It fully caches static pages and the most part of pages with dynamic content. But as always, when your shop needs special customizations – you should take care about extending some of Magento features. Full page caching is not an exception in this case.

Let’s see the example from the real life. Consider that we need a block which should represent some products and these products are different for each customer. This block should be shown on the homepage. So we go to the admin panel, choose the homepage from CMS and insert something like this:

   {{block type="mymodule/products" template="mymodule/products.phtml"}}

We set up special logic for the block class which generates specific products collection for each customer (let’s say using id of currently logged in user). While full page cache is turned off everything is alright. But once you turn it on you will see things a bit different from what you expected. Once first customer comes to the homepage Magento caches the whole content including our special products block and all other customers will see the same items.

How are we going to solve this problem? Fortunately Magento caching mechanism is flexible enough and there is a feature which we may call a “hole” or a “placeholder”. It provides an ability to define blocks on the page which will have special caching logic, different from the rest of the page.
Back to our example, let’s create an xml file called cache.xml under the module etc folder (near to the config.xml):

  <?xml version="1.0" encoding="UTF-8"?>
  <config>
    <placeholders>
        <homepage_products>
        <block>mymodule/products</block>
        <placeholder>HOMEPAGE_PRODUCTS</placeholder>
        <container>Oggetto_Mymodule_Model_PageCache_Container_Homepageproducts</container>
        <cache_lifetime>86400</cache_lifetime>
        </homepage_products>
    </placeholders>
  </config>

You may see that we defined a placeholder which will catch our block mymodule/products and cache it separately using our own container class:

<?php
/**
* Placeholder container for homepage special products block
*
* @category    Mymodule
* @package     Oggetto_Mymodule
*/
class Oggetto_Mymodule_Model_PageCache_Container_Homepageproducts extends Enterprise_PageCache_Model_Container_Abstract
{
/**
* Get customer identifier from cookies
*
* @return string
*/
protected function _getIdentifier()
{
return $this->_getCookieValue(Enterprise_PageCache_Model_Cookie::COOKIE_CUSTOMER, '');
}

/**
* Get cache identifier
*
* @return string
*/
protected function _getCacheId()
{
return 'HOMEPAGE_PRODUCTS' . md5($this->_placeholder->getAttribute('cache_id') . $this->_getIdentifier());
}

/**
* Render block content
*
* @return string
*/
protected function _renderBlock()
{
$blockClass = $this->_placeholder->getAttribute('block');
$template = $this->_placeholder->getAttribute('template');

$block = new $blockClass;
$block->setTemplate($template);
return $block->toHtml();
}
}

Let’s explore this class more detailed:

protected function _renderBlock
This method defines the logic of our block rendering in context of caching. We say that it should work as we defined originally: using that defined block class and template.

protected function _getCacheId
This is more interesting method which defines the cache id which will be used for our special page piece. This is key moment here, because we should use different cache id for each customer. So we set unique prefix HOMEPAGE_PRODUCTS and then call method which will give us a customer identifier:

protected function _geIdentifier
Here we simply retrieve customer id from the cookie.

That’s all. Now each customer will get his own version of cached products list and the rest of cached homepage which is the same for everybody. One important note here is that our product list is actually being cached, this means that this solution will not require any additional queries each time user visits the site. Only first time for each customer, so it won’t kill the performance which is so important for you valuable enterprise clients.

*** Enjoy caching ***