Memcached consistent hashing mechanism

If you are using the Memcache functions through a PECL extension, you can set global runtime configuration options by specifying the values within your php.ini file. One of them is memcache.hash_strategy. This option sets the hashing mechanism used to select and specifies which hash strategy to use: Standard (default) or Consistent.

It’s recommended that you set to Consistent to allow servers to be added or removed from the pool without causing the keys to be remapped to other servers. When set to standard, an older strategy is used that potentially uses different servers for storage.

With PHP, the connections to the memcached instances are kept open as long as the PHP and associated Apache instance remain running. When adding a removing servers from the list in a running instance, the connections will be shared, but the script will only select among the instances explicitly configured within the script.

So, to ensure that changes to the server list within a script do not cause problems, make sure to use the consistent hashing mechanism.



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4 responses to “Memcached consistent hashing mechanism”

  1. I was wondering about this!
    So… if a Memcached server goes down, and you are looking for the key that was cached to it, it just won’t find it? Or if you are trying to save a key and the hashing algorithm wants to place it to a dead server?

  2. Thanks for the link :)
    So sounds like the keys that are “missing” if a server is down or removed, will just get remapped.

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