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A content delivery network (CDN) speeds application content by placing edge servers near users. A browser first reaches a nearby CDN point of presence (POP); if the requested response is cached there, the edge returns it without contacting your origin. That removes a long network round trip and repeated application work. On a cache miss, the edge fetches the response from the origin, returns it, and may store it for the next request according to your cache key, TTL, and response rules.
The request path, from hostname to response
- DNS and connection: Your application hostname resolves to the CDN. The client connects to an edge location selected by latency, routing policy, or provider network design.
- Cache-key evaluation: The edge derives a key from the URL and any configured dimensions, such as query strings, cookies, headers, hostnames, device variants, or authorization state.
- Hit or miss decision: The edge checks its cache for that key and verifies freshness and eligibility.
- Response: A hit is returned immediately. A miss is fetched from the configured origin, then returned to the client and stored only if the cache rules permit it.
Amazon CloudFront describes DNS routing a request to the POP that can best serve the viewer, typically the nearest POP in latency terms. Google Cloud describes a cache hit as the edge finding a cached response and sending it directly to the user. The exact POP and path can change with network conditions, provider routing, and client location.
What happens on a cache hit?
The edge looks up the object by its cache key, checks freshness, and sends the stored headers and body to the client. The origin does not run application code, query a database, or transfer the object for that request. This is the fast path: distance to the user is short and origin processing is removed.
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The edge requests the object from the origin. After receiving a valid response, it can store the object for subsequent requests. The first viewer still pays the origin-fetch latency, so a CDN does not make an uncached object instant. Misses are normal after deployment, expiry, purge, or a cold cache in a low-traffic region.
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Tiered caching
Many providers can place an upper-tier cache between local edges and the origin. A local miss asks the upper tier first; if that tier has the object, the request avoids a direct trip to a distant origin. This reduces origin requests and bandwidth for globally distributed traffic, particularly when each local POP sees too few requests to warm independently.
Why edge caching reduces latency and origin load
Without a CDN, users in every geography repeatedly traverse the path to one origin region. That path can include congested public links and a long physical distance. With a CDN, cacheable copies are distributed around the provider’s edge footprint. A hit removes the repeated long-haul segment and the origin’s repeated storage, template, database, or computation work.
Google Cloud’s documentation states that a cache hit shortens round-trip time and saves the origin from processing the request. Cloudflare’s reference architecture frames the broader CDN goal as reducing latency while improving performance, availability, redundancy, security, and costs. Those benefits are mechanisms, not a guaranteed percentage improvement: the result depends on user geography, object size, cacheability, origin speed, traffic mix, and configuration.
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| Request or content | Typical CDN treatment | Important checks |
|---|---|---|
| JavaScript, CSS, images, fonts, downloads | Strong cache candidates | Use long freshness with versioned filenames or controlled purge. |
| Video and other large media | Often cached at the edge | Confirm range requests, bandwidth pricing, and storage behavior. |
| Public HTML | Cacheable when identical or predictably varied | Keep cookies, authorization, and personalization out of the key unless deliberately designed. |
| Public GET API responses | Cacheable when freshness permits | Define TTL, authorization behavior, query-string policy, and invalidation. |
| Personalized pages or account data | Usually bypassed or narrowly keyed | Prevent one user’s response from being served to another. |
| Writes: POST, PUT, PATCH, OPTIONS, DELETE | Usually sent to origin | CloudFront documents these methods as going directly to the origin rather than regional edge caches. |
A CDN accelerates delivery around application computation; it does not replace your application, database, queue, or write path. Even a GET can miss or bypass when authorization headers, cookies, response cache-control, or provider rules make sharing unsafe.
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Cache keys determine reuse
A cache key that varies on every tracking query parameter, cookie, or header creates many small objects instead of one reusable object. Normalize parameters that do not change the response, and vary only on dimensions that genuinely affect it. Conversely, omitting a response-changing dimension can serve incorrect or private content, so correctness comes before hit ratio.
TTL, revalidation, and invalidation
Long TTLs increase hit rates and reduce origin traffic but can leave old content visible. Short TTLs improve freshness but create more origin requests. Use immutable, versioned asset names for deployments where possible; use purge or invalidation for urgent changes. If your provider supports stale-while-revalidate, it can serve a still-usable object while refreshing it, but verify the exact directive and platform behavior.
Routing, connections, and modern protocols
Anycast, DNS, and provider backbones
Distributed ingress and DNS steer clients toward a suitable edge. After ingress, a provider’s private backbone and regional cache hierarchy can avoid congested public routes. “Nearest” is normally nearest by network latency, not necessarily geographic distance, and the selected location can change.
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HTTP/2 multiplexes requests over a connection and allows reuse, reducing repeated setup. HTTP/3 runs over QUIC and can reduce transport head-of-line effects on lossy networks. TLS 1.3 can shorten secure-connection setup. These gains require support from the client, CDN, and network; a protocol label alone does not guarantee a faster page.
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How to put a CDN in front of an application
- Inventory request classes: Separate immutable assets, public HTML, cacheable API GETs, personalized reads, and writes.
- Choose the origin: Configure the application, object store, load balancer, or multi-cloud endpoint and confirm health checks, TLS, and host headers.
- Set DNS and certificates: Point the public hostname at the CDN as directed by the provider, issue a certificate, and preserve HTTPS end to end.
- Design cache keys: Include only URL, query, cookie, and header values that change the representation. Exclude analytics parameters that do not.
- Set response policy: Use Cache-Control and provider rules for TTL, revalidation, stale behavior, and private/no-store responses.
- Protect private traffic: Keep authorization and session responses out of shared caches unless you have an explicit, tested key and privacy model.
- Handle deployments: Publish assets with content-hashed names; purge HTML or other objects that must change immediately.
- Test geographically: Check first request, repeat request, different POPs, logged-in and logged-out sessions, query strings, and failure responses.
- Observe: Track cache-hit ratio, time to first byte (TTFB), origin offload, origin latency, status codes, bandwidth, and total page or API latency by region and request class.
Measuring whether the CDN is actually faster
Compare the same URL with a warm and cold cache, from the regions where your users live. Segment static assets, HTML, APIs, and writes; a single site-wide average can hide a slow personalized route. Record TTFB, complete transfer time, cache status, origin response time, error rate, and hit ratio. Also inspect browser rendering: a fast first byte does not help if render-blocking JavaScript remains slow.
Use response headers and provider logs to identify hits, misses, bypasses, revalidations, and shield or tier requests. Test cache invalidation after every deployment. For APIs, verify freshness and authorization semantics with automated checks, not only speed.
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Limits, trade-offs, and failure modes
- Cold or low-volume regions: An edge may miss frequently, so users still wait for origin fetches.
- Overly variable keys: Query strings, cookies, or headers can fragment the cache.
- Stale content: Long TTLs require versioning or reliable purge workflows.
- Dynamic computation: Personalization, database reads, and writes still execute at the origin or an edge-compute layer.
- Control-plane complexity: DNS, certificates, rules, invalidations, logs, and provider-specific routing add operational work.
- Cost dimensions: Compare requests, bandwidth or egress, cache fills, invalidations, and any tiered-transfer charges.
- Security is separate from speed: WAF, bot controls, TLS management, and DDoS mitigation may justify a CDN, but they do not turn an uncacheable response into a cache hit.
Common problems and fixes
Hit ratio is unexpectedly low
Inspect the cache key for analytics parameters, cookies, authorization, or varying headers. Normalize non-functional query parameters, reduce unnecessary variation, and verify that the origin does not send private or no-store directives.
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Users receive old files
Check the object’s TTL and purge status. Prefer a new, content-hashed filename for each changed asset; reserve invalidation for URLs that cannot be versioned.
Private data appears in a shared response
Immediately bypass caching for the affected route, purge the object, and review cookies, authorization headers, cache-key fields, and cache-control directives. Re-test authenticated and anonymous requests separately.
First request remains slow
This is expected on a miss. Use tiered caching or a shield where available, warm only predictable high-demand objects, optimize origin processing, and avoid promising hit-like latency for uncached content.
HTTP/3 does not improve results
Confirm client and network support, compare HTTP/2 and HTTP/3 from the same geography, and inspect loss, handshake, and server timing. Protocol negotiation can vary by visitor.
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Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Choosing a CDN provider
| Selection axis | Questions to answer |
|---|---|
| Edge proximity | Are POPs close to your actual users, including smaller or international regions? |
| Cache controls | Can you set precise keys, TTLs, purge, stale-while-revalidate, and versioning rules? |
| Application support | Does it handle APIs, streaming, signed/private content, and required methods? |
| Protocols | Are HTTP/2, HTTP/3, QUIC, TLS versions, IPv6, and connection reuse supported? |
| Origins | Can it integrate with your load balancer, object storage, Kubernetes, or multi-cloud design? |
| Operations | Are routing controls, tiered caches, logs, real-time metrics, and purge APIs sufficient? |
| Security | How do certificates, WAF, bot controls, and DDoS mitigation integrate? |
| Billing | What are the rates for requests, bandwidth, egress, fills, and invalidations in your region? |
Cloudflare CDN, Amazon CloudFront CDN, and Google Cloud CDN are common hosted choices; select based on the axes above and your users’ geography rather than a universal speed claim. A historical Google Cloud blog figure cited more than 120 Google edge nodes in 2021; that dated number should not be treated as a current network count.
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cURL:
curl -G "https://api.screenshotneo.com/v1/shot" -d access_key=YOUR_API_KEY --data-urlencode url=https://stripe.com -o shot.webpPython:
import requests
r = requests.get("https://api.screenshotneo.com/v1/shot", params={"access_key": "YOUR_API_KEY", "url": "https://stripe.com"}, timeout=90)
open("shot.webp", "wb").write(r.content)Node.js:
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const res = await fetch(`https://api.screenshotneo.com/v1/shot?${q}`);See the ScreenshotNeo documentation for the full option set, including full-page and element capture, device and retina settings, PDF output, custom CSS or JavaScript, waits, request blocking, headers and cookies, geolocation, caching, signed links, async webhooks, bulk capture, usage data, and OpenAPI details. The Free plan includes 1,000 screenshots per month with no card; paid plans start at $5 for 3,000. Create a free ScreenshotNeo account.
Frequently Asked Questions
Does a CDN cache every HTTP method?
No. Cache behavior is provider- and rule-dependent; CloudFront documents PUT, POST, PATCH, OPTIONS, and DELETE as going directly to the origin, while GET responses may still bypass for authorization, cookies, or cache-control reasons.
Is a CDN useful if most pages are personalized?
It can still accelerate shared assets and public endpoints, and improve connection and routing paths, but personalized responses generally require origin processing and careful privacy rules.
How should I test a new CDN configuration safely?
Start with immutable static assets, verify anonymous and authenticated behavior separately, test cold and warm requests from representative regions, and monitor hit ratio, TTFB, origin offload, errors, and purge behavior before expanding caching.
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