[cacois]

Lightweight is your problem here, I think, but I've used CouchDB successfully in similar situations. However, its not in-memory like Redis is.

[nurple]

Big fan of couch, wish it was more popular. Its http interface basically removes the whole http translation layer of code most projects put in front of the db.

For such a small need, you might also look at PouchDB. Inspired by couch but simplified to allow it to run in-browser.