Alibaba Cloud CDN:Refresh and prefetch resources

Feature overview

• Refresh: marks all resources that are cached on the points of presence (POPs) as expired. When a POP receives a request, the POP redirects the request to the origin server to retrieve the requested resources. The retrieved resources are returned to the client and cached on the POP. The refresh feature reduces the cache hit ratio.
• Prefetch: prefetches resources from origin servers and caches the resources on the POPs. When a POP receives a request, the POP directly returns the requested resources to the client because the resources are already cached on the POPs. The prefetch feature increases the cache hit ratio.

Common scenarios

Usage notes

• After a refresh task is submitted and completed, specific resources are removed from POPs. When a POP receives a request for the removed resources, the POP forwards the request to the origin server to retrieve the resources. The retrieved resources are returned to the client and cached on the POP. Multiple refresh tasks may cause a large number of resources to be removed from the POPs. This increases the number of requests that are forwarded to the origin server. The back-to-origin routing process consumes more bandwidth resources and the origin server may be overwhelmed.
• A refresh task takes effect 5 to 6 minutes after being submitted. If the TTL for the resources you want to refresh is less than five minutes, you can wait until the TTL expires instead of performing the refresh operation.
• After a refresh task is submitted and completed, the POPs immediately start to retrieve resources from the origin server. Therefore, a large number of refresh tasks cause a large number of concurrent download tasks. This increases the number of requests that are redirected to the origin server. The back-to-origin routing process consumes more bandwidth resources and the origin server may be overwhelmed.
• The time required for prefetch task to complete is proportional to the size of the prefetched file. In actual practice, most prefetch tasks take 5 to 30 minutes to complete.
• Resource Access Management (RAM) users must acquire required permissions before they can refresh or prefetch resources. For more information, see Authorize a RAM user to prefetch and refresh resources.
• URL refresh, directory refresh, and refresh that contains regular expressions support shared cache. If shared cache is configured for a domain name, you can submit a refresh task with the primary domain name or any associated domain name to refresh the cache.

Refresh resources

1. Log on to the Alibaba Cloud CDN console.
2. In the left-side navigation pane, click Refresh & Prefetch.
3. On the Refresh/Prefetch tab, configure the refresh rules.
   

   Parameter	Description
   Operation	Select Refresh.
   Operation Method	Select URL, Directory, or Regular Expression.
   URL	If your daily peak bandwidth exceeds 200 Mbit/s, you can request a quota increase by referring to Quota management. Alibaba Cloud determines whether to approve your application based on your business requirements. Refresh resources based on URLs Each URL must start with http:// or https://. Enter only one URL on each line. Each Alibaba Cloud account can submit up to 10,000 URLs per day and up to 1,000 URLs at a time. You can submit up to 1,000 URLs for each domain name per minute. Refresh resources based on directories Each URL must start with http:// or https:// and end with a forward slash (/). Refresh all files in the root directory: http://example.com/. Refresh all files in the subdirectory /path/test/: http://example.com/path/test/. Enter only one URL on each line. Each Alibaba Cloud account can submit up to 100 directories per day and up to 100 directories at a time. Subdirectories are supported. You can apply for a quota increase. After your application is approved, you can submit up to 1,000 directories at a time. If you set the ObjectType parameter to Directory, the resources in the directory that you want to refresh are marked as expired. You cannot delete the directory. If clients request resources on POPs that are marked as expired, Alibaba Cloud CDN checks whether the resources on your origin server are updated. If resources are updated, Alibaba Cloud CDN retrieves the most recent version of the resources and returns the resources to the clients. Otherwise, the origin server returns the 304 status code. Refresh resources based on regular expressions For more information, see Configure URL refresh rules that contain regular expressions.

4. Click the Submit.
5. Check the refresh progress.
   After you submit a refresh task, you can check the progress and details on the Records tab. The progress depends on the number of objects to be refreshed. It may take some time to complete the task.

   Note If Auto CDN Cache Update is enabled in the Object Storage Service (OSS) console, you cannot check the automatic refresh tasks in the Alibaba Cloud CDN console.

Prefetch resources

1. Log on to the Alibaba Cloud CDN console.
2. In the left-side navigation pane, click Refresh & Prefetch.
3. On the Refresh/Prefetch tab, configure the prefetch rules.
   

   Parameter	Description
   Operation	Select Prefetch.
   Operation Method	Only URL is supported.
   URL	Each URL must start with http:// or https://. Enter only one URL on each line. By default, each Alibaba Cloud account can submit up to 1,000 URLs per day. If the daily peak bandwidth value of your workloads exceeds 200 Mbit/s, you can request a quota increase by referring to Quota management. Alibaba Cloud reviews your application and then increases the quota accordingly. Each Alibaba Cloud account can submit up to 100 URLs in a request. For each Alibaba Cloud account, the prefetch queue can contain up to 50,000 URLs. Content is prefetched based on the time when the URLs are submitted. The URL that is submitted the earliest has the highest priority. If the number of URLs in the queue reaches 50,000, you cannot submit more URLs until the number drops below 50,000. The execution speed of a prefetch task varies based on the average size of the files that you want to prefetch. Smaller average size of files allows you to obtain faster prefetch speeds.

4. Click the Submit.
5. Check the prefetch progress.
   After you submit a prefetch task, you can view the progress and details on the Records tab. The progress depends on the number of objects to be prefetched. It may take some time to complete the task.

API references