implement: cas (check and set) command about crystal-memcached

[comandeo]

Hello @exts

Thank you very much for your comment. I read the link you provided, and it turned out this is about memcached text protocol. This library is based on the "new" binary protocol, which is described here: https://github.com/memcached/memcached/wiki/BinaryProtocolRevamped

As you can see, there is no dedicated CAS command in this protocol. Instead, there is a "version" of a value, which is returned as a result of GET command. In this library is it implemented as get_with_version method. Then, you can pass this version to set method. If the passed version differs from the actual version in memcached, the client will raise BadVersionException. I think it is in accordance with the protocol description:

If the Data Version Check (CAS) is nonzero, the requested operation MUST only succeed if the item exists and has a CAS value identical to the provided value.

Therefore, I assume that if you want to implement compare and set in your application, you should do something like this:

• using get_with_version get a key value and its version (cas token) , and store them somewhere
• when you want to update the value, call set and pass the version among other parameters
• check for BadVersionException; if it is thrown, do something that makes sense for your app

This library is just an a thin wrapper around memcached protocol command. Therefore, I was not sure if it was a good idea to implement some additional CAS logic there. On the other hand, some other clients (dalli, for example) implement cas method. So, it may be a good idea to provide something like this here.