A CDN is a network of geographically dispersed servers used to deliver static content.
CDN servers cache images, videos, CSS, JavaScript files, etc.
Dynamic Content Caching enables the caching of HTML pages based on request path, query strings, cookies, and request headers.
Operation
When a user visits a website, CDN server closest to the user will deliver static content.
The URL’s domain for the specified asset is provided by the CDN provider.
If the CDN server does not have the assets in the cache, the CDN server requests the file from the origin, which can be a web server or online storage.
The origin returns assets to the CDN, which includes optional HTTP header Time-to-Live (TTL), specifying how long the asset is cached.
CDN caches the asset and returns it to User.
The image remains cached in the CDN until the TTL expires.
Other users sending a new request to get the asset will get it from cache as long as the TTL has not expired.
Considerations
Cost
CDNs are run by third-party providers, and you are charged for data transfers in and out of the CDN.
Cache Expiry
For time-sensitive content, setting a cache expiry time is important.
If it is too long, the content might no longer be fresh.
If it is too short, it can cause repeat reloading of content from origin servers to the CDN.
Caching infrequently used assets provides no significant benefits so you should consider moving them out of the CDN.
Fallback
If there is a temporary CDN outage, clients should be able to detect the problem and request resources from the origin.
Asset Invalidation
To remove an asset from CDN before it expires
Invalidate the CDN object using APIs provided by CDN vendors.
Use object versioning to serve a different version of the object.
To version an object, you can add a parameter to the URL, such as a version number.
For example, version number 2 is added to the query string: image.png?v=2.
