denial-of-service attack; protection depends on the provider, plan, traffic type, and configured controls.
Does a CDN replace hosting?
No. A CDN can serve stored responses, but it still needs an origin for cache misses, uncached dynamic requests, uploads, purges, and application logic. The origin may be a cloud virtual machine, container platform, object-storage bucket, or another hosting service. If all content is static and fully pre-published, an object-storage origin plus a CDN can function as the complete delivery stack; that is still hosting, just with little or no application server.
Keep the origin address private where possible and allow it to accept traffic only from the CDN or approved administrative networks. Otherwise, an attacker can bypass the CDN and exhaust the origin directly. Use separate hostnames or signed requests for administrative endpoints, and never cache responses containing another user’s private data.
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What is the difference between DNS, a CDN, and a cloud load balancer?
| Layer | Primary job | Decision or data it handles | Main limitation |
|---|---|---|---|
| DNS | Maps a hostname to an address or service endpoint | Zone records, delegation, TTL, and optional health or geography policy | Resolvers cache answers, so changes are not immediate |
| CDN | Serves cacheable responses from distributed edge locations | HTTP caching, revalidation, purge, TLS termination, and edge routing | Dynamic or private responses usually still require the origin |
| Cloud load balancer | Distributes connections or requests among origin backends | Health checks, backend capacity, locality, and balancing policy | It cannot fix application or database failures by itself |
| Cloud provider network | Supplies redundant compute, storage, and network regions | Regional placement, autoscaling, failover, and operational controls | Resilience depends on how many independent failure domains you use |
These are complementary layers, not interchangeable products. DNS chooses where to start, the CDN handles edge delivery, and the load balancer keeps the origin pool usable.
How should you configure the layers for an outage?
Build from the outside in so every dependency has a tested fallback.
- Delegate DNS to redundant authoritative servers. Confirm the registrar’s name-server records, publish A or AAAA records only for intended endpoints, and choose TTLs that match your change and failover window.
- Put the public hostname behind the CDN. Install the correct certificate, enable HTTP-to-HTTPS redirects deliberately, and verify that the origin hostname is not exposed in page source, DNS, or error messages.
- Define cache rules by content class. Give versioned assets long freshness periods; keep personalized pages, account data, checkout, and most APIs private or uncacheable. Honor response headers unless you have a documented exception.
- Attach a health-checked load balancer to multiple backends. Health checks should test an endpoint that proves the application can serve traffic, not merely that a process accepts TCP connections.
- Spread backends across failure domains. Use separate zones and, when the business requires regional continuity, a second region with replicated data and a deliberate traffic-switching policy.
- Set observability and alerts. Monitor DNS query failures, edge hit ratio, origin latency, 4xx and 5xx rates, health-check status, certificate expiry, and cache purges. Alert on user-visible symptoms, not only infrastructure metrics.
- Exercise the runbook. Test an origin shutdown, a bad release rollback, a cache purge, a DNS rollback, and a regional failover. Record the time each action takes and the caches or resolvers that may delay its effect.
Which cache and TTL choices prevent common incidents?
Version static assets instead of purging everything
Give a changed file a new URL, such as a build hash in its filename. Existing pages can continue using the old immutable object while new pages request the new one. This reduces the need for emergency global purges and avoids a thundering herd of simultaneous origin requests.
Keep HTML and APIs conservative
Shorter freshness windows or revalidation are safer for content that changes frequently. Mark user-specific responses with appropriate privacy directives and vary the cache key only on inputs that genuinely change the representation. A careless cache key can deliver one customer’s response to another.
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Serve-while-stale can keep a news page or documentation site available during a brief origin incident, but stale account balances or inventory can be harmful. Decide which URL classes may serve old data and which must fail closed.
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What failure modes should you troubleshoot first?
- Some users see the old site: Check DNS TTL, browser cache, and CDN freshness. A rollback may require waiting for resolver caches or using a lower-TTL record prepared in advance.
- Everyone receives an origin error: Inspect load-balancer health checks, firewall allowlists, application logs, and dependency status. Purging the CDN will usually make this worse by sending more requests to the failing origin.
- Only uncached URLs are slow: Compare edge hit ratio with origin latency and backend saturation. Add capacity or optimize the origin rather than extending cache time for private data.
- Changes do not appear after deployment: Check the response’s cache-control headers and the CDN cache key, then purge the exact path if necessary. Confirm that the new asset URL is referenced by the HTML.
- The origin is reachable directly: Review DNS records, security-group rules, and leaked hostnames. Restrict origin ingress and rotate exposed addresses or credentials.
- HTTPS fails after a DNS or CDN change: Verify that the certificate covers every hostname, that the CDN is presenting the intended certificate, and that the origin certificate requirements match the selected TLS mode.
How should providers be compared?
Price alone does not describe availability. Evaluate managed DNS and CDN platforms against the traffic and failure modes your site actually has.
| Comparison area | Questions to ask |
|---|---|
| DNS anycast and failover | Are authoritative servers distributed, and can health or geography policies be configured? |
| Edge geography | Are users near an edge location, and is capacity sufficient for regional spikes? |
| Cache control | Can rules honor headers, vary keys safely, revalidate, and purge by URL or tag? |
| Origin health | Are checks configurable, frequent enough, and capable of removing unhealthy backends? |
| Balancing policy | Can traffic be weighted, localized, capacity-aware, or shifted between regions? |
| DDoS and WAF controls | What traffic filtering is included, and which protections require extra configuration or cost? |
| Observability | Do logs and metrics show cache status, origin timing, health changes, and DNS errors? |
| Data residency and support | Can traffic or logs remain in required jurisdictions, and is support available during an incident? |
| Total cost | Include DNS queries, CDN requests and egress, load-balancer processing, origin compute, logging, purge operations, and failover capacity. |
What should you remember about keeping a site online?
Availability comes from independent controls working together: DNS with redundant authoritative service, CDN edges that absorb cacheable demand, health-checked load balancing for dynamic traffic, and origins distributed across failure domains. TTLs and cache headers determine how quickly a change propagates and how long an old response may remain visible. Design each layer for the failure it can actually handle, then test the complete path from a user’s resolver to the application and its data stores.
Frequently Asked Questions
Can DNS alone provide failover when a server goes down?
DNS can return a different address when health-aware policies detect a failure, but recursive resolvers may keep the previous answer until its TTL expires. It is therefore slower and less precise than a load balancer for per-request decisions.
Why did a CDN purge not remove a broken page everywhere?
A purge affects objects stored by the CDN, not browser caches, intermediary proxies, or DNS answers. Purges can also be scoped to a path, host, tag, or cache key, so verify the exact object and check response headers after the operation.
Should API responses ever be cached at the edge?
Only when the response is identical for every authorized caller or is safely partitioned by a complete cache key. Personal data, authentication responses, and rapidly changing state should normally be private, revalidated, or served directly from the origin.
What happens if the CDN provider has an outage?
A CDN outage can affect both cached and uncached requests because the public hostname usually terminates there. A prepared DNS bypass, an independently reachable origin, and tested certificates can provide an emergency route, but DNS caching means the switch will roll out gradually.
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