Response Tagging

The ResponseTagger helps you keep track of tags for a response. It can add the tags as a response header that you can later use to invalidate all cache entries with that tag.

Setup

Note

Make sure to configure your proxy for tagging first.

The response tagger uses an instance of TagHeaderFormatter to know the header name used to mark tags on the content and to format the tags into the correct header value. This library ships with a CommaSeparatedTagHeaderFormatter that formats an array of tags into a comma-separated list. The format for specifying the tags depends on the caching proxy you use and its configuration. The default settings are made to match and work out of the box. If you need to change anything, be aware that the caching proxy is configured separately from your PHP application and the ResponseTagger - it is up to you to make sure the configurations match.

For example, the default configuration of Varnish provided in this library uses the header X-Cache-Tags with a comma-separated list of tags. If you don’t change the TagHeaderFormatter nor the header name, just instantiate the response tagger with its default settings:

use FOS\HttpCache\ResponseTagger;

$responseTagger = new ResponseTagger();

If you need a different behavior, you can provide your own TagHeaderFormatter instance. Don’t forget to also adjust your proxy configuration to match the response. To use xkey tags, instantiate the CommaSeparatedTagHeaderFormatter yourself with the appropriate header and glue, and pass it to the ResponseTagger:

use FOS\HttpCache\ResponseTagger;
use FOS\HttpCache\TagHeaderFormatter;

$formatter = new CommaSeparatedTagHeaderFormatter('xkey', ' ');
$responseTagger = new ResponseTagger(['header_formatter' => $formatter]);

The response tagger validates tags that you set. By default, it simply ignores empty strings and does not add them to the list of tags. You can set the response tagger to strict mode to have it throw an InvalidTagException on empty tags:

$responseTagger = new ResponseTagger(['strict' => true]);

Working with large numbers of tags

Depending on how many tags your system usually generates your tags header value might get pretty long. In that case, again depending on your setup, you might run into server exceptions because the header value is too big to send. Mostly, this value seems to be about 4KB. The only thing you can do in such a case is to split up one header into multiple ones.

Note

Of course, your proxy then has to support multiple header values otherwise you’ll end up with a proxy that only reads the first line of your tags.

This library ships with a MaxHeaderValueLengthFormatter that does the splitting for you. You give it an inner formatter and the maximum length like so:

use FOS\HttpCache\TagHeaderFormatter\CommaSeparatedTagHeaderFormatter;
use FOS\HttpCache\TagHeaderFormatter\MaxHeaderValueLengthFormatter

$inner = new CommaSeparatedTagHeaderFormatter('X-Cache-Tags', ',');
$formatter new MaxHeaderValueLengthFormatter($inner, 4096);

Note

Both, Varnish and Symfony HttpCache support multiple cache tag headers.

Usage

With tags you can group related representations so it becomes easier to invalidate them. You will have to make sure your web application adds the correct tags on all responses. You can add tags to the response using:

$responseTagger->addTags(['tag-two', 'group-a']);

Before any content is sent out, you need to send the tag header:

header(sprintf('%s: %s',
    $responseTagger->getTagsHeaderName(),
    $responseTagger->getTagsHeaderValue()
));
$responseTagger->clear();

The call to clear is only relevant if the same PHP process handles multiple requests. This happens for example when you cache on user context with the Symfony HttpCache.

Tip

If you are using Symfony with the FOSHttpCacheBundle, the tags added to ResponseTagger are added to the response automatically. You also have additional methods of defining tags with annotations and on URL patterns.

Assume you sent four responses:

Response:	`X-Cache-Tags` header:
`/one`	`tag-one`
`/two`	`tag-two, group-a`
`/three`	`tag-three, group-a`
`/four`	`tag-four, group-b`

You can now invalidate some URLs using tags:

$tagHandler->invalidateTags(['group-a', 'tag-four'])->flush();

This will ban all requests having either the tag group-a /or/ tag-four. In the above example, this will invalidate /two, /three and /four. Only /one will stay in the cache.

Note

For further reading on tag invalidation see cache-invalidator page. For changing the cache header, configure your proxy.