This module has been designed to provide the ability to configure response policies that apply per specific Controller.
It comes with a small selection of policies for implementing caching:
- CachingPolicy: rewrite of SilverStripe default.
- CustomHeaderPolicy: allow adding any headers via config system.
- NoopPolicy: allows zero-ing policies in Controllers extending other Controllers.
- BackwardsCompatibleCachingPolicy: pretty much verbatim copy of
An example Page extension PageControlledPolicy is also provided utilising CachingPolicy's ability to customise max-age based on CMS configuration on specific objects.
Let's say we want to apply a caching header of max-age 300 to the HomePage only. This module comes with a
CachingPolicy which by implementing the
ControllerPolicy interface can be applied to anything derived from
Controller. This class can also be configured to specify the custom max-age via (injected) properties.
Using this policy is done via your project-specific config.yml. We configure the pseudo-singleton via
Dependency Injection and apply it directly to
Injector: StandardCachingPolicy: class: CachingPolicy properties: cacheAge: 300 HomePage_Controller: dependencies: Policies: '%$StandardCachingPolicy'
Every policy will set headers on top of the default framework's
HTTP::add_cache_headers, which is exactly what we
want. This allows us to for example customise the
Vary headers per policy, which were previously hardcoded.
If you wish to exclude some domains from the policies completely, you can do the following:
ControllerPolicyRequestFilter: ignoreDomainRegexes: - '/.*\.uat.server.com$/'
This could be useful for example if you wish to disable caching on test servers, or if you are doing aggressive caching and want your editors to see changed resources immediately.
CachingPolicy also allows customisation of Vary headers through the config system:
Injector: StandardCachingPolicy: class: CachingPolicy properties: cacheAge: 300 vary: 'Cookie, X-Forwarded-Protocol, Accept-Language'
Any URL which content depends on an impulse from the visitor should use Vary header to encode this dependency, otherwise caches might serve wrong content to the wrong user (possibly even confidential data!).
Here is a table of some more obvious Vary headers.
CachingPolicy uses a relatively safe combination of
Cookie, X-Forwarded-Protocol. Keep in mind the more of these you specify, the more partitioned the cache, which will nullify potential gains. Use as few as you are confident with.
|Vary on||Description||Cache partitioning impact|
|Accept-Encoding||Vary on content deflate method - Apache will deliver different content depending on accepted encoding. Automatically added by Apache mod_header.||low|
|Cookie||Vary on user session. Pretty much partitions the responses into generic and personalised. Note that for this to work well, the cache needs to purge frontend-only cookies such as __utma from the requests. A sensible addition.||low|
|X-Forwarded-Protocol||Vary on protocol such as http or https - use when the cache is behind a reverse-proxy, as there is often a difference in "BaseURL" which is not reflected in the URL the cache sees. A sensible addition.||low|
|X-Forwarded-Proto||A variation on X-Forwarded-Protocol, choose one appropriate to your reverse-proxy.||low|
|Accept||Vary on the response format. Some URLs, especially the API endpoints, can produce different output depending on what the user accepts: i.e. JSON vs. XML. Avoid if possible, and instead encode the content type in the URL.||medium|
|Accept-Language||Vary on the accepted language, if you are providing different language content depending on the user browser's setting. Avoid if possible, and instead encode the language in the URL.||medium|
|User-Agent||Vary on the user's device. There is so many user strings around this will effectively disable your cache. Avoid at all costs, and instead use responsive themes.||extreme|
If you apply a policy to a certain
Controller it will apply to all inheriting controllers too. For example if we have
FooPage_Controller extends Page_Controller then the
Page_Controller policy will also affect the
You can break that chain easily by applying a policy to the inheriting controller as long as you are not using arrays for configuration (which you ordinarily wouldn't be - but see the "Complex policies" chapter below):
FooPage_Controller: dependencies: Policies: '%$NoopPolicy'
NoopPolicy is a policy that does nothing, so you can use it to "disable" certain controllers. This is useful for example for GET-based multi-step forms (via the silverstripe-multiform) module, where steps are traversed via GET requests, and URIs don't differ - hence preventing your from actually progressing through the form.
Note that you can use any other policy to override the existing one - it doesn't need to be
Here is an example of how to implement CMS capability to override the max-age per specific page. In your config file put the following statements:
Injector: GeneralCachingPolicy: class: CachingPolicy properties: cacheAge: 900 Page_Controller: dependencies: Policies: '%$GeneralCachingPolicy' Page: extensions: - PageControlledPolicy
Here, applying the
PageControlledPolicy extension to the
Page results in a new "MaxAge" field being written into the
DB, and a new tab available ("Caching") which lets the ADMIN user tweak the cache max-age header (denominated in
Complex policies via array-merging
This example illustrates the usage of array-merging capability of the config system, which will enable you to simulate policy inheritance that will reflect your class diagram.
In this example we want to configure a global setting consisting of two policies, one setting the max-age to 300, and second to configure custom header. Then we want to add more specific policy for the home page max-age, while keeping the custom header. Here is how to achieve this using the config system:
Injector: ShortCachingPolicy: class: CachingPolicy properties: cacheAge: 300 LongCachingPolicy: class: CachingPolicy properties: cacheAge: 3600 CustomPolicy: class: CustomHeaderPolicy properties: headers: Custom-Header: "Hello" HomePage_Controller: dependencies: Policies: - '%$LongCachingPolicy' Page_Controller: dependencies: Policies: - '%$ShortCachingPolicy' - '%$CustomPolicy'
Outcome of the array merging for the home page will be as follows:
We handle this array in reverse order, meaning that by default the top policy (most specific Controller) will override the others. This does not mean many Controller policies will trigger - rather, one Controller will apply a merged set.
Caution: you can either use the array syntax, or value syntax. Choose what's easier.
At the moment the
preRequest filters are not too useful because they don't allow us to short-circuit
the execution and allow us to for example return 304 early. We are working on a
pull request to make it possible to return something
else than an
SS_HTTP_Exception from this handler.
Another thing is that the policies will be applied in the Controller order of initialisation, so if multiple Controllers are invoked the
latter will override the former. HOWEVER this is very unlikely and has nothing to do with the inheritance of classes. This relates to how the Controller stack is invoked in SilverStripe. The extension point in
ControllerPolicyApplicator has been chosen such that the
RootURLController do not trigger
application of policies, and it is expected that only one controller will trigger the policy.