Azure CDN Query String & Caching behaviour

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.

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 duration; use the provided cache duration instead. This will not override cache-control: no-cache.
• Set if missing: Honor origin-provided cache-directive headers, if they exist; otherwise, use the provided cache duration.

With Azure Content Delivery Network (CDN), you can control how files are cached for a web request that contains a query string. In a web request with a query string, the query string is that portion of the request that occurs after a question mark (?). A query string can contain one or more key-value pairs, in which the field name and its value are separated by an equals sign (=). Each key-value pair is separated by an ampersand (&). For example, http://www.contoso.com/content.mov?field1=value1&field2=value2. If there is more than one key-value pair in a query string of a request, their order does not matter.

Three query string modes are available:

• Ignore query strings: Default mode. In this mode, the CDN point-of-presence (POP) node passes the query strings from the requestor to the origin server on the first request and caches the asset. All subsequent requests for the asset that are served from the POP ignore the query strings until the cached asset expires.
• Bypass caching for query strings: In this mode, requests with query strings are not cached at the CDN POP node. The POP node retrieves the asset directly from the origin server and passes it to the requestor with each request.
• Cache every unique URL: In this mode, each request with a unique URL, including the query string, is treated as a unique asset with its own cache. For example, the response from the origin server for a request for example.ashx?q=test1 is cached at the POP node and returned for subsequent caches with the same query string. A request for example.ashx?q=test2 is cached as a separate asset with its own time-to-live setting.

source:https://docs.microsoft.com/en-us/azure/cdn/cdn-query-string