How CDNs Handle Caching for Dynamic Versus Static Content

 

When it comes to delivering content quickly and reliably, caching is one of the most powerful tools in a Content Delivery Network (CDN). But not all content is created equal. Static content like images, stylesheets, and videos behaves differently from dynamic content, such as personalized dashboards, shopping carts, or live feeds. Modern CDNs use sophisticated strategies to handle both efficiently, balancing speed, accuracy, and freshness.

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1. Understanding Static and Dynamic Content

• Static Content:
  This is content that does not change frequently and is the same for every user. Examples include:

  • Website logos and images

  • CSS and JavaScript files

  • Pre-recorded videos

  • PDFs and downloadable assets

  Because this content is predictable and consistent, it is ideal for caching on edge servers. Once cached, static content can be served repeatedly without needing to fetch it from the origin server, drastically improving load times and reducing server load.

• Dynamic Content:
  This is content that changes frequently or is personalized per user. Examples include:

  • User account pages or dashboards

  • Shopping carts and order summaries

  • News feeds with constantly updating content

  • Real-time stock prices or sports scores

  Dynamic content is more challenging to cache because it must remain up-to-date and accurate for each user. Improper caching can result in delivering outdated or incorrect information.

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2. Caching Static Content

Static content caching is straightforward:

1. Edge Server Storage: The CDN stores copies of static files on multiple edge servers worldwide.

2. Time-to-Live (TTL): Each cached file is assigned a TTL, determining how long it remains on the edge server before checking for updates from the origin.

3. Cache Hit Delivery: When a user requests a cached static file, the edge server delivers it immediately, without contacting the origin.

4. Cache Purging/Versioning: When updates occur (like a new logo or a revised CSS file), the CDN either purges the old cache or uses versioned URLs to ensure users get the latest content.

The result: faster page loads, reduced bandwidth costs, and minimal origin server load.

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3. Caching Dynamic Content

Dynamic content requires more nuanced handling. Modern CDNs use advanced techniques to cache portions of dynamic content without compromising freshness or personalization. Key strategies include:

1. Edge-Side Includes (ESI):
   ESI allows the CDN to cache static portions of a dynamic page while leaving dynamic parts to be fetched from the origin server. For example, on a personalized dashboard, the header, footer, and navigation menus could be cached, while the personalized content block updates in real-time.

2. Query String and Cookie-Based Caching:
   CDNs can cache dynamic content based on user-specific parameters, such as query strings, cookies, or authentication tokens. This ensures users receive personalized content quickly while maintaining cache efficiency.

3. Short TTL and Revalidation:
   Dynamic content often uses short TTLs so cached content expires quickly, minimizing the risk of serving outdated data. Some CDNs implement conditional requests to the origin server, checking if content has changed before updating the cache.

4. Dynamic Content Acceleration:
   Some CDNs optimize the delivery of dynamic content even without caching, using techniques like:

   • Persistent TCP connections to reduce handshake delays

   • Route optimization to find the fastest path to the origin server

   • Protocol optimizations (HTTP/2, HTTP/3, QUIC) to minimize latency

5. Edge Computing for Personalization:
   Advanced CDNs offer edge computing, where lightweight scripts or functions execute at the edge server itself. This allows for real-time personalization or transformation without needing to always fetch data from the origin. For instance, a CDN could dynamically insert a user’s name or preferred language into a cached template at the edge.

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4. Hybrid Approach: Combining Static and Dynamic Caching

Most modern websites contain a mix of static and dynamic content. CDNs often use a hybrid approach:

• Cache everything that is truly static for long TTLs, including images, scripts, and video segments.

• Cache reusable dynamic fragments (headers, navigation, shared widgets) for shorter TTLs.

• Fetch fully dynamic, user-specific content directly from the origin, or generate it at the edge using serverless functions.

This approach ensures speed without sacrificing accuracy. For example, an e-commerce site can serve product images and site layout from edge servers while generating a live shopping cart for each user dynamically.

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5. Benefits of Proper Caching Strategies

Proper caching for static and dynamic content provides multiple advantages:

• Reduced Latency: Users get content faster because cached elements are served locally.

• Scalability: Origin servers are shielded from heavy traffic, allowing them to handle complex requests efficiently.

• Bandwidth Savings: Fewer requests travel back to the origin, reducing network congestion.

• Improved User Experience: Pages load faster, even under high traffic, and personalized content remains accurate.

• Security: Edge caching can help mitigate some attack vectors, like overwhelming traffic or DDoS attempts.

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6. Real-World Examples

1. Streaming Services: Netflix caches video segments at edge servers for fast playback (static caching), while dynamically adjusting video quality based on user bandwidth and device (dynamic caching and optimization).

2. E-Commerce Sites: Amazon serves static product images and site layout from edge servers while fetching dynamic shopping cart data and personalized recommendations directly from the origin.

3. News Websites: Cached headlines and images allow pages to load quickly, while breaking news articles are fetched or updated dynamically to ensure freshness.

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7. Conclusion

CDNs handle caching differently for static and dynamic content to balance speed, freshness, and personalization:

• Static content is cached extensively on edge servers with long TTLs for maximum efficiency.

• Dynamic content is cached selectively, often in fragments or short-lived caches, with optimizations such as ESI, query-based caching, and edge computing.

By intelligently managing both types of content, CDNs deliver fast, reliable, and personalized experiences to users worldwide, even under heavy traffic, fluctuating network conditions, or high demand scenarios.