How Long Does Cached Data Remain in a CDN Server

The duration for which cached data remains in a CDN server varies based on the cache-control policies set by the website owner, the type of data, and the configurations of the CDN. 

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This can range from a few seconds to several years. The industry standard is a 24 hour expiry, after which, new content is fetched.

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Cache-Control Policies

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Max-Age Directive:

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• The max-age directive in the HTTP header defines the maximum amount of time a resource is considered fresh. For example, max-age=3600 indicates that the data should be cached for one hour (3600 seconds).
• This is the most straightforward method to control caching duration.

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s-maxage Directive:

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• The s-maxage directive is similar to max-age but is specifically for shared caches like CDNs. It overrides the max-age directive if both are present.
• For example, s-maxage=86400 sets the cached data to expire in 24 hours (86400 seconds) for shared caches.

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Expires Header:

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• The Expires header sets a specific date and time for the resource to expire. This is an older method and is less flexible than max-age or s-maxage.
• Example: Expires: Wed, 21 Oct 2024 07:28:00 GMT.

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Cache-Control: No-Cache, No-Store:

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• no-cache indicates that the cached data must be revalidated with the origin server before serving it.
• no-store instructs the cache to never store the data.

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Types of Data

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Static Content:

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• Images, stylesheets, JavaScript files typically have long caching durations because they don’t change frequently.
• Example: Images might be cached for a year (max-age=31536000).
• Check: How to Improve Cache Static Content

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Dynamic Content:

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• Content that changes frequently, like news articles or user-specific data, has much shorter caching durations.
• Example: Dynamic HTML might be cached for a few minutes or even just seconds (max-age=60).

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Fine tuning these also improves cache hit ratio, but that’s more related to the quality rather than the longevity. 

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CDN Configurations

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Default Settings:

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• Many CDNs have default caching policies and cache keys. For instance, if no cache-control header is set, some CDNs might default to caching content for 24 hours.

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Custom Rules:

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• CDNs often allow custom rules based on URL patterns, file types, or other criteria. These rules can override default settings and HTTP headers.

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Browser Caching vs. CDN Caching

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Browser Caching:

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• Browsers cache data locally on the user’s device. The duration is governed by the same cache-control headers but is specific to individual users.
• Example: If a stylesheet has max-age=31536000, the browser will cache it for a year on the user's device.

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Web Caching in CDNs:

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• CDNs cache data across multiple servers to serve it faster to users worldwide. The cache-control headers guide this caching, but CDNs can also revalidate and purge cached content based on specific triggers or manual interventions.

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Calculations

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Let’s look at a scenario for better understanding:

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Example Scenario:

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• A website sets the following header for its images: Cache-Control: public, max-age=604800, s-maxage=2592000.
  • max-age=604800 (1 week) means the browser will cache the images for one week.
  • s-maxage=2592000 (1 month) means the CDN will cache the images for one month.
• Here, users will get the images from their browser cache for up to a week without re-fetching from the CDN. The CDN will serve the same image to different users from its cache for up to a month before re-fetching from the origin server.

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Practical Considerations

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Cache Invalidation:

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• Sometimes, you need to purge cached content before its expiration. CDNs offer mechanisms for cache invalidation, either through their APIs or dashboards.

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Stale-While-Revalidate and Stale-If-Error:

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• These directives allow serving stale content while fetching new data or if the origin server is down, improving reliability.

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Edge Cases:

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• Some content, like personalized data, should not be cached (Cache-Control: private, no-store).

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