"This is not something memcached or redis support out of the box, as far as I know. It would seem to imply a cache manager service that has its own in-memory table of Promises."

Note that it isn't even meaningful to suggest that memcache or Redis should support this, because they aren't responsible for filling cache values for misses. Only something actually generating the value upon the miss can implement this "promise".

And in the general case, you can't serialize promises either, so you can't be "putting a promise into memcached", because memcached only stores bytes. You'd have to wrap a lot of machinery around it, and even then, I personally would say it's the machinery doing it, not memcached.

(I think Redis does have some features that could be pressed into service here for notification, but Redis still wouldn't be doing the actual filling in of the value, since it can't.)