Time-to-live (TTL) is the amount of time that a resource is cached on Alibaba Cloud CDN points of presence (POPs). When the TTL of a cached resource ends, the resource on the POPs expires. Requests that attempt to access expired resources are redirected to the origin server. The retrieved resources are returned to the clients and cached on the POPs. You can create a cache rule for static resources based on file directories or file name extensions.
Usage notes
If you do not configure cache rules on the origin server or POPs, the default TTL is used for cached resources. By default, cached resources are valid for 3,600 seconds.
After you add a domain name, you can modify the TTL value. The amount of origin traffic and the fees incurred vary based on the TTL that you specify. We recommend that you specify a TTL based on your business requirements. The resource TTL affects the origin fetch frequency.
A short TTL may cause frequent origin fetches and increase loads on the origin server. A long TTL may cause resources on the POPs to become outdated.
If a resource that is cached on a POP is infrequently accessed, the resource may be overwritten by frequently accessed resources on the POP before the resource expires.
If a POP retrieves a static file from an origin server, the POP processes the file based on the priorities of the cache rules. For more information, see Default cache rule and priorities of cache rules.
When you update a file on the origin server, we recommend that you add a version number to the file name to differentiate file versions after updates.
This way, each version of the file after an update has a unique name. For example, you can name a file img-v1.0.jpg before the file is updated and img-v2.1.jpg after the file is updated.
Procedure
Log on to the Alibaba Cloud CDN console.
In the left-side navigation pane, click Domain Names.
On the Domain Names page, find the domain name that you want to manage and click Manage in the Actions column.
In the left-side navigation tree of the domain name, click Cache.
On the Cache Expiration tab, click Create Rule.
In the Create Expiration Rule dialog box, configure the parameters. The following table describes the parameters.
Parameter
Description
Type
Select Directory or File Extension.
Directory: creates a cache rule for the resources in the same directory.
File Extension: creates a cache rule for the resources that have the same file name extension.
Object
Specify the directory or file name extension for which you want to add the cache rule.
If you select Directory, take note of the following rules:
You can enter only one directory at a time. You can use a forward slash (/) to specify all directories.
You can enter a full path. The path must start with a forward slash (/). Example: /directory/aaa.
If you select File Extension, take note of the following rules:
You can enter one or more file name extensions. Separate multiple file name extensions with commas (,). Example:
jpg,txt.File name extensions are case-sensitive.
You cannot use an asterisk (*) to specify all file types.
Expire In
Specify the TTL. The maximum TTL is three years. Take note of the following rules:
Specify a TTL of one month or longer for static files that are infrequently updated, such as images and application packages.
Specify a TTL for static files that are frequently updated, such as JavaScript and CSS files, based on your business requirements.
Specify a TTL of 0 seconds to disable caching for dynamic files, such as PHP, JSP, and ASP files.
Weight
Specify a weight for the cache rule. The weight specifies the priority of the cache rule. Valid values: 1 to 99. A larger value specifies a higher priority.
NoteIf you create multiple cache rules, we recommend that you specify a unique weight for each cache rule to define the priorities of the cache rules.
Cache rules that have the same weight are prioritized based on the creation time, regardless of the rule type. The rule that has the earliest creation time takes precedence.
If you have configured multiple cache rules for a cached resource, only the first matched rule takes effect.
Rule Condition
Rule conditions can identify parameters in a request to determine whether a configuration applies to the request.
Do not use conditions
Select the configured rule conditions in Rules Engine. For more information, see Rules engine.
Click OK.
You can click Modify or Delete to modify or delete the cache rule on the Cache Expiration tab.
Default cache rule and priorities of cache rules
After a POP retrieves a static file from an origin server, the POP processes the static file based on the priorities of the following cache rules. A smaller number specifies a higher priority.
If the response carries the
pragma:no-cache,cache-control:no-cache,cache-control:no-store, orcache-control:max-age=0directive, the static file is not cached.Alibaba Cloud CDN follows the TTL for cached resources, or the TTL for HTTP status codes that are configured in the console.
NoteIf a request matches multiple cache rules, only one rule takes effect based on the following order of priority: weight > creation time.
If you create multiple cache rules, we recommend that you specify a unique weight for each cache rule to define the priorities of the cache rules. A higher weight specifies a higher priority.
Cache rules that have the same weight are prioritized based on the creation time, regardless of the rule type. The rule that has the earliest creation time takes precedence.
Alibaba Cloud CDN follows other cache rules set on the origin server. Headers in responses from the origin server are in the following descending order of priority:
Cache-Control>Expires>Last-Modified>ETag.The response carries the
Cache-Controlheader, the directive ismax-ageors-maxage, which is set to a value that is greater than 0, such as Cache-Control:max-age=3600. If both themax-ageands-maxagedirectives exist, the value of thes-maxagedirective prevails.The response carries the
Expiresheader, such as, Expires:Tue, 25 Nov 2031 17:25:43 GMT.If the response carries the
ETagorLast-modifiedheader, the TTL is calculated based on the following rules:If the response carries the
Last-Modifiedheader, TTL = (Current time -Last-Modified) × 0.1. If the result is from 10 seconds to 3,600 seconds, the result applies. If the result is less than 10 seconds, the TTL is 10 seconds. If the result is greater than 3,600 seconds, the TTL is 3,600 seconds.If the response carries only the
ETagheader, the TTL is 10 seconds.
If the response does not carry the
ETag,Last-Modified,Cache-Control, orExpiresheader, the static file is not cached on the POP.
HTTP caching mechanisms
HTTP provides three types of headers that can be used to control caching behaviors.
Configuration examples
Example 1: If you want the POPs to cache TXT files for seven days, create a cache rule for TXT files in the Alibaba Cloud CDN console and set the TTL to seven days.
Example 2: The following cache rules are configured for the accelerated domain name demo.aliyun.com. When the POPs retrieve the resource http://demo.aliyun.com/image/example.png, the two rules are matched. In addition, the rules have the same weight. In this case, the rules are prioritized based on the creation time. The rule that has the earliest creation time has the highest priority. Therefore, the rule that is configured for the /image directory takes effect.