check if an item exists

There is a feature, currently undocumented (and thus unsupported) called stats key. It may be documented/supported soon, but you can certainly try it in a test environment to see if it works for you. This would allow you to get information about a given key without actually retrieving it. No stats on the key would mean it doesn't exist yet.

Of course, an add with expiration 1 is pretty close if you expect it to not really exist. I know that's not quite right though.

What is exactly the name of this undocumented feature, i couldn't find it?
thanks

It's simply an argument to the stats command for the key you want information on.

It's actually documented at a code level right here:
https://github.com/couchbase/ep-engine/blob/master/docs/stats.org#29-sta...

With Open Source, everything is technically documented. :)

Oh, and by the way, you can try it from either your client library or the commandline cbstats.