Control Azure CDN caching behavior with caching rules
Caching rules are available only for Azure CDN Standard from Verizon and Azure CDN Standard from Akamai profiles. For Azure CDN Premium from Verizon profiles, you must use the Azure CDN rules engine in the Manage portal for similar functionality.
Azure Content Delivery Network (CDN) offers two ways to control how your files are cached:
Caching rules: This article describes how you can use content delivery network (CDN) caching rules to set or modify default cache expiration behavior both globally and with custom conditions, such as a URL path and file extension. Azure CDN provides two types of caching rules:
Global caching rules: You can set one global caching rule for each endpoint in your profile, which affects all requests to the endpoint. The global caching rule overrides any HTTP cache-directive headers, if set.
Custom caching rules: You can set one or more custom caching rules for each endpoint in your profile. Custom caching rules match specific paths and file extensions, are processed in order, and override the global caching rule, if set.
Query string caching: You can adjust how the Azure CDN treats caching for requests with query strings. For information, see Control Azure CDN caching behavior with query strings. If the file is not cacheable, the query string caching setting has no effect, based on caching rules and CDN default behaviors.
For information about default caching behavior and caching directive headers, see How caching works.
Accessing Azure CDN caching rules
Open the Azure portal, select a CDN profile, then select an endpoint.
In the left pane under Settings, select Caching rules.
The Caching rules page appears.
Caching behavior settings
For global and custom caching rules, you can specify the following Caching behavior settings:
Bypass cache: Do not cache and ignore origin-provided cache-directive headers.
Override: Ignore origin-provided cache-directive headers; use the provided cache duration instead.
Set if missing: Honor origin-provided cache-directive headers, if they exist; otherwise, use the provided cache duration.
Cache expiration duration
For global and custom caching rules, you can specify the cache expiration duration in days, hours, minutes, and seconds:
For the Override and Set if missing Caching behavior settings, valid cache durations range between 0 seconds and 366 days. For a value of 0 seconds, the CDN caches the content, but must revalidate each request with the origin server.
For the Bypass cache setting, the cache duration is automatically set to 0 seconds and cannot be changed.
Custom caching rules match conditions
For custom cache rules, two match conditions are available:
Path: This condition matches the path of the URL, excluding the domain name, and supports the wildcard symbol (*). For example, /myfile.html, /my/folder/*, and /my/images/*.jpg. The maximum length is 260 characters.
Extension: This condition matches the file extension of the requested file. You can provide a list of comma-separated file extensions to match. For example, .jpg, .mp3, or .png. The maximum number of extensions is 50 and the maximum number of characters per extension is 16.
Global and custom rule processing order
Global and custom caching rules are processed in the following order:
Global caching rules take precedence over the default CDN caching behavior (HTTP cache-directive header settings).
Custom caching rules take precedence over global caching rules, where they apply. Custom caching rules are processed in order from top to bottom. That is, if a request matches both conditions, rules at the bottom of the list take precedence over rules at the top of the list. Therefore, you should place more specific rules lower in the list.
Global caching rule:
- Caching behavior: Override
- Cache expiration duration: 1 day
Custom caching rule #1:
- Match condition: Path
- Match value: /home/*
- Caching behavior: Override
- Cache expiration duration: 2 days
Custom caching rule #2:
- Match condition: Extension
- Match value: .html
- Caching behavior: Set if missing
- Cache expiration duration: 3 days
When these rules are set, a request for <endpoint hostname>.azureedge.net/home/index.html triggers custom caching rule #2, which is set to: Set if missing and 3 days. Therefore, if the index.html file has
Expires HTTP headers, they are honored; otherwise, if these headers are not set, the file is cached for 3 days.
Files that are cached before a rule change maintain their origin cache duration setting. To reset their cache durations, you must purge the file.
Azure CDN configuration changes can take some time to propagate through the network:
- For Azure CDN Standard from Akamai profiles, propagation usually completes within one minute.
- For Azure CDN Standard from Verizon profiles, propagation usually completes in 10 minutes.
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