What is a CDN? A Complete Guide to Content Delivery Networks

A CDN (Content Delivery Network) is a globally distributed network of servers that delivers web content to users more efficiently by bringing that content closer to the user.

Using a CDN can significantly improve website load times and user experience by caching static content like images, CSS, JavaScript files, etc. on edge servers near the user and also help handle spikes in traffic.

In this comprehensive guide, you’ll learn:

• What is a CDN and how do CDNs work?
• Benefits of using a CDN service
• Types of content delivered by CDNs
• Choosing the right CDN provider
• CDN pricing models
• Setting up and configuring your CDN
• CDN limitations to be aware of
• Detailed comparisons of top CDN providers
• Optimizing cache times for top performance
• Using multiple CDNs for failover
• CDN security best practices

What is a CDN and How Does a Content Delivery Network Work?

A content delivery network (CDN) is a globally distributed network of servers designed to deliver web content to users more quickly by bringing that content closer to the user.

CDNs work by storing cached copies of your website’s static content (like images, CSS, Javascript files, etc.) on edge servers that are situated closer to your website’s visitors. This is also referred to as caching content on the network edge.

When a user requests a page from your site, the CDN will serve that cached static content from the nearest edge server to them instead of the originating server, significantly speeding up load times.

If content is not already cached, the CDN will fetch it from the origin server, cache it and then deliver it to the user to optimize performance for future visitors.

Key CDN Terminology

• Edge Server – The geographically distributed servers on the internet backbone that cache and deliver content to users based on proximity.
• Origin Server – The primary source server from where the CDN fetches fresh content if not already cached on edge servers.
• Cache – Temporary storage on CDN edge servers that hold copies of static assets and content.
• Edge Locations – Data centers where edge servers are physically located close to users.
• Points of Presence (PoPs) – Network points allowing interconnection between internet service providers.

Benefits of Using a CDN Service

There are several key reasons why content delivery networks are used by many modern websites and web applications:

1. Faster Page Load Times

By distributing content closer to website visitors using nearby CDN edge servers, page load is significantly faster. Content is loaded from servers geographically closer to users reducing latency.

With a CDN, assets are loaded simultaneously from multiple edge locations rather than just the origin server enabling big speed boosts.

2. Improve Website Performance

CDNs not only reduce load times by distributing content globally but also help handle sudden traffic spikes more effectively preventing slow downs.

Instead of all requests hitting your origin infrastructure, they are absorbed across servers distributed worldwide keeping site performance smooth.

3. Enhanced User Experience

By accelerating content delivery, CDNs directly improve user experience metrics like lower bounce rates, higher pages per session, increased engagement and conversions. Even marginal speed gains positively impact user psychology.

Google uses page speed in their mobile search ranking algorithm so CDNs help improve organic positions also indirectly.

4. Distributed Denial of Service (DDoS) Protection

CDNs provide built-in DDoS mitigation by absorbing and dispersing attack traffic across their global networks helping maintain website uptime.

By serving content from varying geographical edge locations instead of central servers, CDNs inherently make DDoS attacks more difficult to target origin infrastructure.

5. Decreased Bandwidth Costs

Serving content from CDN edge servers located closer to visitors reduces traffic to the origin server which cuts monthly bandwidth costs significantly.

Cost savings from reduced bandwidth usage resulting from caching static assets on edge servers near users can help offset CDN expenses.

What Types of Content Can a CDN Deliver?

CDNs are highly effective for delivering static web content that can be easily cached and does not change frequently. This includes:

• Images – JPG, PNG, GIF, SVG
• Stylesheets – CSS, LESS, SCSS
• JavaScript – JS, JSON, AJAX
• Fonts – WOFF, EOT, TTF
• HTML, text and Markdown files
• Audio files – MP3
• Video files – MP4, WebM

CDNs can also cache some dynamic content and APIs by setting intelligent cache times but work best for completely static assets.

For dynamic content that changes constantly like blog posts, user profiles, shopping carts etc. CDNs are less effective because cached copies need to be continually updated which impacts performance.

Determining Cacheability

Understanding which assets can be effectively cached by CDNs depends on:

• Change frequency – Static assets that rarely change are highly cacheable vs. dynamic content that changes constantly.
• Personalization – Generic assets that are same for all users can be cached easily vs. personalized content.
• Authentication – Public content is highly cacheable vs. private, restricted access content.

Pro tip: Leverage a performance budget for caching only necessary assets.

Choosing the Right CDN Provider

With so many CDN services in the market, selecting the right one for your needs is vital based on your use case, traffic levels and budget.

Top CDN Providers Compared

Cloudflare

• Top rated CDN with very well-rounded free plan.
• Enterprise grade security built-in.
• Very flexible pricing models.
• Best value CDN overall.

Akamai

• A long time market leader focused on large enterprise clients.
• Leading CDN technology innovations.
• Expensive but high performance.

Amazon CloudFront

• Easy to use, highly scalable AWS CDN.
• Well integrated with AWS infrastructure.
• Cost effective bandwidth pricing.

Key CDN Selection Criteria

Consider these aspects when evaluating CDNs for your specific needs:

• Global edge server coverage
• Ease of configuration
• Pricing model
• Performance benchmarks
• DDoS protection
• Caching options and controls
• Streaming delivery support
• SSL availability
• Developer friendly APIs
• Traffic analysis reporting

Pro Tip: Many CDNs offer free tiers to test out features before upgrading.

CDN Pricing Models

CDN pricing is based primarily on volume of traffic served per month along with some additional feature charges:

1. Pay-As-You-Go

• Pay only for exactly the amount of traffic your site uses per month.
• Allows seamless scaling up and down without overpaying.
• Usage costs can vary from less than a dollar for 1GB to over $100 for heavy traffic sites.

2. Committed Plans

• Get discounted rates by committing to a certain amount of traffic per month.
• Overage charges apply if you exceed plan limits before renewal.
• Plans from $20 per month for ~100GB traffic.

3. Unlimited

• Some CDNs offer unlimited monthly traffic plans beyond a certain usage threshold.
• Unlimited plans start from $200 per month for smaller sites.
• Large scalable sites may need custom enterprise level plans.

4. Add-On Charges

• Features like analytics, DDoS protection, caching configurations may cost extra.
• Watch out for hidden fees apart from traffic usage costs.
• Free plans may lack advanced functionality needing upgrades.

How to Set Up and Configure a CDN

Setting up a CDN only requires a few steps for most providers:

1. Select a CDN Provider

Evaluate top CDNs and select one meeting your needs, use case and budget. Consider free tiers for testing.

2. Point DNS

Update your domain DNS settings to point to the assigned CDN subdomain (e.g. cdn.yourdomain.com). This directs traffic to CDN servers.

3. Specify Origin Server

Point your CDN to your web server origin infrastructure. This is where the CDN will fetch fresh content from if not already cached.

4. Asset Upload

Upload CSS, JS, image files, videos and other static assets to your CDN which handles replicating content across its edge servers automatically.

5. Configure Caching Rules

Set global and asset-specific caching rules like expiry times, restriction by location, devices etc. Fine tune caching for optimal performance.

And you’re set! The CDN will now accelerate your website content delivery across the globe via edge servers.

Optimizing Cache Times for Top CDN Performance

Setting appropriate cache expiry times is vital for CDNs to minimize trips back to the origin server while also keeping content fresh for visitors.

Longer cache times ensure fewer requests reach origin infrastructure reducing loads. But can serve stale, outdated content to users.

Shorter cache times result in more origin server hits but content is fresher.

Optimizing caching involves balancing these priorities per asset type depending on:

• How frequently asset content changes
• Importance of content being fresh
• Performance priorities

Set cache times using these best practices:

1. Start by setting longest expiry

Begin with cache times of 1 year for static assets like images, CSS, JS files etc. that change rarely if ever.

2. Add version numbers

For assets that get updated periodically, add version numbers to file names so new copies invalidate old cached versions forcing refreshes.

3. Customize caching rules

For assets needing fine grained controls, set custom expiry rules based on:

• Device types
• Locations
• Times of day
• Session durations
• And many other advanced criteria.

4. Invalidate caches manually

When major website changes happen, have the ability to manually invalidate all cached CDN assets instantly with most providers.

Following caching best practices helps fully optimize CDN performance for your site.

Using Multiple CDNs for Failover

To avoid having a single point of failure, large scale websites and applications use multiple CDNs together for failover redundancy:

1. Active-Active CDNs

• Load is actively balanced across 2 or more CDNs simultaneously.
• If one has an outage, traffic is routed through the other(s) with no interruption.

2. Active-Standby CDNs

• One CDN is active while the other remains on standby.
• If the primary CDN fails, automated health checks switch DNS routing to the standby.

Benefits of multi-CDN failover:

• Eliminates single point of failure risks
• Increases overall traffic capacity
• Provides fallback if a provider has issues
• Allows comparing CDN performance
• Adds flexibility to reduce costs

Just like multicloud infrastructure strategies, leveraging multiple complementary CDNs improves continuity and optimizes costs.

CDN Security Best Practices

While using trusted CDN providers intrinsically improves security, additional best practices further help lock things down:

• Enforce HTTPS – Mandate all requests use HTTPS for secure public key authenticated encryption.
• Validate Uploads – Scan all new uploaded assets for malware before replication on CDN edge servers.
• Access Controls – Use signed URLs, hotlink protection, IP allowlists and authentication to limit asset access.
• Monitor Activity – Review CDN access and delivery logs regularly to catch anomalies.
• Rotate Credentials – Change CDN admin passwords, access keys and secrets periodically.
• Edge Rules – Configure edge directives to handle traffic by country, devices and web security filters.
• Custom Edge Certificates – Use your own SSL certificates on edge servers instead of default CDN ones.

While CDNs provide inherent security from their distributed architecture avoiding centralized risks, implementing these additional controls greatly improves protection further.

Summing Up

Content delivery networks have become essential solutions for delivering websites and applications at scale while providing vital performance and security enhancements.

Evaluating your options to choose the right CDN tailored to your specific use case and budget ensures you extract the most value.

By taking advantage of globally distributed caching on edge servers closer to users, CDNs massively accelerate site load times leading to big boosts in user experience.

Neil Beckett

Neil is an accomplished web, designer and developer with over 15 years of experience in creating and optimizing websites for small businesses and online entrepreneurs. Read full bio

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