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As it is best practice to group related keys that are frequently retrieved together (using multiGet) on a single server for optimum performance, I have a couple questions regarding the implicit mechanics employed by the client functions built for doing this.

I have seen two different approaches for serving what I assume is the same purpose using libmemcache (php-memcached specifically). The first and most obvious approach is to use getByKey/setByKey to map keys to servers and the second is to use the option OPT_PREFIX_KEY (there is a simple example posted in the php documentation under memcached::_construct), which according to the documentation is "used to create a 'domain' for your item keys". The caveat of the second approach is that it can only be set on a per-instance basis, which may or may not be a good thing.

So unless I am completely mistaken, and these two approaches don't actually serve the same purpose; is that any clear benefit for going with approach over the other?

And while I'm on this topic my other question would be: What are the implications, if any, to mapping keys to servers in a consistently hashed scenario? I'm assuming that if a node were to fail, the freeform key would simply be remapped to a new server without any issue..

Thanks!

Adam
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If these keys are really almost always retrieved together you probably want to cache them together in a single key/value pair, for example by sorting and concatenating keys and storing values serialized as a dictionary in JSON or similar format.

Returning to your question:

  • OPT_PREFIX_KEY has almost nothing to do with grouping values by key, it just prefixes all keys used by this particular client, so "1" becomes "foo1" and is distributed by consistent hashing using this new value, without any grouping by "foo".
  • getByKey/setByKey does the closest thing to what you want, since it can pass different keys to libketama (used to choose server) and memcached server. If you specify same first key and different second keys - they will end up on same memcached server, but won't overwrite each other.

Premature optimization is the root of all evil

Ivan Blinkov
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