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Latest commit 5218ebc Feb 19, 2016 @tractorcow tractorcow Merge pull request #14 from helpfulrobot/add-standard-scrutinizer-config
Added standard Scrutinizer config

Controller Policy

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 HTTP::add_cache_headers.

An example Page extension PageControlledPolicy is also provided utilising CachingPolicy's ability to customise max-age based on CMS configuration on specific objects.

Simple policy

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 HomePage_Controller:

    class: CachingPolicy
      cacheAge: 300
    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.

Ignoring domains

If you wish to exclude some domains from the policies completely, you can do the following:

    - '/.*\$/'

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.

Vary headers

CachingPolicy also allows customisation of Vary headers through the config system:

    class: CachingPolicy
      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

Overriding policies

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 FooPage_Controller.

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):

    Policies: '%$NoopPolicy'

The 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 NoopPolicy.


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:

    class: CachingPolicy
      cacheAge: 900
    Policies: '%$GeneralCachingPolicy'
    - 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 minutes).

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:

    class: CachingPolicy
      cacheAge: 300
    class: CachingPolicy
      cacheAge: 3600
     class: CustomHeaderPolicy
         Custom-Header: "Hello"
      - '%$LongCachingPolicy'
      - '%$ShortCachingPolicy'
      - '%$CustomPolicy'

Outcome of the array merging for the home page will be as follows:

  • LongCachingPolicy
  • ShortCachingPolicy
  • CustomPolicy

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.

Developer notes

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 ModelAsController and RootURLController do not trigger application of policies, and it is expected that only one controller will trigger the policy.