Magento cache regeneration lock

Avoid DOS when Magento regenerates its cache

The problem

Let's say you reset Magento's cache, on the next run the cache gets recreated but... what if you're website has a lot of traffic and, before the cache regeneration finishes, you got more requests?

What happens is that all the concurrent requests will trigger cache regeneration, sometimes these requests will create a spike in the server(s) load making your website unavailable.

Why is it happening?

Magento, by default, doesn't use a locking mechanism to prevent multiple concurrent cache regenerations. You could check it in the Mage_Core_Model_App class.

Solution

Overriding Mage_Core_Model_App class in your "local" codepool implementing a locking mechanism.

Why can't we use a standard Magento model rewrite?

Cause Mage_Core_Model_App is a special class that's not instanced (by the Magento core) using the Mage::getModel(), it's instead instanced with a "new Mage_Core_Model_App()" in app/Mage.php.

This is why I couldn't create a Magento module for this purpose.

Side benefit

This solution should also avoid multiple concurrent runs of the resource setup scripts.

Installation

If you're on Linux or Mac OSX use these lines of code:

cd magento-root-directory
mkdir -p app/code/local/Mage/Core/Model
cp app/code/core/Mage/Core/Model/App.php app/code/local/Mage/Core/Model/App.php
wget https://raw.githubusercontent.com/fballiano/magento-cache-regeneration-lock/master/fballiano-magento-cache-regeneration-lock.patch
patch -p1 < fballiano-magento-cache-regeneration-lock.patch
rm fballiano-magento-cache-regeneration-lock.patch app/code/local/Mage/Core/Model/App.php.orig

If you're on Windows simply copy core/Mage/Core/Model/App.php to the local pool and apply the patch in this repository.

Why a patch and not directly the patched file?

To have a better compatibility across multiple Magento (1.x) versions, both CE and EE.

Why are you directly instancing the Mage_Index_Model_Resource_Helper_Mysql4 class?

At that point in the run time it's not possible to successfully call Mage::getModel() or Mage::getResourceModel() methods, only a limited set of configurations are loaded and the Magento's class name resolution can't work.

##Compatibility Tested on Magento CE 1.9 and EE 1.13.1 (not working on 1.13.0).

It will not work on Magento < 1.7 cause it doesn't have Mage::getEdition() method.

The file based flock version can only work on singe server projects or on multiserver projects where the document_root (actually the var directory) is shared via NFS. This solution will not work on a truly separated multiserver environment. I'm working on a different approach for that situation.

##History The first version of the patch (available at http://bit.ly/1INibTw) used file based flock and could work only on a single server environment (or on multiserver with document root shared via NFS).

The newer version uses Magento's Mage_Index_Model_Resource_Helper_Mysql4 core class which locks on the database, solving the multiserver environment issue with much less custom code.

##Support If you have any issues with this extension, open an issue on GitHub.

##Contribution Any contributions are highly appreciated. The best way to contribute code is to open a pull request on GitHub.

##Developer Fabrizio Balliano
http://fabrizioballiano.com
@fballiano

##Licence OSL - Open Software Licence 3.0

##Copyright (c) Fabrizio Balliano

vaimo-wilko / magento-cache-regeneration-lock