Magento – How to prevent Redis cache race conditions

cacheredis

We have a multi server Magento installation using Redis as cache backend.

Every time I flush Redis (FLUSHDB or FLUSHALL) under traffic Redis runs into race conditions.

Calling 

redis-cli -h xxx.xxx.xxx.xxx monitor | egrep set | cut -c 1-120

I get lots of lines like this

1444810735.659889 [0 int.ern.ip.3:55588] "hmset" "zc:k:my_cache_prefix_CONFIG_GLOBAL_STORES" "d" "gz:\...
1444810735.779509 [0 int.ern.ip.0:38044] "hmset" "zc:k:my_cache_prefix_CONFIG_GLOBAL_STORES_PL" "d" ""gz:\...
1444810735.787633 [0 int.ern.ip.7:51360] "hmset" "zc:k:my_cache_prefix_CONFIG_GLOBAL_STORES_SE" "d" ""gz:\...
1444810735.795483 [0 int.ern.ip.4:33149] "hmset" "zc:k:my_cache_prefix_CONFIG_GLOBAL_STORES_EU" "d" ""gz:\...
1444810735.803374 [0 int.ern.ip.4:33142] "hmset" "zc:k:my_cache_prefix_CONFIG_GLOBAL_STORES_CZ" "d" ""gz:\...
1444810735.831377 [0 int.ern.ip.1:40537] "hmset" "zc:k:my_cache_prefix_CONFIG_GLOBAL_STORES_UK" "d" ""gz:\...
1444810735.840627 [0 int.ern.ip.7:51359] "hmset" "zc:k:my_cache_prefix_CONFIG_GLOBAL_STORES_EU" "d" ""gz:\...
1444810735.847732 [0 int.ern.ip.7:51356] "hmset" "zc:k:my_cache_prefix_CONFIG_GLOBAL_STORES_AT" "d" ""gz:\...
1444810735.854708 [0 int.ern.ip.1:40535] "hmset" "zc:k:my_cache_prefix_CONFIG_GLOBAL_STORES_FR" "d" ""gz:\...
1444810735.864083 [0 int.ern.ip.7:51345] "hmset" "zc:k:my_cache_prefix_CONFIG_GLOBAL_STORES_PL" "d" ""gz:\...
1444810735.872110 [0 int.ern.ip.0:38046] "hmset" "zc:k:my_cache_prefix_CONFIG_GLOBAL_STORES_CH" "d" ""gz:\...
1444810735.880313 [0 int.ern.ip.4:33145] "hmset" "zc:k:my_cache_prefix_CONFIG_GLOBAL_STORES_CZ" "d" ""gz:\...
1444810735.887541 [0 int.ern.ip.4:33148] "hmset" "zc:k:my_cache_prefix_CONFIG_GLOBAL_STORES_EN" "d" ""gz:\...
1444810735.897101 [0 int.ern.ip.0:38044] "hmset" "zc:k:my_cache_prefix_CONFIG_GLOBAL_STORES_FI" "d" ""gz:\...
1444810735.904982 [0 int.ern.ip.4:33149] "hmset" "zc:k:my_cache_prefix_CONFIG_GLOBAL_STORES_AT" "d" ""gz:\...
1444810735.912321 [0 int.ern.ip.4:33142] "hmset" "zc:k:my_cache_prefix_CONFIG_GLOBAL_STORES_PL" "d" ""gz:\...
1444810735.919390 [0 int.ern.ip.7:51360] "hmset" "zc:k:my_cache_prefix_CONFIG_GLOBAL_STORES_EN" "d" ""gz:\...
1444810735.929803 [0 int.ern.ip.7:51359] "hmset" "zc:k:my_cache_prefix_CONFIG_GLOBAL_STORES_AT" "d" ""gz:\...

again and again. Sometime after some minutes the global caches could be written and everything is ok. But under heavy traffic I have to take the shop offline and warm the cache manually.

My Redis config:

   <backend>Cm_Cache_Backend_Redis</backend>
    <backend_options>
        <server>xxx.xxx.xxx.xxx</server>                          <!-- or absolute path to unix socket for better performance -->
        <port>6379</port>
        <database>0</database>
        <persistent></persistent>
        <force_standalone>0</force_standalone>                   <!-- 0 for phpredis, 1 for standalone PHP -->
        <automatic_cleaning_factor>0</automatic_cleaning_factor> <!-- Disabled by default -->
        <compress_data>1</compress_data>                         <!-- 0-9 for compression level, recommended: 0 or 1 -->
        <compress_tags>1</compress_tags>                         <!-- 0-9 for compression level, recommended: 0 or 1 -->
        <compress_threshold>20480</compress_threshold>           <!-- Strings below this size will not be compressed -->
        <compression_lib>gzip</compression_lib>                  <!-- Supports gzip, lzf and snappy -->
    </backend_options>

Any idea to prevent such race conditions?

Best Answer

The 'cache is corrupt' there is to short time to rebuild the core config cache, it's to big.

More information or explanation and the credits; https://github.com/AmpersandHQ/magento-ce-ee-config-corruption-bug

The fix is easy, in app/code/core/Mage/Core/Model/Config.php

change the function: public function init($options=array())

To :

$cacheLoad = $this->loadModulesCache();
if ($cacheLoad) {
    return $this;
}
// Cache fix 
$this->_useCache = false;
// Fix End
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