HTTP Cache Headers#

Overview#

By default, SilverStripe sends headers which signal to HTTP caches that the response should be not considered cacheable. HTTP caches can either be intermediary caches (e.g. CDNs and proxies), or clients (e.g. browsers). The cache headers sent are Cache-Control: no-store, no-cache, must-revalidate;

HTTP caching can be a great way to speed up your website, but needs to be properly applied. Getting it wrong can accidentally expose draft pages or other protected content. The Google Web Fundamentals are a great way to learn about HTTP caching.

Cache Control Headers#

Overview#

In order to support developers in making safe choices around HTTP caching, we're using a HTTPCacheControl class to control if a response should be considered public or private. This is an abstraction on existing lowlevel APIs like HTTP::add_cache_headers() and SS_HTTPResponse->addHeader().

The HTTPCacheControl API makes it easier to express your caching preferences without running the risk of overriding essential core safety measures. Most commonly, these APIs will prevent HTTP caching of draft content.

It will also prevent caching of content generated with an active session, since the system can't tell whether session data was used to vary the output. In this case, it's up to the developer to opt-in to caching, after ensuring that certain execution paths are safe despite of using sessions.

The system behaviour does not guard against accidentally caching "private" content, since there are too many variations under which output could be considered private (e.g. a custom "approval" flag on a comment object). It is up to the developer to ensure caching is used appropriately there.

The HTTPCacheControl class supplements the HTTP helper class. It comes with methods which let developers safely interact with the Cache-Control header.

disableCache()#

Simple way to set cache control header to a non-cacheable state. Use this method over privateCache() if you are unsure about caching details. Takes precendence over unforced enableCache(), privateCache() or publicCache() calls.

Removes all state and replaces it with no-cache, no-store, must-revalidate. Although no-store is sufficient the others are added under recommendation from Mozilla

Does not set private directive, use privateCache() if this is explicitly required (details)

enableCache()#

Simple way to set cache control header to a cacheable state. Use this method over publicCache() if you are unsure about caching details.

Removes no-store and no-cache directives; other directives will remain in place. Use alongside setMaxAge() to indicate caching.

Does not set public directive. Usually, setMaxAge() is sufficient. Use publicCache() if this is explicitly required (details)

privateCache()#

Advanced way to set cache control header to a non-cacheable state. Indicates that the response is intended for a single user and must not be stored by a shared cache. A private cache (e.g. Web Browser) may store the response. Also removes public as this is a contradictory directive.

publicCache()#

Advanced way to set cache control header to a cacheable state. Indicates that the response may be cached by any cache. (eg: CDNs, Proxies, Web browsers) Also removes private as this is a contradictory directive

Priority#

Each of these highlevel methods has a boolean $force parameter which determines their application priority regardless of execution order. The priority order is as followed, sorted in descending order (earlier items will overrule later items):

• disableCache($force=true)
• privateCache($force=true)
• publicCache($force=true)
• enableCache($force=true)
• disableCache()
• privateCache()
• publicCache()
• enableCache()

Cache Control Examples#

Global opt-in for page content#

Enable caching for all page content (through Page_Controller).

Note: SilverStripe will still override this preference when a session is active,
a [CSRF token](/developer_guides/forms/form_security) token is present,
or draft content has been requested.

### Opt-out for a particular controller action

If a controller output relies on session data, cookies,
permission checks or other triggers for conditional output,
you can disable caching either on a controller level
(through `init()`) or for a particular action.

Note: SilverStripe will still override this preference when a session is active, a CSRF token token is present, or draft content has been requested.

Global opt-in, ignoring session (advanced)#

This can be helpful in situations where forms are embedded on the website. SilverStripe will still override this preference when draft content has been requested. CAUTION: This mode relies on a developer examining each execution path to ensure that no session data is used to vary output.

Use case: By default, forms include a CSRF token which starts a session with a value that's unique to the visitor, which makes the output uncacheable. But any subsequent requests by this visitor will also carry a session, leading to uncacheable output for this visitor. This is the case even if the output does not contain any forms, and does not vary for this particular visitor.

## Defaults

By default, PHP adds caching headers that make the page appear purely dynamic. This isn't usually appropriate for most 
sites, even ones that are updated reasonably frequently. SilverStripe overrides the default settings with the following 
headers:

  * The `Last-Modified` date is set to be most recent modification date of any database record queried in the generation 
  of the page.
  * The `Expiry` date is set by taking the age of the page and adding that to the current time.
  * `Cache-Control` is set to `max-age=86400, must-revalidate`
  * Since a visitor cookie is set, the site won't be cached by proxies.
  * Ajax requests are never cached.

## Max Age

The cache age determines the lifetime of your cache, in seconds.
It only takes effect if you instruct the cache control
that your response is public in the first place (via `enableCache()` or via modifying the `HTTP.cache_control` defaults).

```php
	HTTPCacheControl::singleton()
	    ->setMaxAge(60)

Last Modified#

Used to set the modification date to something more recent than the default. DataObject::__construct calls HTTP::register_modification_date( whenever a record comes from the database ensuring the newest date is present.

	HTTP::register_modification_date('2014-10-10');

A Vary header tells caches which aspects of the response should be considered when calculating a cache key, usually in addition to the full URL path. By default, SilverStripe will output a Vary header with the following content:

To change the value of the `Vary` header, you can change this value by specifying the header in configuration.

Note that if you use Director::is_ajax() on cached pages then you should add X-Requested-With to the vary header.