implement: cas (check and set) command

[exts]

I don't know much about this feature aside from a library in another language uses it and I'm trying to offload some work using crystal, but there's no current memcache libraries that support the check and set/compare and swap functionality for memcached.

This was something I found (ignore the site's name, it has a lot of important information at the beginning) along the way, but I've never worked with sockets or byte code much so I'm not even sure where to start.

Hopefully that helps someone with implementation of said feature.

[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.