HTTP cache headers
By default, Silverstripe CMS 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-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 HTTPCacheControlMiddleware
class to control if a response
should be considered public or private. This is an abstraction on the
HTTPResponse->addHeader()
lowlevel API.
The HTTPCacheControlMiddleware
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 HTTPCacheControlMiddleware
class replaces
(deprecated) caching methods in 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 precedence 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 the no-store
directive unless a max-age
is set; other directives will remain in place.
Use alongside setMaxAge()
to activate 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 (e.g. 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($force=true)
disableCache()
privateCache()
publicCache()
enableCache()
Cache control examples
Global opt-in for page content
Enable caching for all page content (through PageController
).
namespace {
use SilverStripe\CMS\Controllers\ContentController;
use SilverStripe\Control\Middleware\HTTPCacheControlMiddleware;
class PageController extends ContentController
{
public function init()
{
HTTPCacheControlMiddleware::singleton()
->enableCache()
// 1 minute
->setMaxAge(60);
parent::init();
}
}
}
Note: Silverstripe CMS will still override this preference when a session is active, a CSRF token token is present, or draft content has been requested.
Opt-out for a particular controller action
If a controller output relies on session data, cookies,
permission checks or other triggers for conditional output,
you can disable caching either on a controller level
(through init()
) or for a particular action.
namespace {
use SilverStripe\CMS\Controllers\ContentController;
use SilverStripe\Control\Middleware\HTTPCacheControlMiddleware;
class PageController extends ContentController
{
public function myprivateaction($request)
{
HTTPCacheControlMiddleware::singleton()
->disableCache();
return $this->myPrivateResponse();
}
}
}
Note: Silverstripe CMS will still override this preference when a session is active, a CSRF token token is present, or draft content has been requested.
Global opt-in, ignoring session (advanced)
This can be helpful in situations where forms are embedded on the website. Silverstripe CMS will still override this preference when draft content has been requested. CAUTION: This mode relies on a developer examining each execution path to ensure that no session data is used to vary output.
Use case: By default, forms include a CSRF token which starts a session with a value that's unique to the visitor, which makes the output uncacheable. But any subsequent requests by this visitor will also carry a session, leading to uncacheable output for this visitor. This is the case even if the output does not contain any forms, and does not vary for this particular visitor. Forms can also contain submission data when they're redisplayed after a validation error.
namespace {
use SilverStripe\CMS\Controllers\ContentController;
use SilverStripe\Control\Middleware\HTTPCacheControlMiddleware;
class PageController extends ContentController
{
public function init()
{
HTTPCacheControlMiddleware::singleton()
// DANGER ZONE
->enableCache($force = true)
// 1 minute
->setMaxAge(60);
parent::init();
}
}
}
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 cacheable in the first place
(via enableCache()
, publicCache()
or privateCache()
),
or via modifying the HTTP.cache_control
defaults).
use SilverStripe\Control\Middleware\HTTPCacheControlMiddleware;
HTTPCacheControlMiddleware::singleton()
->setMaxAge(60)
Note that setMaxAge(0)
is NOT sufficient to disable caching in all cases,
use disableCache()
instead.
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.
use SilverStripe\Control\HTTP;
HTTP::register_modification_date('2014-10-10');
Vary
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.
By default, Silverstripe CMS will output a Vary
header with the following content:
Vary: X-Forwarded-Protocol
To change the value of the Vary
header, you can change this value by specifying the header in configuration.
SilverStripe\Control\HTTP:
vary: ""
Note that if you use Director::is_ajax()
on cached pages
then you should add X-Requested-With
to the vary header.
Testing
HTTP Cache headers are disabled in developer environments by default to prevent any confusion around content not updating. To enable HTTP Cache Headers in dev mode you can add the following in YAML config.
---
Name: 'app_httpconfig'
After: '#httpconfig-dev'
Only:
environment: dev
---
SilverStripe\Control\Middleware\HTTPCacheControlMiddleware:
defaultState: 'enabled'
defaultForcingLevel: 0