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Introduction intro

Traffic passes through the CDN to an Apache web server layer, which supports modules including Dispatcher. To increase performance, Dispatcher is used primarily as a cache to limit processing on the publish nodes.
Rules can be applied to the Dispatcher configuration to modify any default cache expiration settings, resulting in caching at the CDN. Dispatcher also respects the resulting cache expiration headers if enableTTL is enabled in the Dispatcher configuration, implying that it refreshes specific content even outside of content being republished.

This page also describes how the Dispatcher cache is invalidated and how caching works at the browser level regarding client-side libraries.

Caching caching

Caching of HTTP responses in AEM as a Cloud Service鈥檚 CDN is controlled by the following HTTP response headers from the origin: Cache-Control, Surrogate-Control, or Expires.

These cache headers are typically set in AEM Dispatcher vhost configurations using mod_headers, but can also be set in custom Java鈩� code running in AEM Publish itself (see How to enable CDN caching).

The cache key for CDN resources contains the full request url, including query parameters, so every different query parameter will produce a different cache entry. Consider removing unwanted query parameters; see below for improving cache hit ratio.

Origin responses that contain private, no-cache or no-store in Cache-Control are not cached by the AEM as a Cloud Service鈥檚 CDN (see How to disable CDN cachingfor more details). Also, responses that are setting cookies, i.e. have a Set-Cookie response header are not cached by the CDN.

HTML/Text html-text

Dispatcher configuration sets some default caching headers for text/html content type.

Define DISABLE_DEFAULT_CACHING

This method is useful, for example, when your business logic requires fine-tuning of the age header (with a value based on calendar day) since by default the age header is set to 0. That said, exercise caution when turning off default caching.

Client-Side libraries (js,css) client-side-libraries

Images and any content large enough to be stored in blob storage images

The default behavior for programs created after mid-May 2022 (specifically, for program ids that are higher than 65000) is to cache by default,while also respecting the request鈥檚 authentication context. Older programs (program ids equal or lower than 65000) do not cache blob content by default.

In both cases, the caching headers can be overridden on a finer grained level at the Apache/Dispatcher layer by using the Apache mod_headers directives, for example:

   <LocationMatch "^/content/.*\.(jpeg|jpg)$">
     Header set Cache-Control "max-age=222"
     Header set Age 0
   </LocationMatch>

When modifying the caching headers at the Dispatcher layer, be cautious not to cache too widely. See the discussion in the HTML/text section above. Also, make sure that assets that are meant to be kept private (rather than cached) are not part of the LocationMatch directive filters.

JCR resources (bigger than 16KB) that are stored in blob store are typically served as 302 redirects by AEM. These redirects are intercepted and followed by CDN and the content is delivered directly from the blob store. Only a limited set of headers can be customised on these responses. For example, to customise Content-Disposition you should use the dispatcher directives as follows:

<LocationMatch "\.(?i:pdf)$">
  ForceType application/pdf
  Header set Content-Disposition inline
  </LocationMatch>

The list of headers that can be be customised on blob responses are:

content-security-policy
x-frame-options
x-xss-protection
x-content-type-options
x-robots-tag
access-control-allow-origin
content-disposition
permissions-policy
referrer-policy
x-vhost
content-disposition
cache-control
vary

New default caching behavior new-caching-behavior

The AEM layer sets cache headers depending on whether the cache header has already been set and the value of the request type. If no cache control header is set, public content is cached, and authenticated traffic is set to private. If a cache control header is set, the cache headers are left untouched.

While not recommended, it is possible to change the new default behavior to follow the older behavior (program ids equal or lower than 65000) by setting the Cloud Manager environment variable AEM_BLOB_ENABLE_CACHING_HEADERS to false.

Older default caching behavior old-caching-behavior

The AEM layer does not cache blob content by default.

Now, images in blob storage that are marked private cannot be cached at the Dispatcher using Permission Sensitive Caching. The image is always requested from the AEM origin and served if the user is authorized.

Other content file types in node store other-content

Further Optimizations further-optimizations