silverstripe-versionfeed/docs/en/developer.md

# Version Feed

## Development

### Feed actions

Creating functions called `changes` or `allchanges` on any of your page types or controllers will cause confusion with
the extensions defined on the extension.

### Enabling / Disabling

By default the `allchanges` and `changes` feed are disabled.

The `allchanges` feed can be enabled by setting the `SilverStripe\VersionFeed\VersionFeed.allchanges_enabled` config to true. If this is true, then the allchanges feed can still be disabled by unchecking the "All page changes" checkbox in the "Settings" section in the CMS. 

Likewise, the `changes` feed for each page can be globally enabled by setting the `SilverStripe\VersionFeed\VersionFeed.changes_enabled`
config to true. If this is true, then each page can still be individually disabled by unchecking the
'Make history public' checkbox in the CMS under page settings.

See [user documentation on enabling / disabling](userguide/index.md#enabling--disabling).

### Default RSS action

Templates can offer a "Subscribe" link with a link to the most relevant RSS feed. This will default to the changes feed
for the current page. You can override this behaviour by defining the `getDefaultRSSLink` function in your page type
and returning the URL of your desired RSS feed:

```php
class MyPage extends Page
{
	public function getDefaultRSSLink()
	{
		return $this->Link('myrssfeed');
	}
}
```

This can be used in templates as `$DefaultRSSLink`.

### Rate limiting and caching

By default all content is filtered based on the rules specified in `vendor/silverstripe/versionfeed/_config/versionfeed.yml`.
Two filters are applied on top of one another:

 * `SilverStripe\VersionFeed\Filters\CachedContentFilter` provides caching of versions based on an identifier built up of the record ID and the 
   most recently saved version number. There is no configuration required for this class.
 * `SilverStripe\VersionFeed\Filters\RateLimitFilter` provides rate limiting to ensure that requests to uncached data does not overload the 
   server. This filter will only be applied if the `SilverStripe\VersionFeed\Filters\CachedContentFilter` does not have any cached record
  for a request.

Either one of these can be replaced, added to, or removed, by adjusting the `SilverStripe\VersionFeed\VersionFeedController.dependencies`
config to point to a replacement (or no) filter.

For smaller servers where it's reasonable to apply a strict approach to rate limiting the default
settings should be sufficient. The `SilverStripe\VersionFeed\Filters\RateLimitFilter.lock_bypage` config defaults to false, meaning that a
single limit will be applied to all URLs. If set to true, then each URL will have its own rate limit,
and on smaller servers with lots of concurrent requests this can still overwhelm capacity. This will
also leave smaller servers vulnerable to DDoS attacks which target many URLs simultaneously.
This config will have no effect on the `allchanges` method.

`SilverStripe\VersionFeed\Filters\RateLimitFilter.lock_byuserip` can be set to true in order to prevent requests from different users
interfering with one another. However, this can provide an ineffective safeguard against malicious DDoS attacks
which use multiple IP addresses.

Another important variable is the `SilverStripe\VersionFeed\Filters\RateLimitFilter.lock_timeout` config, which is set to 5 seconds by default.
This should be increased on sites which may be slow to generate page versions, whether due to lower
server capacity or volume of content (number of page versions). Requests to this page after the timeout
will not trigger any rate limit safeguard, so you should be sure that this is set to an appropriate level.

You can set the `SilverStripe\VersionFeed\Filters\ContentFilter.cache_lifetime` config in order to control the maximum age of the cache.
This is an integer value in seconds, and defaults to 300 (five minutes). Set it to 0 or null to make this
cache unlimited.