The update strategy of the CDN (Content Delivery Network) cache server is key because it directly affects the freshness, performance and user experience of the content. Different types of content may require different update strategies. Here are some common CDN cache server update strategies:

　　Regular updates:

　　Under this strategy, the CDN cache server regularly updates cached content at predetermined intervals. This works well when content changes infrequently and has a fixed update frequency. For example, a news website could refresh news articles every hour to ensure users get the latest news.

　　Based on expiration header:

　　HTTP responses often include an "Expires" or "Cache-Control" header to indicate how long content should remain in the cache. CDN cache servers can manage cached content based on instructions from these headers. When the content expires, the CDN will request the latest content and update the cache.

　　Refresh the cache:

　　For some specific cases, you may want to immediately clear specific content from the CDN cache server. This can be accomplished by sending an HTTP PURGE request or using the CDN provider's management interface. This is useful for emergency updates or bug fixes.

　　Original server notification:

　　Some CDN providers support origin servers notifying the CDN when content is updated. This can be achieved via Webhooks or API. When the origin server notifies the CDN, the CDN will immediately request the new content and update the cache.

　　version control:

　　To ensure the freshness of content, you can use version control strategies. Whenever content is updated, change the version number or query parameters in the URL. This will force the CDN cache server to re-fetch the content as it will be treated as a different resource.

　　Dynamic content caching strategy:

　　For dynamic content, such as user-generated comments or social media aggregations, shorter cache times can be used to ensure the timeliness of the content. This can be achieved via "no-cache" or "max-age" in the "Cache-Control" header.

　　Regional updates:

　　For a globally distributed CDN, you can update the cache based on user needs in different geographical locations. For example, users in Europe may want different content than in the United States. The CDN can update the cache based on geographical location needs.

　　Content prefetching:

　　Prefetching means that CDN obtains and caches content that has not yet been requested in advance. This can be achieved through scheduled or periodic prefetch tasks to ensure that content is already in the cache when it is first requested.

　　It should be noted that the update strategy of the CDN cache server needs to be formulated according to your specific needs. Different types of content may require different strategies. At the same time, CDN providers often provide tools and options to help you manage updates to cached content. When designing your strategy, be sure to balance performance and freshness to provide the best user experience.