I have a database with files which can be searched, browsed and have multiple copies on multiple servers.
I cache searches, browse pages and server locations (urls). Say I delete a file, what’s a good way to invalidate all searches, browse data and urls for this file? Or if a file server goes down, and I need to invalidate all urls pointing to this server?
Essentially I’m looking for something similar to memcache-tags, but with standard memcache and php components. (Without having to change anything on the web server itself). I need some sort of many to many relation (one server has many files, and one file has multiple servers) in between keys, but can’t seem to figure out a good way to accomplish this. In some situations stale cache is acceptable (minor updates etc.), but in some cases it’s not (typically delete, and server down) where I need to invalidate all cache items containing references to it.
Some approaches I have looked at:
$ns_key = $memcache->get("foo_namespace_key");
// if not set, initialize it
if($ns_key===false) $memcache->set("foo_namespace_key", rand(1, 10000));
// cleverly use the ns_key
$my_key = "foo_".$ns_key."_12345";
$my_val = $memcache->get($my_key);
//To clear the namespace do:
$memcache->increment("foo_namespace_key");
- Restricts the cache key to a single namespace
$files = array('file1','file2');
// Cache all files as single entries
foreach ($files as $file) {
$memcache->set($file.'_key');
}
$search = array('file1_key','file2_key');
// Retrieve all items found by search (typically cached as file ids)
foreach ($search as $item) {
$memcache->get($item);
}
- Gives a problem if a file server is down, and all keys containing urls to this server should be invalidated (E.G large number of small cache items would be needed which in turn requires a large amount of requests against cache)- breaks any chance of caching full objects and resultsets
class KeyEnabled_Memcached extends Zend_Cache_Backend_Memcached
{
private function getTagListId()
{
return "MyTagArrayCacheKey";
}
private function getTags()
{
if(!$tags = $this->_memcache->get($this->getTagListId()))
{
$tags = array();
}
return $tags;
}
private function saveTags($id, $tags)
{
// First get the tags
$siteTags = $this->getTags();
foreach($tags as $tag)
{
$siteTags[$tag][] = $id;
}
$this->_memcache->set($this->getTagListId(), $siteTags);
}
private function getItemsByTag($tag)
{
$siteTags = $this->_memcache->get($this->getTagListId());
return isset($siteTags[$tag]) ? $siteTags[$tag] : false;
}
/**
* Save some string datas into a cache record
*
* Note : $data is always "string" (serialization is done by the
* core not by the backend)
*
* @param string $data Datas to cache
* @param string $id Cache id
* @param array $tags Array of strings, the cache record will be tagged by each string entry
* @param int $specificLifetime If != false, set a specific lifetime for this cache record (null => infinite lifetime)
* @return boolean True if no problem
*/
public function save($data, $id, $tags = array(), $specificLifetime = false)
{
$lifetime = $this->getLifetime($specificLifetime);
if ($this->_options['compression']) {
$flag = MEMCACHE_COMPRESSED;
} else {
$flag = 0;
}
$result = $this->_memcache->set($id, array($data, time()), $flag, $lifetime);
if (count($tags) > 0) {
$this->saveTags($id, $tags);
}
return $result;
}
/**
* Clean some cache records
*
* Available modes are :
* 'all' (default) => remove all cache entries ($tags is not used)
* 'old' => remove too old cache entries ($tags is not used)
* 'matchingTag' => remove cache entries matching all given tags
* ($tags can be an array of strings or a single string)
* 'notMatchingTag' => remove cache entries not matching one of the given tags
* ($tags can be an array of strings or a single string)
*
* @param string $mode Clean mode
* @param array $tags Array of tags
* @return boolean True if no problem
*/
public function clean($mode = Zend_Cache::CLEANING_MODE_ALL, $tags = array())
{
if ($mode==Zend_Cache::CLEANING_MODE_ALL) {
return $this->_memcache->flush();
}
if ($mode==Zend_Cache::CLEANING_MODE_OLD) {
$this->_log("Zend_Cache_Backend_Memcached::clean() : CLEANING_MODE_OLD is unsupported by the Memcached backend");
}
if ($mode==Zend_Cache::CLEANING_MODE_MATCHING_TAG) {
$siteTags = $newTags = $this->getTags();
if(count($siteTags))
{
foreach($tags as $tag)
{
if(isset($siteTags[$tag]))
{
foreach($siteTags[$tag] as $item)
{
// We call delete directly here because the ID in the cache is already specific for this site
$this->_memcache->delete($item);
}
unset($newTags[$tag]);
}
}
$this->_memcache->set($this->getTagListId(),$newTags);
}
}
if ($mode==Zend_Cache::CLEANING_MODE_NOT_MATCHING_TAG) {
$siteTags = $newTags = $this->getTags();
if(count($siteTags))
{
foreach($siteTags as $siteTag => $items)
{
if(array_search($siteTag,$tags) === false)
{
foreach($items as $item)
{
$this->_memcache->delete($item);
}
unset($newTags[$siteTag]);
}
}
$this->_memcache->set($this->getTagListId(),$newTags);
}
}
}
}
- No control over what keys are invalidated when, due to internal memcache key dropping, can drop a tag key which in turn invalidate a large number of actual valid keys (which would still exist)
- Issues with write concurrency
// Having one slow, and one fast cache mechanism where the slow cache is reliable storage containing a copy of tag versions
$cache_using_file['tag1'] = 'version1';
$cache_using_memcache['key'] = array('data' = 'abc', 'tags' => array('tag1' => 'version1');
- Potential bottleneck using disk/mysql etc. for the slow cache
- Issues with write concurrency
Having come accross the comment here, which explains the logic of evicting existing keys, I believe tags can be implemented reliable by the version flags approach mentioned in: PHP memcache design patterns
I actually implemented this logic once already, but discarded it as unreliable due to memcache eviction of elements before they expire. You can find my initial implementation here. I do however, believe this is a reliable tags pattern because:
Please correct me if I’m wrong! 🙂