Intro
This section explains content delivery networks in practical terms. Focus areas are delivery, performance, security, pricing, SEO, and integration.
Material is organized as quick answers with links to deeper pages. Each answer uses concrete mechanisms and conservative guidance.
Quick answers
What is a CDN and how does it work?
A content delivery network is an overlay of edge servers designed to bring content closer to clients. Each edge accepts connections, terminates TLS, enforces policy, and serves cached responses. The edge fetches from origin only when a response is missing or expired. The response is stored under a cache key for reuse. The cache key typically includes method and path, and a small set of safe vary dimensions such as Accept and Accept-Encoding. Cookies and volatile headers should not be part of the key unless required.
Traffic reaches an edge by DNS and by routing. DNS maps a hostname to an IP that is close in network terms. Anycast BGP advertises the same IP from many sites so routers select a nearby path under current conditions. Many providers blend DNS steering and anycast. The goal is stable reachability and low round trip times. Some providers also use mid tiers or origin shields to reduce long-haul fetch fan-out and protect fragile origins.
Freshness relies on TTLs and validators. TTLs set reuse windows. ETag and Last-Modified enable conditional fetches that avoid full transfers. Stale-while-revalidate hides refresh latency while a new copy is fetched. Negative caching reduces repeat errors when an origin is unwell. Control exists at multiple layers: DNS selects the site, edge rules manage headers and cache policy, and origin responses define cache control and validators. The CDN is a delivery system. Durable state and business logic stay at the origin. Read more: What a CDN is and how it works.
What are the benefits of using a CDN?
Lower latency is the first gain. Shorter paths and fewer congested links reduce round trips. TLS handshakes complete faster when the terminator is near clients. Time to first byte falls for cache hits because the origin leg is removed. Even cache misses gain from warm, persistent connections on the origin side. Steadier transport and fewer cross-continent trips lower p95 and p99 delays.
Origin offload is the second gain. Each cache hit avoids application work, database queries, and origin egress. Offload raises headroom and reduces variance. Tail latencies shrink because long trips occur less often. Offload also smooths periodic load such as product launches or media releases. Tiered caching and origin shielding reduce burst pressure further and confine fetches to predictable paths.
Resilience is the third gain. The edge absorbs abusive traffic and incidental surges before they cross long haul links. Filters, rate limits, and token checks run close to clients. Incidents are smaller and shorter when noise is stopped early. The delivery plane remains steady while the origin focuses on core logic. Global purging and rule rollout allow fast mitigation actions.
Three core benefits often guide adoption:
- Faster delivery through proximity and reuse.
- Lower origin load through cache hits and shielding.
- Higher resilience through edge filtering and surge absorption.
Benefits are not automatic. A CDN adds another system with its own rules and failure modes. Consistency depends on sound TTLs and accurate purging. Debugging spans client, edge, and origin logs. With the basics in place, gains in latency, stability, and cost compound over time. Read more: Benefits and trade-offs of a CDN.
How does a CDN improve website speed?
Speed improves when round trips are removed or shortened. A cache hit at the edge removes the origin leg. The connection is local, so TCP and TLS complete quickly. First byte arrives sooner and parsing starts earlier. Even when a fetch occurs, persistent connections and tuned congestion control reduce the cost of the long path. Shielding concentrates fetches into a few stable tunnels instead of many scattered ones.
Transport protocols matter. HTTP/2 multiplexes many streams over one TCP connection. This reduces socket overhead and limits head of line impact at the application layer. HTTP/3 over QUIC changes the transport model. Packet loss on one stream does not block others. Connection migration helps on mobile networks where IPs change during a session. Both protocols benefit from nearby termination because handshakes and recovery complete faster.
TLS setup is another lever. TLS 1.3 and session resumption reduce round trips. A nearby edge makes handshakes inexpensive. The edge keeps warm connections to origins to avoid repeat setups on the server side. Connection pooling and keepalive settings affect tail latency when many small objects are fetched. Early hints can prepare critical asset retrieval without heavy-handed pushes.
Payload size is the next lever. The edge applies Brotli or gzip for text. Client hints enable image formats such as AVIF or WebP when supported. Range requests transfer only the required media segments. Cache key design also matters. Keep the key tight and predictable, with normalized parameters and minimal vary. The result is faster first byte, fewer bytes, and fewer long-haul trips. Read more: Performance fundamentals with a CDN.
What types of content can a CDN deliver?
Static assets are the baseline. CSS, JS, fonts, and images cache well with explicit TTLs. Filenames that embed a content hash allow very long TTLs with safe rollouts on deploy. Browsers and edges both benefit from immutable assets. Large downloads gain from range requests, resumable transfers, and partial caching that avoids re-downloading earlier segments.
HTML can be cached when safe. Many sites can strip nonessential cookies, set a small TTL, and purge on content change. Personalized pages can cache fragments or select among a small set of variants at the edge. Keep vary policies simple and observable. Complex variation creates fragmentation and cache misses. Validators such as ETag and Last-Modified help keep freshness without full fetches.
APIs are mixed. Idempotent GET responses cache when identifiers live in the path or in stable query parameters. Authorization headers usually force pass through. Public and semi public data can still be cached with care. POST, PUT, and DELETE pass by default. Some platforms support surrogate keys or signed exchanges that allow reuse with strict controls. These patterns need careful review and good telemetry to avoid stale or unauthorized deliveries.
Images and media use established methods. The edge resizes, transcodes, and compresses images within guardrails. Format negotiation picks the best option per device. Video uses HLS or DASH with segment caching. Origin shielding keeps load steady when many viewers arrive at once. Live streaming adds timing constraints that reward simple, predictable policy. Vary policies should be explicit and minimal. Safe dimensions include method, path, normalized query parameters, and limited headers like Accept and Accept-Encoding. Device or geo variation increases risk for SEO and cache fragmentation and should be used sparingly. Read more: What a CDN can deliver.
How does a CDN enhance security?
Edges terminate TLS and own certificate lifecycle at scale. This centralization reduces failed renewals and weak cipher drift. TLS 1.3, modern curves, and HSTS can be enforced broadly without waiting for origin changes. OCSP stapling and session tickets improve connection setup without new application code. Key management becomes consistent instead of per-origin scatter.
Request filtering operates before origin exposure. A WAF inspects requests for common attack patterns. Managed signatures handle known issues. Custom rules address application specific paths. Staged rollouts and detailed logs reduce false positives. Token checks at the edge protect hot paths such as checkout and download endpoints. Bot mitigations and rate limits reduce automated abuse without blocking legitimate sessions.
DDoS protections function at several layers. Network layer controls drop floods and reflections. Transport and application controls mitigate connection floods and request spikes. Rate limits and challenges reduce bot traffic while preserving sessions for real users. Placing these defenses close to clients avoids saturating long-haul links or origin bandwidth and preserves upstream capacity for useful work.
Origin lockdown reduces blast radius. Origins should accept traffic only from the CDN. Methods include IP allow lists, private interconnects, and mutual TLS. Direct DNS exposure of origin hosts should be avoided. Signed URLs and headers prevent hotlinking and control access to expensive resources. These measures ensure the CDN is the only viable path to origin and that edge policies are enforced consistently.
Security must respect privacy and compliance. Logging should minimize sensitive fields. Data residency features may be required for regulated regions. Secrets and certificates need rotation. Configuration should live in version control with peer review and change control. Regular drills keep teams ready to tighten rules or fall back safely. Read more: Security at the edge.
What is the difference between a CDN and web hosting?
Hosting provides compute, storage, and databases. It runs application code and holds durable state. A CDN is a delivery layer in front of hosting. It manages connection termination, caching, and request filtering. The CDN complements the origin. It does not replace it. Separation of concerns keeps systems understandable and reduces risk.
State separation is the main boundary. Durable writes and authoritative reads belong at the origin. The CDN holds transient state. Examples include cache entries, TLS sessions, and rate limit counters. Edge compute features can run lightweight logic near users. These functions should avoid authoritative state and focus on shaping requests and responses. When state boundaries blur, debugging becomes difficult and failure modes expand.
Consistency follows from this split. Changes at the origin may require cache invalidation to take effect globally. TTLs trade freshness for load and safety. Long TTLs fit immutable assets or paths that can be purged on change. Dynamic pages need short TTLs or validation on each request. Teams should document how changes propagate across layers and confirm that purges reach all tiers including shields.
Operations differ. Hosting teams manage deployments, databases, and storage. CDN teams manage edge configuration, rules, certificates, and purging. Incidents have distinct signatures. A cache stampede looks different from a database lock. Logs and metrics arrive from separate systems and need correlation. Coordinated releases and clear ownership reduce risk. Read more: CDN vs hosting.
How to choose the right CDN provider?
Start with audience and geography. Map users by country, region, and network type. Mobile and last mile conditions vary widely. A footprint that matches those realities is more important than raw PoP counts. Anycast reachability is not the same as consistent performance. Field measurements are required. Presence on relevant eyeball networks often matters more than global averages.
Collect real user data first. RUM shows latency and errors by region and device. Build simple segments and track outcomes over time. Add controlled A or B splits to compare providers or configurations under live traffic. Keep the split stable for a full cycle so caches warm and patterns settle. Use synthetic tests to analyze cold edge behavior and route stability. Treat synthetic ranks as advisory signals only.
Evaluate feature fit. Validate purge APIs, log delivery, and header control. Check image optimization, media pipelines, and HTTP protocol support. Review WAF scope, bot controls, and rate limiting. Confirm mTLS or private links for origin lockdown. Assess IaC options and SDK maturity. Operational quality becomes the daily experience and should be part of the trial.
Plan a bakeoff with clear exit criteria. Define KPIs such as p50 and p95 TTFB, cache hit ratio, and error rates by region. Include origin offload and a rough cost view. Limit the trial to representative domains and paths. Keep change control tight to avoid confounding variables. Negotiate only after data supports the decision and aligns with growth forecasts. Read more: How to choose a CDN provider.
What is the cost of using a CDN?
Costs break into transfer, requests, features, and logs. Transfer is billed per GB and often varies by region. Requests are billed per thousand or per million and may be split by cacheable and dynamic classes. Security, bot controls, and image or media processing add event or transformation charges. Log delivery and storage can be material at scale and often rise faster than expected.
A simple model starts with demand. Estimate monthly GB by region from analytics. Count requests by class such as HTML, APIs, images, and downloads. Apply published rates for a baseline. Then adjust with two factors. First, expected cache hit ratio by class. Second, origin egress savings that move into CDN transfer. This yields a total delivery cost rather than a narrow CDN-only view. Include log delivery and storage, which frequently change the economic balance.
Tiered pricing interacts with traffic shape. A surge into a higher tier can raise effective rates across all traffic. Regional skews raise cost even when the global total is flat. Free allowances often exclude certain regions or request classes. Read the small print for security features and for logs. Commit discounts lower unit rates but introduce use-it-or-lose-it pressure.
Negotiate only after measurement stabilizes. Align commit levels with observed traffic and realistic growth. Include safeguards for unknowns such as crawler surges or media events. Avoid folding many add-ons into a single opaque price. The useful lens is total cost at target performance and reliability, including origin savings from higher cache hit ratios. Read more: CDN pricing models.
Can a CDN improve SEO?
Delivery speed affects user experience signals. Faster first byte and smaller payloads improve Core Web Vitals when applied carefully. A CDN reduces round trips and enables compression and modern formats. Stable performance reduces variability in measured metrics. Gains come from removing slow paths rather than micro tuning single timers. Reducing long-tail delays often helps more than shaving a few milliseconds from medians.
Crawler behavior needs attention. Crawlers follow redirects, respect robots rules, and cache responses. Device or geo based variation can confuse indexing if HTML differs. Keep markup stable across variants. Use canonical links and predictable redirects. Avoid vary policies that fragment caches and yield inconsistent content to bots and users. Serve the same primary content to crawlers and normal visitors to avoid cloaking concerns.
Edge features can help. Image optimization reduces Largest Contentful Paint by shrinking hero assets. Brotli for text cuts transfer size without application changes. HTTP/2 and HTTP/3 keep connections warm and avoid slow starts. Stale-while-revalidate hides occasional origin delays. These tools are safe when rolled out with guardrails and measured in the field, not just in lab tests.
Headers and cache control matter. Public pages should enable reuse while keeping freshness through validators. Avoid caching HTML that embeds volatile user fragments. Keep cookies off public paths when possible so caches can be shared. Pair changes with monitoring of crawl stats and index coverage. Use controlled rollouts to confirm improvements across regions and devices. Read more: SEO and CDNs.
How to set up and integrate a CDN?
Begin with DNS and naming. Point the desired hostname to the CDN using a CNAME or an apex method provided by the vendor. Confirm TTLs and plan a controlled cutover. Prepare TLS certificates that cover all hostnames. Test SNI and chain completeness from multiple regions. Decide on key management and renewal cadence and store configurations under version control.
Define cache keys and TTLs per path class. Keep the key tight. Include method and path. Include only meaningful query parameters. Include only safe headers such as Accept and Accept-Encoding. Set explicit TTLs for static, HTML, APIs, and media. Add validators for dynamic paths. Plan purge by surrogate keys or by path. Keep purges targeted so caches stay warm. Normalize headers to keep behavior predictable.
Build a staged rollout. Start with a canary that routes a small slice of traffic. Watch cache hit ratio, origin status codes, and error rates. Expand in steps while keeping an easy rollback. Avoid coupling CDN rollout with large application releases. Keep a small set of flags to toggle features without deploys. Document runbooks for warming, bypassing, and purging.
Lock down the origin. Restrict access to CDN addresses or to a private link. Add mutual TLS if supported. Confirm that health checks and management access still work through allowed paths. Test failure cases such as a region outage or a purge storm. Integration is complete when traffic is stable, origin load is predictable, and teams can operate calmly with clear observability. Read more: Integrating a CDN.
Related reading
For complex traffic or regional gaps, consider combining providers. See the Multi-CDN guide. Recommended next pages include What a CDN is and how it works, Performance fundamentals with a CDN, Security at the edge, CDN pricing models, and Integrating a CDN.