Magento's REQUEST_PATH FPC Tag

Published: February 6, 2017

Tags:

When Magento saves data to cache, it has the option to add “tags” to the data it is saving. This feature, which is built into Zend Framework 1, is particularly useful as it allows the application easily invalidate all data associated with a particular tag.

For example, when the name of a product changes, all pages on which that product was displayed can be invalidated by passing the appropriate tag to the cache instance’s clean method. If the product with entity_id 1 had changed, cleaning all cache data would look like this…

Enterprise_PageCache_Model_Cache::getCacheInstance()
    ->clean('CATALOG_PRODUCT_1');

One tag that Magento’s Enterprise_PageCache module makes uses of is the REQUEST_PATH tag. In this post, we’ll first explore how Magento uses this tag out-of-box. Then, we’ll take a look at how we can take advantage of this tag.

How It Gets Added

If you look at the processRequestResponse method in Enterprise_PageCache_Model_Processor the first line of code you’ll see looks something like this…

$this->addRequestTag(
    Enterprise_PageCache_Helper_Url::prepareRequestPathTag(
        $request->getOriginalPathInfo()
    )
);

addRequestTag simply updates the protected $_requestTags property with the specified tag(s).

public function addRequestTag($tag)
{
    if (!is_array($tag)) {
        $tag = array($tag);
    }
    foreach ($tag as $value) {
        if (!in_array($value, $this->_requestTags)) {
            $this->_requestTags[] = $value;
        }
    }
    return $this;
}

Later, when Enterprise_PageCache saves the response, it will tag it with all the $_requestTags.

$cacheInstance->save(
    $content,
    $cacheId,
    $this->getRequestTags()
);

Composition of the tag

Here’s what prepareRequestPathTag looks like…

public static function prepareRequestPathTag($path)
{
    $path = trim((string)$path, '/ ');
    return Enterprise_PageCache_Model_Processor::REQUEST_PATH_PREFIX . md5($path);
}

Essentially, it just md5s the path portion of the URL. In other words…

User requests www.example.com/hello
“hello” is extracted from URL and md5-ed
Response cache entry is tagged with REQUEST_PATH_5d41402abc4b2a76b9719d911017c592

Notably, the query string is not considered when generating the REQUEST_PATH tag.

What Does Magento Do With This?

If we look in Enterprise_PageCache_Model_Observer we’ll see the following method…

public function clearRequestCacheByTag(Varien_Event_Observer $observer)
{
    if (!$this->isCacheEnabled()) {
        return $this;
    }
    $redirect = $observer->getEvent()->getRedirect();
    $this->_cacheInstance->clean(
        array(
            Enterprise_PageCache_Helper_Url::prepareRequestPathTag($redirect->getData('identifier')),
            Enterprise_PageCache_Helper_Url::prepareRequestPathTag($redirect->getData('target_path')),
            Enterprise_PageCache_Helper_Url::prepareRequestPathTag($redirect->getOrigData('identifier')),
            Enterprise_PageCache_Helper_Url::prepareRequestPathTag($redirect->getOrigData('target_path'))
        )
    );
    return $this;
}

Looking at the config.xml file in Enterprise_PageCache we can see that this method will be called when the following events fire…

rewrite_url_partial_reindex
redirect_save_commit_after
redirect_delete_commit_after

Magento leverages this tag to clean the cache entries when URL redirects change.

Why Should We Care About This?

Knowing about the REQUEST_PATH tag is useful as we can use it to flush a single page from the cache. For example, if we wanted to flush www.example.com/hello, we’d just have to do the following…

$tag = Enterprise_PageCache_Helper_Url::prepareRequestPathTag('hello');
Enterprise_PageCache_Model_Cache::getCacheInstance()->clean($tag);

This trick can avoid blowing away the entire full page cache in some cases.

Conclusion

Hopefully some of you found learning about the the REQUEST_PATH tag to be interesting and useful. If you have any questions or comments, feel free to drop a note below, or, as always, you can reach me on Twitter as well.