Disabling FPC

[knowj]

The result of this module seems allow design exceptions to be used however it prevents 100% of FPC hits making the easier option being just to disable FPC all together.

If you look in app/etc/enterprise.xml Enterprise_Pacgecache_Model_Processor is statically typed within the cache request_processors which is then passed into Mage_Core_Model_Cache this Model is then created as a new object within Mage_Core_Model_Cache::processRequest() Mage_Core_Model_Cache::_getProcessor($processor).

When the new processor object is created it calls $this->_createRequestIds(); within Enterprise_PageCache_Model_Processor bypassing the Rewrite you have in place but then continues with the cache miss to load the page without FPC.

The DevBlog_MobileFpcFix_Model_Processor::_createRequestIds() is only triggered on the creation of the FPC Cachefile. Which follows the cache miss (every request) and creates the illusion of a working process.

It's frustrating that magento doesn't even cover their own base functionality within the FPC module. I'm currently looking into the best way to get around this, simply changing the ee request_processor isn't enough (generates a 404 with a quick test).

Within Enterprise_PageCache_Model_Processor::_getDesignPackage() the following line looks like it should handle design exceptions (but obviously doesn't)
$this->_designExceptionExistsInCache = $cacheInstance->getFrontend()->test(self::DESIGN_EXCEPTION_KEY);

The issue is still persistent in 1.13, I've not checked 1.14

[knowj]

Update:
The reason the 404 is thrown if you set the DevBlog_MobileFpcFix_Model_Processor as the ee request_processor in the config is when you getTheme it tries to get the current store. Because Mage isn't properly init the current store doesn't exist resulting in an exception being throw in:

Mage_Core_Model_App::getStore($id)
if (!isset($id)) {
$this->throwStoreException();
}

I'm going to look further into the DESIGN_EXCEPTION_KEY and whether it's used at all and if/how it should be working.

Edit: Further update:
In DevBlog_MobileFpcFix_Model_Processor::_createRequestIds() The line
$theme = Mage::getSingleton('core/design_package')->getTheme('frontend'); causes the issue because it is trying to load from something that doesn't yet exist and to maintain the benefit of FPC we don't want to.
The only logical way I can think to obtain a valid value here without init Magento would be to use either a cookie or request data to determine what the user should be displayed based on their rules.

[bubach]

Great information! Did you ever solve this and could provide a patch/pull request? I'm guessing I'm not the only one finding this through Google. I've gotten as far as using a cookie to decide which theme to use inside the processor, but I'm not sure about the part with "changing the ee request_processor". If I get this thing going I'll try to provide more information or a patch.

EDIT:
I have discovered that you also need to override Mage_Core_Model_Cache::_getProcessor to instantiate your own Processor.php there. Overrides to Mage_Core_Model_Cache needs to be directly under /app/etc/*.xml to be loaded in time, so I put it in local.xml
EDIT2:
The Enterprise_PageCache_Model_Processor::__construct method seems to be a good idea to override too.

[knowj]

Because I don't have EE 1.11 I wasn't sure approach I've taken is possible so I didn't pull request it. I have uploaded the approach I implemented here https://github.com/knowj/DesignExceptionsFix.

On EE1.12+ there's a _getDesignPackage method within the enterprise cache processor that takes into account package exceptions but not templates, layout, skin. I extended the observer and processor to take into account these additions.

The Observer stores the exceptions into the cache as a single serialized array this is then unserialized on each read request and compared against the users HTTP_USER_AGENT to build an array of exception rule themes. The array is then implode into a string and appended to the requestId generating a key for both the read and write.

To ensure you get the same read/write key you need to define the ee fpc processor in the local.xml due to the read processor object being created from the statically typed class defined in enterprise.xml (no rewrites take place)

<request_processors>
Jh_DesignExceptionsFix_Model_PageCache_Processor
</request_processors>

Unfortunately you can't use core/cookie to define a cookie on the read request due to it requiring the store to be defined (which doesn't exist on a read request) trying to call the set() method on the cookie throws a core store error. The overhead of the device detection is so small I didn't spend the time resolving this issue.

I'd happily fork and submit the changes as a pull request if you're happy with the approach.

[benjy]

I don't use Magento anymore so i'm not following this issue. @knowj, If you want the repository transferring across to yourself, let me know.

[knowj]

@Benjy14 If you want put a link in the readme to the repo I created for anyone trying to address the same issue. With the focus on responsive web design the use of design exceptions isn't as common. Hopefully it's been addressed with the theme changes in 1.14